Flotation of Oxidized Ores

About three years ago John Hays Hammond took over the control of the Eureka Metallurgical Co., at Salt Lake City, Utah. Funds were advanced for investigating the process invented by R. V. Smith, for concentrating oxidized lead ores. A great deal of laboratory work was done and a reagent was developed by C. M. Nokes of Salt Lake, which was found to be particularly suitable for oxidized copper ores, refractory manganese silver ores, and oxidized gold ores. After sufficient laboratory work had been

performed to demonstrate the fundamental principles of these inventions, a test mill, having a capacity, of 20 tons of ore daily, was erected at Murray, Utah.

In this test mill approximately 1200 tons of various types of ores have been concentrated by means of the above-mentioned process and reagents, and this large-scale work as well, as laboratory work is being continued at present. Details of these mill tests are given in Table 1, and a representative group of laboratory tests are shown in Table 2.

The process requires no special machinery or apparatus. Its success depends entirely on the method of preparing the pulp and its subsequent flotation in clean water, as outlined in the Smith patent. The reasons for the success of the Nokes special reagent are not definitely known. The outstanding feature, which seems to be of extreme importance, is that when the concentrates are examined under the microscope, each particle of mineral appears to have a particle of paraffin attached to it. This paraffin-mineral combination seems to be particularly suitable for flotation and it follows that any coating oils readily attach themselves to the paraffin-mineral combination.

The costs for this process are the usual milling and flotation costs, with an additional charge due to the use of sodium sulfide and larger quantities of oils. This additional charge will vary from 30 to 75 c. per ton of ore treated. If paraffin-sodium sulfide combination is used, a large amount of paraffin is subsequently recovered for re-use by merely heating the concentrate, suspended in water, whereupon the paraffin rises to the top and can be skimmed.

Extracts from the patent specifications of the two above-mentioned inventions will possibly be interesting.

Charles M. Nokes Patent (U. S. No. 1,444,552; issued Feb. 6, 1923)

In a typical instance of the use of my process, I employ in the preparation stage, a solid hydrocarbon, such as paraffin, and an alkaline sulfide, such as sodium-sulfide, the ore under treatment being a non-sulfide ore, or a mixture of sulfide and non-sulfide ores. The paraffin and the sodium-sulfide are fused or mixed together at a temperature sufficiently high to permit of the liquefaction of the hydrocarbon. To effect this I may pulverize the sodium-sulfide to say 100 to 250 mesh. I next melt the paraffin, and then make a paste by mixing the paraffin and the pulverized sodium-sulfide. This paste, as it cools, solidifies into an apparently homogeneous mass. If this be allowed to solidify without stirring, it becomes hard and flinty and must be pulverized before addition to the tube mill. But stirring seems to give it a more or less granular condition, somewhat lumpy, and suitable, when broken into fragments, for the tube mill. It is introduced into the tube mill along with the ore, to be ground into pulp, or it may be introduced into the pulp after the ore has been ground and passed to emulsifiers.

It should be understood that this mixture is not a froth or scum maker, or “lifter,” and therefore other oils are added, such as pine oils, to expedite the formation of a certain amount of froth or scum wherein the particles of the mixture collect and with which they may be removed from the flotation cell. The function of the paraffin-sodium-sulfide mixture is one of preparation, and not one of flotation or frothing. It acts upon non-sulfide or oxidized ores so that in the flotation stage they float, while the gangue remains submerged. Certain lifting oils, like Yaryan pine, which will not lift oxidized ores direct, will do so after the

addition of the paraffin mixture. If the lifting oils were omitted, the paraffin-sodium-sulfide mixture alone would give flotation in the flotation stage, but the scum would be too sparse to remove with sufficient rapidity; it would be very sparse and very rich, made up of a vast number of particles of paraffin of minutest size, each one solid and with the mineral particles or values clinging to it.

The amounts of the mixture employed, and the ratios between paraffin and sodium-sulfide vary. On oxidized copper ores I have used six to ten pounds, per ton of ore, of sodium sulfide, and the same amount of paraffin. On other ore I have used ten pounds of the sodium sulfide and seven and one-half pounds of paraffin. In this case I used thirteen pounds per ton of ore of the lifting oils, etc.

Reinold V. Smith Patent (U. S. No. 1,459,167; issued June 19, 1923)

The ore to be treated is prepared as usual, that is to say, it is broken up and crushed to the desired condition of fineness, and if desired the disintegrated material may be separated into granular and slimes portions, these being subjected to substantially the same treatment, thereafter, but separately.

One of the important features of the process is in the fact that the finely divided ore, in a thick water pulp, is treated with an oil or other substance, such as petroleum sludge, for instance, in the presence of a soluble sulfide, with the object of preparing the pulp so that the particles that it is desired to concentrate may be floated in the subsequent steps of the process. Where oil is used it may be added to the thick pulp in a suitable apparatus.

After the pulp has been treated as described in the preceding paragraph, the water, including such preparation substances as remain with it, is separated, the thoroughly mixed dewatered pulp is diluted with several times its volume of new water which is not effectively contaminated by the preparation substances, and is then discharged into a flotation cell, wherein concentration is accomplished in the usual way.

It will be understood that the process divides itself naturally into two parts, i.e., preparation of the finely divided ore in a thick pulp, and flotation of mineral-particle components thereof in a dilute pulp. Further, instead of removing the water of preparation I may form the thin flotation pulp by dilution with quantities of new water, sufficient to substantially eliminate the deterrent effect of remanent sulfidizing agents.

The granular portions of the ore, and the slimes, may be treated separately by the steps above described, and the preparation water from the sands pulp be re-used in the preparation of a fresh supply of slimes, while that from the slimes pulp, is re-used in the preparation of fresh supplies of sands.

The test mill of the company is equipped with a 4-ft. Hardinge tube mill; drag classifier; ball-mill classifier; small tube mill; two Janney flotation machines; a Fahrenwald flotation machine, and a small American filter. There are also two Dorr thickeners. Preliminary crushing for all test work is done at the local ore testing plant. The mill is so equipped that the flowsheet can be varied to every possible combination. Work up to date has clearly demonstrated that all laboratory results can be duplicated in the test mill and that the quantities of oils and chemicals required in the laboratory machine can be reduced about 40 per cent, when the same ore is put through the mill. In nearly all cases the concentrate produced in the mill is of higher grade than that in the laboratory tests.


Mining Methods

These mines, which belong to the Burma Corporation, Ltd., formerly a London company now incorporated in Rangoon, Burma, are situated in the semi-independent state of Tawng-Peng, one of the small divisions comprising the northern Shan states and erroneously known as part of Upper Burma. Bawdwin is approximately 23° 6′ N. latitude and 97° 20′ E. longitude, 450 miles north of Rangoon, 169 miles northeast of Mandalay and 50 miles south and west of the Province of Yunnan, China (see Fig. 1).

Although the mines were worked by the Chinese during the Ming Dynasty, 1412 A. D., the most extensive operations took place between 1796 and 1851, when the mines contributed largely to the silver market of China. During the reign of Tung Chik, 1868 A. D., they were abandoned partly because of the Mohammedan rebellion in Yunnan, which made life and property insecure, but largely because of the difficulty of operating the mines on account of water and poor ventilation. Since that time practically nothing was done, although the Burmese kings are said to have made several sporadic attempts to work the mines, until 1891 when Europeans were attracted by the great slag dumps (assaying about 40 per cent, lead) which the Chinese had left after extracting the silver.

After a railroad and smelter had been built and 200,000 to 300,000 tons of slag smelted for lead, exploration work was started and old workings cleaned out in order to locate the remnants of the orebody that it was supposed the Chinese had left. After two years of most discouraging work, the remains of a large orebody was discovered by the Dead Chinaman Tunnel or what is the 171-ft., or No. 2 level adit. From then on the development was rapid and today the Chinaman is considered one of the largest high-grade silver-lead-zinc orebodies in the world. The total ore reserves on Jan. 1, 1922, were:

Of this, only a little over one-tenth is designated as probable ore. No mineral with a value of less than 20 per cent, combined lead and zinc is considered commercial ore unless it is part of the 335,681 tons of copper ore that averages silver 23.2 ounces, lead 12.8 per cent., zinc 7.7 per cent., copper 11.0 per cent.

The mineral lease comprises an area 5 miles long by 2 wide and covers a period of 30 years from Jan. 1, 1920. The royalty payable to the government is 2½ per cent, of 30 per cent, of the gross value of the metal contents of the ore. The company, as reorganized in Rangoon, has a total authorized capitalization of 20,000,000 shares at Rs. 10 each, of which 13,541,682 shares have been issued. There is also a first mortgage of 8 per cent, convertible debenture stock of £1,000,000 at Rs. 10 to the pound.


The rocks at Bawdwin are of two classes: volcanics (comprising rhyolite tuffs, breccias and flows) and unfossiliferous sediments comprising quartzite, sandstone and shales, which are probably Ordovician. Rhyolite tuff (the ore-bearing formation) forms a wide band running northwest and southeast that has been exposed by the erosion of the overlying sediments. Through this band the main ore fissure passes longitudinally and forms the lode by metasomatic replacement of the rhyolite tuff by ascending ore-bearing solutions. Replacement has taken place parallel to the strike of the fissure, as shown by the laminated structure of the ore and its dimunition in value and density as it passes into low-grade ore, mineralized material, and finally barren tuff on approaching the east, or foot-wall, side.

In the Chinaman lode, there is a well-defined hanging wall, which makes a sharp demarcation between ore and waste; but there is no such foot-wall, the limit of the orebody in that direction is considered to be the limit of what is defined as commercial ore. On some levels of the Chinaman section, the solid sulfide is 50 ft. wide for over 1000 ft. along the strike and in places has reached a width of 140 ft., see Fig. 2. The faulted portion of the Chinaman lode is known as the Shan and is much narrower. This narrowing is partly accounted for by the character of the rock. In the Chinaman the fissure cuts through coarse rhyolite tuff with large feldspar crystals, which were easily dissolved by the ascending solutions making the rock more porous and favorable for the deposition of a large orebody, while in the Shan the tuff is more compact, siliceous, and fine-grained and consequently gives a clean-cut narrow fissure vein. The orebody has a tendency to finger out and become much smaller at the outcrop, as compared to the body below.

The ore is an intimate mixture of galena and sphalerite and in many places also of chalcopyrite, although the latter is often found in parallel bands alongside the former as pure unmixed chalcopyrite. The mixture of galena and sphalerite contains approximately 1 oz. of silver for every per cent, lead; it is generally considered that the silver accompanies the galena, but the ore is so complex and the crystals so intimately inter-grown that it is most difficult to make a first-class separation of the metal by mechanical means. Taken as a whole, the southern end of the Chinaman section predominates in zinc-lead ore; the middle in more equal quantities of both; and in the northern end the zinc is partly replaced by copper. In practically all sections, the ore along the hanging wall is the highest grade, with the lead predominating over the zinc, but toward the center or the foot wall the zinc contents increase until, in many sections, the zinc predominates. Still farther toward the foot wall the ore becomes lower grade and below what is classified as ore until it is only mineralized: the lead however predominates and often is found as pure crystals of galena. These conditions, together with the following, have been instrumental in determining the method of mining:

(a) The Chinese worked the mine from the surface to 50 ft. below the 171-ft. level for the silver alone and consequently did not want lead ore, high in zinc or low-grade ore; so that there is a large remnant of

the orebody on the upper levels mixed with Chinese filled stopes and workings. This remnant must be preserved for future requirements, as a considerable part of it is not commercial ore today.
(b) The banded structure of the ore; for instance, there are bands of chalcopyrite running parallel to the lead-zinc ore. These bands must be mined separately in order to save the copper.
(c) There is only one pronounced wall, the hanging wall. The ore starts from it as a solid mass high in lead and gradually passes through zinc and chalcopyrite bands into low-grade ore and, finally, into mineralized ground. That is, there is no definite stoping limit toward the foot-wall side except what is arbitrarily fixed as the limit of commercial ore (20 per cent, combined lead-zinc with whatever silver it contains). Later, this arbitrary value may be lowered and another 2,000,000 tons of low-grade ore added to the reserve. Any stoping method must take this low-grade material into consideration and not leave it in such condition that it cannot be economically mined.

General Conditions that Influence Methods of Mining

The country is very rough. There are no roads; the only means of travel are a 2-ft. gage railway with steep gradients and sharp curves, and trails along the ridges of the hills.

From November until May (the dry season), this region is healthy and enjoyable, but during the wet, or the remaining, months the climate is depressing. Many employees suffer from malaria and kindred ailments. The coolies, especially those from the high cold regions of China, suffer considerably from malaria as they do not take the necessary precautions.

The upper levels of the mines, on account of the many workings, including all the old Chinese stopes, drives, etc., in which a large surface of ore is exposed to oxidation, are quite hot. During the wet season, water percolates through and increases the oxidation and also the humidity of the air.

The rain falls in heavy showers; the fall amounting to about 70 in. in six months, making the mine quite wet and the ground heavy. It is also difficult to keep the railway free from landslides and washouts.

The mine is situated 3100 ft. above sea level, which is very favorable to the health. The air is clear and the dense fogs found 1000 to 2000 ft. lower do not prevail.

Water, Fuel, Timber, and Other Supplies

Good water, in abundance, is piped to all bungalows and to all the levels in the mine. This is a treat to one who has lived in the tropics as, in most cases, water must be boiled to insure no contamination. Partly furnished bungalows, electric lighting, and water are supplied to all Europeans and fuel to all employees. This fuel is brought in by pack mules a distance of 8 miles and costs Rs. 14 per stack (108 cu. ft.). Local timber, which is hardwood and heavier than water, is shipped in by rail from the company’s timber reserves, about 20 miles away, at the following rates:

Mine logs……………………………………………………..Rs. 41 per ton (50 cu. ft.)
Local sawn timber……………………………………Rs. 90 per ton (50 cu. ft.)
8 in. by 2 in. by 6 ft. lagging…………………………Rs. 74 per 100 pieces
8 in. by 3 in. by 7 ft. 4 in. lining boards…….Rs. 120 per 100 pieces

The laggings and lining boards are hand cut in the jungle by Chinese. Bamboo, 4-in. and 5-in., used for lining the sides of stopes preparatory to filling, cost Rs. 5 per 100 laid down in Bawdwin. High-grade ingyin

timber is shipped from Mandalay at the rate of Rs. 140 per ton (all charges). This dense, hard, local timber, which a person might think would last for years in the mine, has a short grain and breaks without any warning under no great load. Some of this timber breaks sharply across the grain and looks as though it had been sawn through. It lasts, on the average, in this humid atmosphere about three years. In all permanent workings, the timber is now creosoted, which we believe will double its life. It is difficult to frame, and a nail cannot be driven into it unless a hole is first bored.

Coke and coal are brought from India across the Bay of Bengal and then carried by rail 450 miles; coal costs at the mine, approximately, Rs. 56; coke, Rs. 79.

Fuel oil, from the Yenangyoung oil fields in Burma, is shipped from Rangoon and costs Rs. 0.3 per gallon.

Electric Power

The corporation has installed a 3000-hp. hydro-electric plant at Mansam Falls, which supplies the mine, smelter, and mill. A Diesel oil-engine plant capable of delivering 1800-hp. is available for emergencies. From the hydro-electric plant, the power is transmitted 38 miles over high-tension wires at 33,000 volts to the mine substation where it is stepped down to 550 volts for mine use. The mine uses 270,000 kw.-hr. per month and is charged Rs. 0.0106 per kw.-hr. An additional expense on the low-tension side, of motor operators, repairs, and maintenance, increases the cost to Rs. 0.0142 per kw.-hr., which is the charge applied to the mine operation. No depreciation charges are included in this cost of power, this being cared for by the head office.


Chinese from Yunnan, China, form the nucleus of the underground labor with a number of Gurkhas (from the independent state of Nepal, Upper India) and a small number of other Indians. Practically no natives of Burma work in the mine. On the surface, there are men from nearly every neighboring country. Chinamen from Shanghai, Canton, Indo-China, and neighboring Chinese provinces, many of whom cannot converse with one another; Chinese-Shans from the northern Shan states; men from all parts of India even Nepal and Afghanistan. Men of all castes, creeds, and languages. The laboring class comprises men from nearly all neighboring countries; the Burman makes a good clerk and a fair carpenter, but will not do hard labor. The other natives of the country will not work in the mine nor do manual labor on the surface under any conditions; they prefer to cultivate their small farms though they eke out a bare existence.

All technical supervision must be supplied by men from England, Australia, or the United States. It will be many years before the Anglo-Indians or Indians can be trained for this technical work and take the place of a European in the more responsible positions.

The Chinese and the Chinese-Shan coolies are the best laborers and when one makes his home in Burma, taking his family there, he makes an excellent miner. However, a large part of the labor is seasonal. The Chinaman prefers to come over at the beginning of the dry season and goes home before the wet season in order to plant his rice. It often takes 20 days for him to walk in from remote places in Yunnan. He is honest, compared to the Indian coolies, and is easy to handle if treated with reasonable justice. On an average, two Chinamen will do the work of one European, and one Chinaman that of two Indians. As a large part of the labor is seasonal, it is not efficient. The turnover is very large, averaging 11 per cent, for six months. During March, as many as 20 per cent, of the men leave for their homes. It is most discouraging to teach a coolie to run a drill or timber a stope, just to have him leave when he becomes proficient.


The pumping problem is simple, as practically all the water is handled by a gravitysystem through three adits—No. 1 level, No. 2 level, and No. 6 level—except the development below the latter which is opened by two winzes. These require two sinking pumps (a No. 7 and a No. 9 B Cameron) to pump the water to the No. 6 level. This No. 6 level adit, known as Tiger tunnel, takes care of 40,000 gal. per hour.


The property is accessible from Rangoon, the main port in Burma, by the Burma Railways (a meter-gage line) that connects with the company’s private road at Nam-Yao. From this point, a 2-ft. gage line leads over the mountains to Bawdwin, a distance of 45 miles.

Exploration, Sampling, and Estimating Methods

Exploration is today chiefly done by drives, crosscuts, and winzes. In the upper part of the orebody, down to 50 ft. below the 171-ft. level, the orebody is riddled with Chinese adits and workings; but as they are small, tortuous, and filled with mud and water, new adits, drives, and crosscuts were made to explore the orebody. A small amount of diamond drilling was done; but on account of the character of the ore, the drillers were continually losing the water or the bit so that it is more satisfactory to develop by actual workings. For development purposes, drives were originally run in the ore parallel to the strike and crosscuts put out to the extremities of the ore every 100 ft. with occasional rises in ore from these crosscuts to the upper level. During the later development of the mine, the drives were made in the country rock foot wall and crosscuts run to and through the ore. These drives require no timber and being outside and parallel to the orebody become the main extraction drives for the present stoping method. Two winzes, or inside shafts, were sunk from the No. 2 adit level to open up and develop the levels below and also facilitate the driving of No. 6 adit from each end.


Each crosscut is sampled every 5 ft. on both sides and the average of the two taken for calculations. Drives and raises are also sampled every 5 ft. but these samples are not included in the calculations of the orebody but are used to prove the continuity of the ore and grade only. As most of the ore is friable and soft, it is not difficult to cut a small trench of even width with hammer and moil. However, there are occasional patches of low-grade siliceous ore, which require the aid of a Jackhammer. Samples, when assayed, are entered in an assay ledger, one page for each crosscut, drive, etc. Using a width factor of 1 for every 5 ft. of sample and carrying a running total of width factor times ounces or per cent, facilitates the averaging of the values of any drive, crosscut, etc., for its entire length, or between any two points (see Fig. 6).


After several levels had been opened up by crosscuts and raises every 100 ft., it was apparent from the character of the orebody—that the size and grade were fairly uniform from level to level and crosscut to crosscut and the continuity and transition of values from higher to lower grade were so gradual—that it was permissible to estimate the orebody on the crosscuts alone; that is, the width and value of the ore in the various crosscuts on any particular level (with the aid of geological evidence in faulted country) would give the area and grade of that level. For the purpose of estimating, complete assay plans are made of each level and by referring to the assay ledger the average of all 5-ft. samples (both sides) is plotted and the values marked on the map, showing location, width, and assay values. Values of samples from all crosscuts, drives, etc., are shown on this assay plan for every 5 ft., or less, notwithstanding the material is mineralized only or waste. For estimating ore and tonnage, a graphic system is employed and another plan, called the estimating plan, is made of each level (see Fig. 7).

The next question to consider, and possibly the most important, is: what is commercial ore now and what will be commercial ore during the life of the property? This question is most difficult to answer because the mine is in a state of development and no one knows its life, or only approximately so, and furthermore due consideration must be given to the possibility of new metallurgical treatment and the fluctuation in price of the products. However, the result is only an estimate based on available data. The advisers for this mine decided on 20 per cent, combined lead and zinc, in whatever combination, with its accompanying silver; or in the case of copper ore, any of the combination containing lead and zinc with 3 per cent, copper and its accompanying silver.

By referring to the assay ledger, in preference to the assay plans, the limits of commercial ore can be marked on each crosscut. Due consideration must be given to the stoping method to be applied in order to know just what samples to include. Lines are then drawn on the plan from crosscut to crosscut enclosing the body of commercial ore. The area is divided into triangles, as shown in Fig. 7 and, with the aid of a scale, the base and altitude of each triangle is measured. The areas of triangles in the adjoining half of the space between the crosscuts (that is nominally 50 ft. on either side) are multiplied by the average value of the crosscut and give as a product ounces times square feet and per cent, times square feet. The sum of all ounces times feet and per cent, times feet divided by the total of all the areas between the crosscuts, which is the area of the level, gives the average value of that level. By calculating the Pb in form of PbS, Zn in form of ZnS, and the Cu in form of CuFeS2, the total percentage of sulfides is obtained and the remainder is considered quartz. By deducting 5 per cent, for voids and using the specific gravity of the various minerals comprising the ore, the number of cubic feet per ton is obtained. The sum total of all the areas divided by this factor gives the number of ton-feet. Ton-feet multiplied by the dis¬tance up and down, which is generally half way to the next level, gives the total tonnage and value of the level; see Fig. 8. As the ratio of cubic feet to tons depends on the relative specific gravity, porosity, and moisture of the ore, the average value of the crosscuts as calculated is not exact, as they were calculated on volume and not tonnage. It is impossible and too expensive to get the specific gravity of each sample, so that this factor cuts down the metal contents per actual tons of ore 7 to 10 per cent.

Ore is classified as proved and probable. Proved is considered ore of practically no risk in estimating its value or continuity. As development work has shown the orebody to be fairly uniform from level to level with no abrupt changes in grade or size, as proved by raises every 100 ft., all ore between lines drawn from the boundaries of proved ore on one level to those above or below is considered proved; also ore 25 ft. above the

upper level and 25 ft. below the lowest. Probable ore is considered as containing some risk on account of the lagging behind of development on a level, particularly at the extremities of the orebody. From 25 to 100 ft. below the lowest level is considered probable. Probable ore would also be assumed 50 ft. ahead of the last crosscut, where geological evidence is favorable for continuity. It would be calculated on the value of the last crosscut and on an area equal to a triangle, having an altitude of 50 ft. and a base the width of the ore in the crosscut.

Accuracy of Methods of Estimation

As the mine is still in its infancy, it is not possible to check the method of estimating by actual mill yield plus tailings. In most mines, there is a loss of 10 per cent, or more in actual value. By the present method of estimating, using the algebraic average of a section, the metal contents per actual ton of ore is cut down 7 to 10 per cent, dependent on whether the high-grade or the low-grade predominates. This should take care of any mining loss, and dilution will add to the values recovered as the dilution will be low-grade non-commercial ore, instead of barren waste.

History of Mining Methods

The first method of stoping tried was the common flat-back square-set stope, which was carried very wide and long with a number of ore passes to shovel into. Fortunately, this method was stopped in time, as it is impossible to prevent such a large stope from caving and the cost of keeping so many timbered ore passes is excessive. Furthermore, the filling was laborious and expensive as all waste had to be wheeled and shoveled into position.

A narrow Gilman slice rill stope was tried in the hardest and most favorable ore, with only timber on the sides to confine the waste from the adjacent stopes. This was a failure and a death trap, as the solid sulfide ore (9 and 10 cu. ft. to the ton) would drop without any warning in large masses; and in other places the friable ore would run up in the form of a chimney and cause the stope to cave.

Present Mining Methods and Reasons for Adoption


The upper levels of the mine have been riddled by Chinese workings; consequently the ore left is mixed with old filled stopes and gives a low-grade oxidized ore. The company does not care to mine and mill this ore at present as it plans to open-cut and quarry a large portion of it in connection with the regular waste-filling plan now being used to fill the stopes. Any method of mining the ore, must leave these upper levels undisturbed until the company is prepared to handle this ore. This eliminates several systems of stoping, for instance mill hole, top slice, and caving systems, and leaves only some form of a timbered and filled stope. The ore is so heavy and friable that it must be closely timbered to support any opening and the orebody is so wide that it requires mining in sections.

By elimination we have arrived at the square-set system of stoping with all its modifications. A flat-back square-set stope is costly and slow, on account of the laborious work in handling the ore and waste, and is not easy to ventilate. To meet these disadvantages, an efficient combination square-set rill system, of a variable width and length to meet any conditions that arise has been adopted, (see Fig. 11).

To lay out the system to the best advantage for longitudinal stoping, as required by the grades and class of ore, the development drives should be run outside and parallel to the orebody in the hard foot-wall rock and not in the ore, as were some of the earlier ones. At every 100 ft., crosscuts are driven to and through the ore to the hanging wall and continuous raises put up from level to level, and finally to the surface, so that waste may be taken directly into the stopes.

When commencing stoping operations, half way between the main crosscuts, auxiliary ones are put in with a 25-ft. radius curve to the main or extraction drive. The 50-ft. blocks on either side, with the exception of a 12-ft. pillar directly over the crosscut, comprise the stope. These rill stopes have their apexes at the main passes and slope down to the extraction crosscuts. In the early work, no pillar was left at the toe of the two stopes and one ore chute sufficed for two stopes. This toe, or section around the chute, would become very heavy, and it was difficult to prevent this from caving. A 12-ft. pillar is now left between the two stopes and individual ore chutes are carried up. The pillar is mined after the two stopes are finished.

Stopes are carried three or four sets wide, depending on the character of the ground. Four sets is the maximum width that the best ground will stand; and as the ground becomes heavier the stope is brought in to three, and sometimes two sets in exceedingly bad ground, but the length remains the same.

Where the vein is wide, a longitudinal section is first taken along the hanging wall; and when this is completed a second and third section alongside, retreating toward the foot wall. Under this arrangement, the crosscuts and drives are never in bad ground, under worked-out stopes, but in the solid ore or country rock. The part of the crosscut under the completed stope can be filled, thus doing away with all expensive repairs maintaining openings under old stoped areas. In order to get the proper rill and still not make it too steep to climb up, square sets must be of proper size and height. Those at Bawdwin mine are 5½ ft. square and 7 ft. 4 in. high.

Manways should always be on the hanging-wall side of chute and not along one side, as is the general custom. This gives access to either stope, from the manway or the chute without interfering with entance to the other. As all ore is dropped to the lowest level or adit, from which it is hauled out by electric locomotives, any system of continuous raises can be used as an ore pass, provided the raise is not being used for passing waste for a stope below. However, it has since been found cheaper to run up continuous untimbered ore passes from level to level in the foot wall when suitable hard rock can be found; this leaves the main passes for waste only. The lower repair cost of maintaining timbered chutes for the soft waste fill only and not for the heavy ore offsets the increased cost of tramming ore on the various levels to the (all rock) ore passes. Waste filling is obtained from the surface by quarrying or mill-holing around the top of these continuous mullock passes placed at intervals of 100 ft. along the strike of the orebody so that the filling can be passed directly into the stopes without any tramming.

Mine Openings, Shafts and Tunnels

The main opening to the mine, as at present developed, is the No 6. level adit, commonly known as Tiger tunnel; see Fig. 12. This is the

main haulage and drainage adit. It is nearly 2 miles long and double-tracked from the portal to the inside shaft, a distance of 7400 ft. The section is 9 ft. wide by 8 ft. high in the clear. The ditch is carried in the middle in the space between the two tracks; long sleepers pass completely over it and support both tracks. The grade of the tunnel started at 0.6 per cent, but was increased to 0.7 per cent., to accommodate the large volume of water passing through the ditch, which had a tendency to silt up on the lower grade. The average flow is 40,000 gal. per hr. but this has been exceeded many times.

The double-track part of the tunnel was run from both ends; it was begun in April, 1914, and finished Sept. 21, 1916. The length of survey was 23,300 ft., consisting of forty-nine readings of the theodolite, exclusive of those in the tunnel itself. Of these readings, three were lines less than 10 ft. in length, four between 10 and 20 ft., and six between 20 and 30 ft. The actual error of closure was 1.26 ft.; giving an error of 1 ft. in 18,500. The distance between the center lines of the two ends at right angles to their length was 0.35 ft. In elevation, the error of closure was 0.35 ft. or 1 ft. in 100,000, the distance leveled being approximately 34,000 feet.

Tiger Tunnel Construction

The work was originally started with an outfit, picked up in the country, consisting of a Class A Sargent straight-line compressor (steam cylinder 22 by 24 in., air cylinder 22¼ by 24 in., giving approximately 960 cu. ft.) and two second-hand boilers run on wood fuel; 2½-in. piston machines were used for drilling. During the first year, on account of lack of organization, poor equipment and raw coolie labor, only 2448 ft. was driven—an average of 204 ft. per month. Work was then reorganized. A new equipment was obtained and eleven trained white men were employed to supervise as follows: One foreman, three shift bosses, one mechanic, one steel sharpener, one combined track layer and pipeman, and four miners. The new equipment comprised the following, with necessary accessories:

1 new Ingersoll-Rand compressor belt-driven, Imperial type 10, 950 cu. ft.
1 Garrett semiportable boiler and engine combined.
1 No. 5 Leyner drill sharpener.
6 No. 18A Leyner drills (three machines on a bar), replacing the piston machines.
1 British Westinghouse dynamo, direct-current 9 kw.
1 Root blower.

During the following 12 months and 21 days, the outside heading advanced 3814 ft. at the rate of 300 ft. per month, the highest footage for any month being 502 ft. An average month of 300 ft. required daily 227 men, working as follows: 151 underground, 40 mechanical department, 36 surface, all working in four 6-hr. shifts; of this number eight to eleven were white men acting as supervisors. There was 3050 ft. of tunnel timbered. Many large bursts of water occurred, the flow through the tunnel being from 40,000 to 73,000 gal. per hr. The total cost of the 7400 ft. of double-track tunnel amounted to Rs. 731,000, including all charges for labor and supplies and 8 per cent, for depreciation on the capital cost of equipment. As soon as the tunnel was finished the equipment was transferred to the mine account.

All running and heavy ground was timbered, but within a year after the tunnel was completed, the rapid deterioration of the local timber made necessary a large amount of repairs and many caves and obstructions to traffic occurred. It was decided to replace the timber with masonry, as good sandstone could be had on the railway line about ½ mile from the portal. Work was started and a 15-in. wall and arch replaced the timber. Any space above the arch was filled with dry rubble. As a large proportion of the timbered sections was in spiling ground, progress was slow. Only one set of timber could be removed at a time, for the iron forms had to be put in and masonry built around them and allowed to set before the next set of timber could be taken out. Nine steel forms were used 50 ft. or more apart, and an advance of 150 to 200 ft. per month was made. The forms were so constructed and work so managed that there was no serious delay to the mine haulage, and the trolley and lighting wires were never cut. The cost of the masonry was as follows:

All ore mined is dropped to this level and hauled out by electric locomotives. Three smaller adits on the upper levels, namely Zero, No. 1 and No. 2 levels, are used for drainage and transportation of supplies. As there are only three levels that have no adits and as no ore or waste is hoisted or lowered in cages, an elaborate shaft and hoist equipment is not required until stoping operations begin below the lowest adit, or Tiger tunnel.

Two inside shafts, one of two and the other of three compartments 4 by 8 ft. and 4½ by 11 ft. in the clear, respectively, are used for hoisting and lowering men and supplies. The 8 by 8-in. timbers in the two-compartment shaft have shown considerable pressure and have been replaced by 10 by 10-in. Mandalay ingyin timber. The framing of the set has also been changed by making a bevel on the inside corners of the set; this prevents the wall plates crushing and splitting at the end plates. These shafts were sunk in the orebody as a means of develop-

ment before the lower adit was driven, consequently they are only temporary and will be replaced by a large circular shaft 14 ft. in diameter and masonry lined, Fig. 13. This will be located in the hanging wall in country rock and at some distance from the orebody.

In sinking the three-compartment shaft, one European was put in charge of the coolie labor and advanced the shaft 30 to 35 ft. per month; 5000 to 5500 gal. of water was pumped per hour. An advance of 33 ft. per month required 461 coolies divided into three shifts.

The cost per foot was:

Of the local wood in Burma, only ingyin and teak are satisfactory for shaft work and the latter is too expensive. The three-compartment shaft, which is of ingyin timber, has been completed over 6 years and the timber is still in good preservation, although a number of sets have had to be replaced on account of rock pressure. This is partly accounted for by the shaft being wet with acid water impregnated with zinc sulfate and there being a good circulation of air. Timber has been taken out of the submerged lower levels of the Chinese workings that is 50 to 100 years old and in a good state of preservation. When this old timber is brought to the surface, the absorbed water evaporates and leaves a 1/8-in. coating of zinc sulfate over the entire stick. The smaller shaft, which was sunk about the same time and timbered with local sawn timber, has been practically retimbered twice on account of the rapid decay, as this shaft is dry. The second time 10 by 10-in. Mandalay ingyin timber replaced the local timber and we expect this to remain in good shape for two or three years, but not as long as if it were wet with the acid mine water.

Underground Development

In Figs. 14 and 15 are shown longitudinal sections of the different levels.

Drilling and Blasting

The corporation has standardized on the following drills, for the following reasons:

(a) The first ones were purchased about 7 or 8 years ago and tried out with the raw coolie labor; although they are subject to much abuse they stand up well.
(b) It has taken considerable time to teach the coolies to operate and repair these particular drills efficiently.
(c) On account of the long distance Burma is from the source of supply, a large stock of repair parts is necessary; this is quite an expense when about 100 drills are operating.
(d) By using only one type that has been found satisfactory, much labor is saved that would otherwise be wasted on a new type of drill, and the stock of repairs is kept to the minimum.
(e) Burma is not the country nor the Chinaman the man to try out new machines; consequently we leave that to others.

The drills in use are: 40 I. R. B. C. 21 stopers, 40 B. C. R. 430 Jackhamers, 13 18A Leyner drifters.

For Jackhamers and stopers 7/8-in. hexagonal hollow and 1-in. cruciform solid steel are used; 1 1/8-in. round hollow steel is used for the Leyners. Ordinary cross bits are used with as few changes and as large a clearance as possible, because the coolie persists in running the steel until the machine is entirely cranked out. Drills are tempered by a separate fire by giving them a short heat not farther up than 1 in. from cutting point and then standing them on a grating, resting ½ in. below the surface of a tank of water. Sharpening is done on two Ingersoll-Rand No. 5 Leyner drill sharpeners in conjunction with oil furnaces. The average rock is not hard and requires no special bit; and as the cross bit

is the simplest and easiest to sharpen it is used on account of coolie labor. The simplest rounds are used as the placing of holes to the best advantage is the most difficult part of mining to teach the coolie.

Fifty-per cent. Nobels or Curtis & Harveys gelignite is used. This explosive is comparatively safe and only one fatal accident and two minor ones have occurred in the mine during the last 5 years and these were due to carelessness. Gelignite, a modification of blasting gelatine, resists the action of water, is safer than gelatine, and has all the plastic advantages of the latter over dynamite. It is a nitroglycerine compound to which is added nitrocotton, nitrate of potash, and wood meal. It, however, lacks the rapidity of explosion of the dynamite and does not produce the local shattering but has a larger rending action in being slower. Dynamite is more effective for bulldozing, etc., where gelignite is more effective for drill holes. Double tape fuse is used for dry work and gutta percha tripple tape for wet work. No. 7 detonators are used entirely. The coolies have very little trouble in spitting the fuse, as they roll a small piece of gelignite into a small pencil, which is quite effective especially in wet and drafty workings.

Drifting and Storing

The ordinary methods of driving and timbering are used. In going through running ground, spiling with false set and face boards is used. The latter are, however, made in two pieces. One piece has two bolts and the other a slot to accommodate them. First, room is made for the half with the two bolts and the end shoved in front of the point of the side spiling, while the other end is supported by a temporary brace. Room is then made for the other half, and the board is put in with the slot in line with the two bolts. Large washers are slipped over the bolts and the nuts tightened. This makes a rigid face board that is easy to put in and take out.

For stoping, hardwood is the only available timber. This wood is so hard that nails cannot be driven into it unless the holes have first been drilled. Wire nails are of little value, so cut iron spikes must be used. The timber is heavier than water; consequently every stope is supplied with Holman or tugger hoists. Instead of the ordinary 2-in. lagging generally used to line the inside of a stope preparatory to filling, 4-in. bamboos 11 ft. long are used, much the same as pole lagging.


Special timber is framed by hand but all other by machine.

All framing is done on the surface and all timber, except that required for stopes, goes to the preserving plant. This consists as follows:

One creosote storage tank 5½ by 12 ft. in diameter.
Two open-treatment steel tanks, one for hot and the other for cold solution, size 5 by 5½ by 12 ft.
Three 1½-ton chain blocks and overhead crawl.
One vertical boiler, 9 hp., 100 lb. pressure.
The boiler is connected to a series of 1-in. pipe coils laid in the bottom of the hot tank, which raises the liquid to 180° or 220°.

The timber is left in this tank for 6 or 7 hr. The heat causes the wood to expand, expelling the air and moisture, and to absorb a small amount of creosote in the cellular spaces. The wood is then put in the cold tank where it is left over night. This causes contraction and a condensation of moisture, and the creosote is absorbed and forced into the wood. The wood is very hard and dense and only a ½-in. penetration is obtained. As most of the timber is thoroughly seasoned, the hot tank is not necessary except for green timber; by leaving the seasoned timber in the cold tank 12 to 24 hr. a penetration of ½ in. is obtained. The process has not been in operation more than 2 years so that we are not able to tell just how much longer life is given to the timber by the treatment. There is reason to believe from the experience of two years that the treatment will at least double the life of the timber, which will bring the chemical life of the timber close to its mechanical life, and that is as far as one is justified in going in expenditure for treatment.

The local Burmese timber is as follows:

Thitsee…………………………………….3150 lb. per 50 cu. ft.
Yindike…………………………………….3000 lb. per 50 cu. ft.
Thaukkyan………………………………3075 lb. per 50 cu. ft.
Thumalum……………………………….3037 lb. per 50 cu. ft.
Kadee………………………………………..2925 lb. per 50 cu. ft.
Yang………………………………………….3900 lb. per 50 cu. ft.
Cedar………………………………………..1500 lb. {not suitable for
Cotton wood…………………………..1440 lb. mining purposes.}

All timber is peeled in the forest, as the space between the bark and wood affords a starting place for fungi and is unsuitable for creosoting. Most of the timber is seasoned by cutting it the previous year and stacking it along the railway line; however, not more than enough for one

season should be cut as the timber decays very rapidly in the forest and is badly damaged by ants and borers.

No timber is recovered from stoping operations, as the ground is heavy and the timber takes weight very rapidly; we are satisfied if the timber will hold until the filling is completed without requiring the additional expense of putting in doubling up sets and angle braces. Timber is loaded on special timber trucks 6½ by 3 ft. wide, 2-ft. gage and is hauled into the mine in special trains, by electric locomotives, to the inside shaft and winzes, where it is loaded into cages for transportation to the various levels. In the new circular shaft, these trucks will be run onto a large cage and hoisted to the various levels without any additional handling.

Underground Sampling

Sampling has been described under exploration work. As a check, crosscuts used for estimating the ore reserve are resampled by an independent sampler and have so far closely checked the original samples.

Every set of ground mined is located by the number of the stope and the number of sets north and south of the nearest 100-ft. crosscut and east or west of the drive and with respect to the floor above the level. For example, “4-14 S. stope 6th floor 3 W. & 4 S.” locates a definite set of ground in the mine. This is interpreted as No. 4 level, 1400 S. coordinate, sixth floor, 3 west of drive, and 4 south of 1400 S. crosscut. There is great difficulty in sampling across the middle of the back of stope sets, as it is high up and, unless one is on the spot when the ground is taken out and before the lagging is placed, it is impossible to take the sample correctly. Often two or three sets fall out, leaving a dangerous hole, and the sampler thinks more of protecting his head than of getting a good sample. To avoid all trouble, danger, and careless work, samples are cut across the vein on the exposed side of the set about 4 ft. above the floor and after the timber is placed and lagged over; this arrangement gives the sampler two or three days time to cut his sample before the next set ahead is taken out.

Each level is divided into convenient sections and stope plans prepared of each floor in that section, showing all the ground taken out with its respective value. As the ore is thoroughly mixed in passing through continuous rock passes from the upper to the haulage level, ordinary hand sampling gives fairly close results. Each car is sampled as it leaves the portal of the mine. The ore then passes through the tipple, and by travelling belt, to the bins where it is discharged into 20-ton railway cars. Here it is again sampled by hand; and as it has been thoroughly mixed in going through the rock passes, cars, tipple plant, and bins, a fairly accurate result is obtained, which checks closely the sample taken at the concentrator.

Tramming and Haulage

On all levels, except Tiger tunnel or the haulage level, small ¾-ton cars of 20-in. gage are used to tram the ore from the stope chute to the nearest ore pass, which is seldom more than 200 ft. As the ore is very heavy it requires two men to a car, especially in tipping. Turntables were used at the intersection of the crosscuts and drives, as no extra ground had to be taken out for switches. As the tables used were unsatisfactory and a continual source of trouble, they were scrapped and switches put in. The mine is very heavy, and additional expense was incurred in continually replacing the long caps at the curve. However, a very satisfactory turntable was discovered at this smelter and installed in the mine, although it is more expensive than most others; it requires no repairs and is never out of order. For hand tramming, all right-angle turnings in heavy ground are provided with these turntables, which have simplified the timbering and cut down the repairs.

On the haulage level, 4-ton (56 cu. ft.) tipple cars are used. They are 6½ ft. long by 3 ft. wide by 2 ft. 11 in. deep and equipped with standard railway journals and brasses and the small size U. S. M. C. B. railway coupling. They are hauled, in trains of ten cars, by 4-ton Baldwin-Westinghouse electric locomotives of 2-ft. gage. Four of these locomotives handle the ore and supplies. Current is supplied by an overhead trolley line (one over each track), which is fed by two 42-kw. Westinghouse motor-generator sets. These sets take alternating current at 500 volts 63.3 amp., and deliver direct current at 250 volts 168 amp. The rails are 50 lb. and are bonded by General Electric twin-stud, flexible cable bonds, which have proved quite satisfactory both for safety from theft and for service. Gage of the track is 2 ft. and the grade is 0.7 per cent, in favor of the loaded train. On this grade no power is required to take out the loaded trains but considerable is necessary to take in the empties. A 4-ton locomotive will haul a train of ten empty (56 cu. ft.) cars up this grade without undue heating. A lighter grade would have been more efficient from the haulage standpoint but insufficient for drainage purposes.


As the loaded train of ten cars (40 tons of ore) reaches the portal, it is pushed to the tipple. There are two revolving tipple drums 6 ft. 9 in. in diameter by 34 ft. long, each, geared to an Allis Chalmers, direct-current, 15-hp., 58-amp., 200-volt motor, which is fed from the trolley wire. Each drum is capable of receiving four 4-ton cars and discharging their contents in one revolution into the bin below. Four special revolving drums operated by 7.5-hp. motors are used for gates in delivering the ore from the bin to the incline belt conveyor. This belt, which conveys the ore to the storage bin, is 20 in. wide and 323 ft. long and runs on an angle of 16° 30′; it requires a 15-hp. motor and discharges on to a 20-in. distributor belt 406 ft. long equipped with an automatic tripper, which requires a 7.5-hp. motor. From the tripper, the ore is discharged into various storage bins, which supply the railway that hauls the ore to the mill at Namtu 7 miles away. As the ore passes over the end pulley of the elevator belt (just before it is discharged), it passes under a Cutler Hammer, type B, size 18, 2.75 amp. hot, 220-volt magnet, which takes out all iron.


As no ore is hoisted the hoisting plant, though small, is adequate and will suffice until the new 14-ft. circular shaft is sunk and equipped. Only timber supplies and coolies are hoisted at present. The hoisting is facilitated by the adits on Nos. 6, 2, 1, and 0 levels. The main inside shaft is equipped with a motor-driven double-drum electric hoist made by Allis Chalmers: one fixed and one loose drum; drum reel 4 ft. diameter by 2½ ft. inside flange, drum speed 34 r.p.m. or 408 ft. per min.; motor 50 cycles, three-phase; speed 725 r.p.m., 70 amp.; 500 volts, 60 horsepower.

The hoist is equipped with a solenoid brake on the motor and a Lilly brake (overwinding and speeding device) on the loose drum. The hoist motor is protected by an oil switch with an overload and no-volt release trip, which opens the circuit in case of a dangerous overload or stoppage of current and applies the solenoid brake on the motor. The Lilly brake controller is also connected to the no-volt release; and in case of overwinding or overspeeding, it not only applies the brakes on the hoist itself but also those on the motor. In addition, two limit switches located in the head frame open the circuit and apply all the brakes in case the Lilly brake controller does not function in an overwind. Close to the operator is a hand emergency switch for cutting off power and applying all brakes. The one fixed drum, instead of two loose ones, is a decided advantage when using coolie labor as there is just one-half the chance for an accident.

Double-cylinder 10 by 12-in. second-motion air hoists are used for operating winzes and 6 by 12-in. double-cylinder hoists for sinking. The large Holman stretcher-bar hoist is often used for the first 100 ft. in sinking. Small Holman 3 by 5-in. double-cylinder stretcher bar hoists and Little Tugger hoists are used for the stopes and rises.

Air Compressor

Compressed air is supplied by the following compressors:

1 Robey compressor, rope drive, 800 cu. ft. free air per min.; air cylinders 12 and 20½ by 30 in.
1 Ingersoll-Rand, imperial, type 10, 900 cu. ft. free air per min.; high-pressure cylinder 12 by 16 in., low-pressure 20 by 16 in.; belt driven.
1 Ingersoll-Rand, class P.R.E. 2, low-pressure cylinder 25 by 18 in., high-pressure 15¾ by 18 in., speed 214 r.p.m., motor 344 hp.; capacity of air in three stages: Full load 2370 cu. ft., 432 hp.; three-fourth load 1778 cu. ft., 321 hp.; one-half load 1185 cu. ft.; 255 hp.

The motors for the compressor plant are supplied from the low side of a substation receiving 33,000 volts and stepping it down to 550 volts; they are as follows:

On account of the large number of induction motors on the circuit of the mine, mill and smelter, this synchronous motor has been most beneficial and has brought up the power factor considerably. Another large synchronous motor has been ordered. One 350-kva. transformer at Tiger camp and two 350- and one 700-kva. at Bawdwin transform the necessary power for the mine.

Air is delivered into two air receivers (5 ft. 3 in. in diameter and 12 ft. long and 5 ft. in diameter and 13 ft. long) and then through a 10-in. steel pipe into the mine.


The ventilation of the mine is by natural draft assisted by mechanical means. Practically all air enters the mine through the Tiger tunnel and a shaft at the extreme north end of the Chinaman orebody, which is connected to each level. As the air enters the portal of the tunnel, it travels 7200 ft. before entering the actual mine workings. Here the current splits and passes up the various raises and winzes, spaced at least every 100 ft. along the vein, from level to level until it reaches No. 1 level, where it is accelerated by a 100-in. Sturtevant fan and discharged from the mine. This fan simply accelerates the natural draft and consequently increases the volume of air. It exhausts 70,000 cu. ft. per min. running at 579 rev. and driven by a 70-hp. motor. Although it is doing the required work, its speed is too great for efficiency and consequently consumes too much power.

As space is limited, the fan, which is a 100-in. double-inlet single-width, will be replaced by a 110-in. double-width occupying practically the same space and decreasing the power by about 50 per cent, for the same volume. At the same time it will be able to take care of a larger volume of air if required.


Three General Electric lighting transformers, each 15 kva. 200 volts, located underground and at different parts of the plant supply the necessary electric lighting for bungalows, surface plant, main adits, haulage levels, shaft stations, and underground stores. The bulbs of all lamps are etched with the name of the corporation to prevent theft and sale in the local bazaars. Carbide lamps are used underground by the European shift bosses and candles by the coolies. These candles are of good quality and are made in the country by the Burmah Oil Co. They are colored green to prevent theft and sale but without avail. They cost Rs. 0.42 per pound.

The coolies were originally provided with three candles for an 8-hr. shift, but as they work in twos and fours it was observed that a group of four Chinamen working together would seldom have more than two candles burning at one time. The extra candles were taken home, collected, and finally sold in the bazaars of local towns. It is safe to say that the mine was furnishing light for the surrounding country. Candles have now been cut down to two for each coolie and still they have enough left to light their own homes and sell some to their neighbors. In a mine of cheap labor, the lighting account is large. In this particular mine it is the fourth largest separate account, being next to repairs and maintenance.


A telephone system is installed at the supply stores on each level. A clerk is continually on duty and prevents meddling by the coolies. Originally, all the telephones were on the same circuit but this was found unsatisfactory for when one line became broken, which is often the case in a mine of heavy ground, all the telephones were out of commission; when defects or short circuits appeared they were more difficult to locate; the level clerks were continually using the phones for social conversation and it was difficult to get a message through. A small exchange was placed at No. 2 level store and separate lines run to each level; this system, although requiring a larger expenditure, is highly satisfactory.

Timekeeping and Store Checking

As many coolies have the same name and look alike to the new arrival (Europeans), it is necessary to give them a number; this is a round metal tag upon which the number is stamped. When he receives it, he is told in Chinese what number it bears and rarely does he forget his number or the appearance of the numerals on it. In a day or two, he can pick his number out of several disks and often will be able to call it out in English. Together with the brass disk, he is given a ticket bearing his number, rate of pay, and 15 spaces for the days of the half month. Each morning, before beginning his work, he appears at the time office and calls out his number; the disk is taken off the board and given to him. After the shift has passed through the time office, the number of tags remaining on the boards shows the timekeeper those who are absent and he can then make up his payroll sheet for subsequent checking. Checkers go underground and again check the coolies in their working places and make the required shift allocation. On coming off shift, the coolie is searched by the police (as he will steal anything no matter how small, as everything is of value to him if he gets it back to China) and presents himself again at the time office, where he hands in both his metal disk and his ticket. The former is put on the board and the latter (ticket) is punched for that day. In this way he can see at any time how many days he has worked and arguments at pay day are avoided. However, he is not paid on the ticket, that would be to his liking however, but from the actual pay roll which is kept up daily. The ticket is for his identification; if he loses it without reporting the fact, the company is not responsible if the pay envelope is delivered over to the presenter. New coolies use many ingenious schcmes for punching their own tickets when absent, thinking they will be paid according to these, but when they find out it does not work and calls forth a penalty they do not do it the second time.

Tools of all kinds, dynamite, fuse, caps, etc. are of great value to the Chinese coolie when he goes home, consequently on issuing these articles certain precautions must be taken or they will not be returned. On every level there is a supply store where the coolie can get any tool or article he requires, but he must deliver to the issuing clerk his pay ticket as security until he returns the articles at the end of the shift. If the tools are not returned, his pay ticket is turned into the office with the list of tools and he is fined, or the matter is brought to the attention of the mine foreman.

The system of timekeeping and underground store checking has been gradually developed from our experiences, and is now the simplest and most satisfactory to all concerned. The Governments of India and Burma have stringent laws concerning the use of explosives and any quantity found in the possession of coolies is investigated; and the mine management is held responsible for any that gets out of its possession. Notwithstanding the punishment (which is both corporal and imprisonment) coolies are caught every week by the police guard, attempting to take explosives out of the mine.

Records of Unit Production

An average month is taken as the basis of the following calculations (14,643 long tons per month).

Stopemen are paid on a bonus system on the number of sets of ground taken out and timbered regardless of the tonnage contained therein as the ore may be low or high grade. Development is also paid on the bonus system, but on the footage advanced and timbered; other excavations, on the sedrum (100 cu. ft.); loading ore from bins, on tonnage.

It requires the following men (8-hr. shifts) to mine and timber one set of ground and muck the contents into the chute:

17.4 miners per set One set in ordinary ground removes 270
6.6 muckers per set cu. ft. or 27 long tons of ore.
24.0 men per set.
1.55 long tons per miner in stope for 8 hr.
4.10 long tons per shoveler in stope for 8 hr.
1.10 long tons per each man in stope for 8 hr.
11.00 long tons per each man on ore and rock in development for 8 hr.
0.54 long tons per each man underground for 8 hr.
1.16 long tons per each man engaged on surface including office.
0.37 long tons per each man of total organization.

Classification of Labor

As already mentioned, a large proportion of the labor is seasonal. The Chinese in large numbers come over at the beginning of the dry and go away at the beginning of the wet season. The turnover during the 6 months, November to April, is generally over 11 per cent.

Records of Units of Supplies used per Ton of Ore Produced

The explosives used per long ton of ore produced are 0.36 lb. 50 per cent, gelignite, 3 ft. Bickfords fuse, 0.55 No. 7 detonators.

The timber required per long ton of ore produced is 0.38 cu. ft. logs, 0.33 cu. ft. sawn timber, 0.30 cu. ft. 8 by 2-in. lagging, 0.27 cu. ft. 8 by 3-in. lining boards and cribbing or a total of 1.28 cu. ft. of timber.

The power required is as follows:

Also, 10,000 cu. ft. free air compressed to 90 lb. were required per ton of ore

Safety and Welfare Work

Notwithstanding so many coolies are employed who have never handled explosives or worked underground before, the fatal accidents are very few; there has been only one fatal accident due to explosives in five years.

No safety engineer is employed; but by making each European shift boss responsible for his level and by providing him with all necessary material and instructing him that the safety of the coolies comes first, the mine has been made comparatively safe. Extreme sanitary precautions are taken to avoid epidemics, which are so prevalent in the tropics—such as plague, cholera, relapsing fever, and other contagious diseases which wipe out villages in a few days. Fresh drinking water is piped to every level and many of the working places, and at suitable places on each level latrines are provided. At first, great difficulty was encountered in getting the different types of Indians and Chinamen to use the same latrines, but by most persistent work on the part of the European staff, with the aid of heavy fines and dismissals, the mine has been brought to a stage where for safety and sanitation it will rank with the best mines employing Europeans only.

Free medical attention is given to all natives and employees on the corporation property, and hospital accommodation for those that require it. The hospital and sanitary department is one of the largest and best in Burma.

For recreation, the corporation has provided clubs with tennis courts, both for Europeans and Asiatics, and is now trying to become a member of a cinematograph circuit. A first-class race-course has been built, to which everybody migrates on the only two holidays of the year— Christmas and Boxing Day. All the employees with their families are there: from the iron mines, a day’s journey by train, from all parts of the forest, from the power plant (100 miles away), from Bawdwin (the lead mines), the mill, smelter and all the stations along the corporation’s railway. They travel by train, ox-cart, pony, and afoot. Many of the Europeans and natives own their own racing ponies.


Intermetallic Compounds

Nature and Behavior of Intermetallic Compounds

The nature of intermetallic compounds and their behavior, both as solutes and solvents, is a wide subject and I can only hope to touch its fringes. A good deal depends on the definition we adopt for intermetallic compound. If we define it merely as a homogeneous crystalline alloy in which the two metals are present in such a ratio as to conform to the law of multiple proportions, it is practically impossible to distinguish between solid solutions of certain compositions and compounds. On the other hand, the study of alloy systems has revealed the existence of a number of bodies possessing well-defined characteristic properties and located in the equilibrium diagram in one of a few special ways, which we must certainly regard as definite compounds possessing a nature essentially different from that of solid solutions. Unfortunately, direct recourse to results of x-ray analysis fails us here, for there are no published data giving the lattice structure of a well-defined intermetallic compound.

We might not be on safe ground if we argue from analogy with ordinary crystalline compounds of inorganic chemistry, such as chlorides and fluorides, for several of which the lattice structures have been worked out, were it not that in the case of at least one typical intermetallic compound CuAl2, a characteristic x-ray spectrum has been worked out by Owen and Preston. The interpretation of this spectrum is difficult and a corresponding lattice structure has not been worked out, but the fact that this compound exhibits a spectrum totally different from that of either copper or aluminum strongly supports the view that this compound resembles such bodies as chlorides, etc., in having a more complex lattice structure than a pure metal. By analogy, we may, therefore, expect intermetallic compounds to possess, like inorganic compounds, a lattice structure that may be regarded as consisting of two or more interpenetrating lattices, in which the arrangement of the atoms is much less symmetrical than in the regular pure metals.

By the substitution theory, a solid solution cannot be regarded as having anything but a single lattice, that of the solvent metal, and here we have an important and critical distinction. This distinction is reflected in one of the most striking properties of compounds, their brittleness or inability to undergo plastic deformation by slip. When one finds this brittleness in compounds of pairs of such ductile metals as gold and aluminum, copper-aluminum, copper-tin, etc., a fundamental difference of internal structure is suggested; a difference not shared by solid solutions that, though harder and less ductile than the solvent metals, are far from being really brittle. On the other hand there are pure metals, like antimony and bismuth, that are quite as brittle as typical compounds.

There is an evident common explanation for these striking differences. Plastic deformation occurs by slip within the lattice, and such slip, if it is to occur without loss of cohesion, must be accompanied by a rapid “handing on” of atomic bond to atomic bond as the moving atoms slide past one another. The explanation suggested is that, where the atoms are evenly, or almost evenly spaced, such handing on will occur easily, particularly if the lattice is not initially unduly strained. On the other hand, if the atomic spacing is uneven, any handing on process would require a rapid readjustment of the atomic bonds in the act of transfer from one atom to another placed farther away or out of the straight line of motion. It may be, that the maximum distance over which an interatomic bond can be stretched is exceeded when movement begins. In either case slip without loss of cohesion cannot occur unless the two slipping surfaces are held in contact by powerful external pressure long enough for fresh bonding to occur in any new position that rows of atoms may assume. Stressed in the ordinary way, such a crystal would be brittle, but under a heavy external pressure it might prove ductile.

Such uneven interatomic spacing must occur whenever we have a lattice structure of lower symmetry; consequently, it is in accordance with expectations that the metals whose structures show the highest degree of symmetry, those having a cubic lattice, are by far the most ductile. The hexagonal group is markedly less ductile (zinc being a typical example). Those having still lower symmetry, such as bismuth and antimony, are brittle in the ordinary way but can flow under heavy hydrostatic pressure, as in extrusion. Intermetallic compounds, and indeed most inorganic crystalline compounds, are typically brittle, although some of the latter are also capable of flow under pressure. Certain exceptions to this rule may be found both in metals and other substances, for the reason that the structure resulting from the interlaced lattices of a compound may quite closely approach the symmetrical lattice of a simple substance if the two component lattices are very similar and are symmetrically situated. Such a lattice as that of potassium chloride might be cited as an example, while in alloys the compounds of copper and zinc (in so far as they are true compounds at all) are much less brittle than many others.

Behavior of Intermetallic Compounds

Can intermetallic compounds enter into solid solutions as compounds, that is, in molecular association? All the considerations discussed here point to the view that, when the solvent is one of the constituents of the compound, we cannot think of the compound going into solid solution in molecular association. Such a molecule as CuAl2, for instance, would be too large to find room upon the space lattice of aluminum. If it did exist in the solid solution, even in only molecular aggregation, it would exhibit a typical diffraction spectrum under the x-rays and no such spectrum is found. The x-ray spectrum of the solid solution of copper in aluminum is substantially the same as that of pure aluminum even when 4 per cent, of copper is present. We must conclude, therefore, that in the solid solution we deal with individual copper atoms and that the compound CuAl2 is formed from groups of atoms at the moment when it comes into existence as a separate crystalline phase. It is not at all certain that this conclusion holds in the case of all compounds, although where the solvent metal enters into the compound one cannot accept the idea of so relatively large a thing as the compound molecule occupying the space ordinarily assigned to any one of its constituent atoms. Where the compound does not contain an atom of the solvent material, a different case may arise; the molecule of the compound may be better fitted to occupy a place on the lattice than its atomic constituents. Whether or not such cases actually exist remains to be determined. In most cases, we must regard the compound as dissociated when in solid solution. But this dissociation may not be similar to that which, in some cases, occurs in liquid solutions. In the compound CuAl2, it is possible to regard each copper atom as in some way specially attached to two of the adjacent aluminum atoms on the space lattice: there may be some difference in the manner of the interatomic linking in these cases, which may bring with it some special disturbance of the lattice structure. These are details upon which it is not very fruitful to speculate, but the possibilities must be borne in mind if false conclusions are to be avoided.

Intermetallic compounds quite well defined in character may act as solvents in the formation of solid solutions. The cases so far studied relate to solid solutions in which one of the component metals of the compound is the solute. Here we meet with a curious consideration. If the substitution theory holds in these cases, in a compound AB of metals A and B, metal A passing into solid solution in the compound obviously cannot replace atoms of A already present on the compound lattice and must therefore replace atoms of B. The atomic similarity of A to B must therefore govern the solubility of A in the compound AB as it governs its solubility in B itself. The degree of solubility is not likely to be quantitatively identical, for the behavior of B in the compound lattice is likely to be different from that of B on its own space lattice, but we should expect to find a distinct correspondence between the solubility of a metal in the pure metal at the extreme end of a binary series and in an intermediate compound. A much fuller knowledge of limiting solubilities in the solid than we possess is required to allow this generalization to be tested. In a few cases it appears to be correct.

The question arises whether all solid solutions in metallic alloys have as their basis, or solvent lattice, either the normal lattice of a pure metal or of a definite compound. While it seems probable that this is so in most cases there is no reason to suppose that other possibilities do not exist. From our consideration of the behavior of atoms arranged on a space lattice we arrived at the general view that when a lattice becomes distorted or expanded beyond a certain definite point, it tends to break up, with the formation of a second phase—this may be a liquid, as in melting, an amorphous solid, or congealed liquid, as in plastic deformation, or a second solid phase, when the limit of solid solubility is passed. But it does not follow that the second solid phase must be either a saturated solid solution of the second metal or an intermetallic compound. It is conceivable that the atoms of the solvent metal may be capable of assuming an arrangement on a lattice slightly different from that which is normal to the pure metal. In that case the alternative lattice is probably less stable, i.e., contains more potential energy, than the usual lattice when the metal is pure. It is not difficult to imagine that in the presence of a number of solute atoms sufficient to strain the usual lattice to its limit, the second or alternative lattice becomes more stable, that is, capable of accommodating a larger proportion of solute atoms before disruption occurs. If this is the case, the second phase will still be a solid solution of the solute in the original solvent metal but based on a different space lattice. The solvent metal may be regarded as taking up this rather less stable arrangement by the inducement due to the presence of the solute. In a sense, this alternative lattice may be regarded as representing an allotropic form of the pure metal and cases are known where an allotropic modification that normally undergoes transformation at a definite temperature is maintained in a stable condition beyond that temperature by the presence of solute. The effect of nickel in lowering the transformation temperature of gamma into alpha iron is a case in point. In other cases, the allotropic modification may not exist independently, but merely constitutes a latent possibility, which is brought into action by the solute. A case of this kind may be found in the beta body of the copper-zinc alloys and in the beta body of the zinc-aluminum alloys.

If the second or, as it is usually termed, the beta phase in an alloy system is based not on any definite compound but on an alternative, or allotropic, space lattice of the solvent metal, it may be expected that such a beta solid solution will not be as brittle as one based on a compound. This anticipation is borne out by the properties of the beta solid solution in the two best known cases of this kind, which occur in the copper-zinc and the zinc-aluminum systems. Here, again, we are not in position to seek proof among known facts and, therefore, having pointed out the possibilities, or even probabilities of the case, we may leave the matter for future investigation.

I have left to the end one of the most difficult and most important points: the change of solubility with change of temperature. The importance of these differences of solubility has only recently been recognized, but it is now to be regarded as the determining factor in regard to the power that an alloy may possess of undergoing hardening as the result of sudden cooling followed by tempering. It is, therefore, a matter requiring careful consideration from the point of view of any theory that claims to explain the phenomena of solid solutions. Looking at the question from the point of view adopted here, it would seem that the thermal expansions of the two metals concerned should prove decisive. Thus with a solvent A and a solute B, if B is a smaller atom than A but has a greater rate of expansion than A, the two atoms would become increasingly similar and solid solubility should increase with rising temperature. But it does not seem justifiable to suppose that the thermal expansion of the atoms of B, isolated among atoms of A, will necessarily be the same as that of a crystal of B. Further, the matter does not depend solely on the dimensions of the atom, and change of temperature may and probably does bring with it changes in the atom other than those of the lattice constant.

The intensity of the interatomic attraction or bond, the ease with which it can be deflected from its normal position, and even the range over which it extends may be appreciably changed. That changes of this kind actually occur in some metals is suggested by the occurrence of allotropic transformations at definite temperatures, usually accompanied by large and abrupt changes of solid solubility. It is not surprising, therefore, to find that comparison of coefficients of thermal expansion alone does not always indicate correctly the manner in which solid solubility varies with changing temperature. A change in the stiffness of the solvent lattice with changing temperature is likely to have a determining influence in this matter, for it will affect the energy content of the solid solution for a given concentration of solute. Another effect must arise from the nature of the second solid phase formed when a previously saturated solid solution becomes supersaturated and then breaks down; the stiffness of a second type of lattice, often that of an intermetallic compound, enters into the question in this way. It may be hoped that when this whole matter of solid solubilities and their changes with temperature has been as fully investigated as it deserves, a satisfactory explanation may be forthcoming.

If we try to sum up the general result of the consideration in regard to solid solutions which I have presented here, it may appear that there is much that is speculative about the various suggestions and arguments put forward. On the other hand, there is a definite, although limited, amount of fact that bears out the fundamental assumption of the substitution theory. Further, the resulting inferences in several instances lead us to recognize groupings of known facts that almost deserve the name of laws and these fit in with extreme clearness and simplicity with the available data. What is still more important, new aspects of known facts have been brought to light and a large number of lines of new inquiry have been suggested. This furnishes ample justification for putting this theory before you. Further research alone can establish it on a firm foundation. Certainly something more than a prima facie case has been made out for the theory and it will, I hope, stimulate thought on whole ranges of phenomena that we have hitherto been content to study in an empiric manner.

X-Ray Analysis

The investigations of Laue, Debye, Scherrer, Sir William and Prof. W. L. Bragg, Hull, Westgren and other x-ray analysts have given us a large amount of precise and detailed data where our knowledge had been largely qualitative. The truly crystalline character of metals had been recognized and demonstrated by metallographers and so had the persistence of crystalline structure after moderate amounts of plastic strain, as well as the whole mechanism by which crystals undergo plastic deformation. To one who investigated these problems more than twenty years ago, the present-day confirmation of many of the inferences drawn by means of much cruder methods is particularly gratifying and satisfactory.

To most of those interested in the deeper problems of physical metallurgy, the knowledge of the internal structure of crystals that has been obtained by x-ray analysis is probably familiar. The x-ray analyst measures the diffraction or scattering that occurs when a homogeneous beam of x-ray falls upon the surface of either a single crystal or an aggregate of a large number of minute crystals. His procedure is similar to that used when the “spacing” of a ruled grating is measured by means of the diffraction spectrum it produces upon an incident beam of visible light. If the wave length of the light is known the width of separation of the lines of the grating can be determined, and vice versa. It is, however, essential that the grating spacing should be proportionate in its dimensions to the wave length of the light used; otherwise the diffraction spectra become unduly weak. The wave lengths of the characteristic x-radiations of various substances are now well known and the spacing of the rows or planes of atoms in a crystal is so proportioned to the x-ray wave length as to furnish strong diffraction spectra; consequently, the spacing, of the crystal planes can be determined from the x-ray spectrum in much the same way as the line spacing of a grating from a visible spectrum. The crystal, however, is not a plane reflector so that the diffracting objects are spaced out in three dimensions, which implicates the calculations but the principle remains the same. We need discuss the various experimental methods that have been successively used, beyond saying that it is possible to obtain clearly defined and easily interpreted spectra in an x-ray spectrometer from an aggregate of small crystals so that a large single crystal is no longer needed. It is why these latest methods that some of the measurements referred to here have been made by my colleagues at the National Physical Laboratory—
Doctor Owen and G. D. Preston.

The net result of x-ray crystal analysis may be briefly summed up. Metallic crystals yield very well-defined diffraction spectra from which, in all the simpler cases, the arrangement of the atoms can be determined. The results are most readily understood in terms of what is known as “space lattices.” A space lattice is simply a system of lines running in three directions in space and crossing one another in a regular manner; at some or all of these intersection points are located the centers of the atoms. For instance, a lattice might consist of three sets of uniformly spaced lines at right angles to one another intersecting in such a way as to divide space into a number of equal and adjacent cubes. This would constitute the simplest possible cubic space lattice, but crystals built on such a lattice are not found among simple metals. By adding lines parallel to the three original sets, and placing atoms upon certain of the additional points of intersection thus provided, we may obtain the two types of cubic lattice most commonly met in metals. In one case, in addition to an atom at the corner of each cube, there is an atom at the center of each cube face; this gives the “face-centered cubic lattice,” which is illustrated diagrammatically in Fig. 5. If instead of the atoms at the centers of the cube faces, there is an additional atom at the center of the cube itself, the “body-centered cubic lattice” is obtained; this form also is found in metals. By a slight shift of alternate sets of lines, the cubic system can be transformed into the “close-packed hexagonal” often found in metals. More complex types of lattice, possessing a lower degree of symmetry, are only rarely found in simple metals, of which bismuth and antimony are examples. In the case of intermetallic compounds, however, more complex types of arrangement are to be anticipated, although the exact structure of an intermetallic compound has not been worked out.

Reduction of Melting Point

A property of solid solutions, even more typical, perhaps, than hardness, which also finds ready explanation on the basis of the “substitution” theory is the lowering of the melting point and the spreading of melting and freezing over a range of temperature. This difference is indicated in the equilibrium diagrams of Figs. 2, 3, and 4. To arrive at the explanation which our theory affords for these phenomena, let us consider a crystal of a pure metal in which the atoms are arranged in perfect regularity upon the normal space lattice. With thermal expansion there is, inevitably, a corresponding expansion of the lattice. This has been observed, although in certain cases the lattice does not expand equally in all crystallographic directions; in the case of graphite there is a slight contraction of the lattice in one direction compensated by a correspondingly greater expansion in another. In a cubic or close-packed hexagonal lattice, however, there is no reason to suspect this and the fact that Westgren has found the characteristic lattice of gamma iron at 1100° C. to be a face-centered cube, while delta iron at 1450° C. shows the typical diffraction lines of a body-centered cubic lattice, provides direct evidence that in this metal there is no serious distortion of the symmetry of a cubic lattice by very considerable thermal expansion. We may, then, assume that the cubic metals undergo a regular and symmetrical lattice expansion proportional to their mass-coefficient of expansion.

We return, now, to the conception that for every lattice there is a maximum distortion (in most cases an extension) that cannot be exceeded without causing the lattice to break down with the resultant formation of a new phase. We cannot, however, assume that the actual maximum extension that can occur is independent of the nature of the second phase which results from a breakdown of the lattice. Considerations to be discussed later, make it certain that the maximum lattice extension is considerably dependent on the nature of the alternative structure that arises when that maximum is exceeded. With this reservation in mind, it is possible to derive from the “substitution” theory of solid solutions a complete explanation of the behavior of these bodies on melting and freezing.

A crystal of a pure metal possesses a perfectly uniform lattice, which expands uniformly with rising temperature, at all events in the case of cubic lattices. The ground for the latter assertion is that a number of lattices of this kind have been studied at quite high temperatures; for instance, the lattices of gamma and delta iron at 1200° and 1450° C., respectively, by Westgren, which, at those high temperatures, have shown the characteristics of a cubic lattice—an observation that disposes of any serious degree of unequal expansion in different directions in the lattice. In the case of graphite, it has been found that the expansion in one direction is much greater than that to be anticipated from the mass-coefficient of expansion; while in another direction there is a small contraction. In view of the other observations, however, this seems to be a special case and not to be applicable to cubic lattices. The result of uniform thermal expansion on a simple regular lattice must, therefore, be that at a definite temperature the whole of the lattice attains the limit of its extensibility; a further rise of temperature leads to the breakdown of the lattice and the formation of liquid—the whole of the metal melts at one uniform temperature. This conception of melting is in accord with the well-known fact that a high melting point is definitely associated with a low coefficient of expansion, while the harder and stronger metals are, in general, those with high melting points.

On the basis of the present theory, a crystal of solid solution possesses a lattice that is slightly extended throughout and, in addition, in the neighborhood of solute atoms, there are regions of local and more marked extension. If such a crystal is heated, the locally distended regions around solute atoms will reach the limit of lattice extension first, and melting will begin at those places at a temperature at which the bulk of the lattice is still stable. We thus have local melting with the formation of a liquid whose content of solute is higher than the average of the whole crystal. As the temperature rises, other regions of less distorted lattice will melt, until the whole crystal has melted. The highest temperature required to bring this about will be lower than in the case of the pure metal, because the entire lattice is extended and therefore reaches its limiting extension after a smaller thermal expansion. The limiting extension, however, is not constant throughout a series of solid solutions, as it must depend on the concentration of the liquid formed. The mechanism of this dependence, which can be accounted for in terms of energy content of the two phases, is probably connected with the internal, or rather the osmotic, pressure of the liquid solution formed. This pressure may act by restraining the expansion of the lattice under thermal expansion, but that point cannot be easily tested. The reality of the effect, however, is undoubted.

From the preceding, it is possible to draw interesting inferences as to the behavior of solid solutions on melting and freezing, as the freezing operation is exactly the reverse of the melting process as described. In a solid solution, two lines in the equilibrium diagram, known as the solidus and liquidus, determine the beginning and the completion of melting. On the basis of the substitution theory, the beginning of melting (the position of the solidus) is determined by the maximum local extension produced in the lattice by the presence of solute atoms; while the completion of melting (the liquidus) depends essentially on the general expansion of the lattice. If this is correct, we should find that where local distortion is large and general extension small, the solidus would fall rapidly with increasing concentration of the solid solution, while the liquidus will be much less steeply sloped. Further, the slope of the solidus will be increasingly steep the lower the solid solubility of the solute in the solvent metal. This latter point is borne out by the well-established equilibrium diagrams. Wherever there is a long range of solid solutions, the solidus falls slowly and does not lie far below the liquidus; on the other hand where solubility is low the solidus maybe nearly vertical while the slope of the liquidus is often relatively gentle.

Nature of Solid-Solution Crystal

The nature of the solid-solution crystal may be considered from two points of view: its behavior in regard to melting and in regard to solidification. In the case of alloys following one of the modifications of the type shown in Fig. 3, the melting temperature falls with increasing concentration of the solid solution. This means that the amount of kinetic energy that must be communicated to the constituent atoms or molecules of the crystal to bring about disruption or melting is lessened by increasing concentration of the solid solution; therefore, in some form or other these crystals already hold a store of energy that tends to assist the kinetic energy of rising temperature in bringing about disruption of the crystal. We are thus led to the assumption that the solid-solution crystal is a self-strained structure in which potentially disruptive energy is stored and the limit of solubility is determined by the amount of such energy that can be stored in the internal structure of the crystal.

If we think of the manner of formation of a solid-solution crystal, or rather of a duplex aggregate in an alloy containing just too much of the solute metal to form a homogeneous solid solution, we are led to much the same conclusion. The solid solution is formed from the liquid solution because it constitutes that arrangement, in the solid state, which contains the least amount of stored potential energy. If any other arrangement, such as separation into an aggregate of two kinds of crystals, offered a means of reducing the total amount of energy stored, we know (from elementary thermodynamic principles) that such an alternative would be instantly adopted. We conclude, therefore, that the solid solution is still formed, in alloys of increasing concentration, until a point is reached where the total energy content of a more highly saturated solid solution would be greater than that implied by a duplex structure, whether eutectic or otherwise. This implies that increasing concentration of the solid solution brings with it an increasing storage of internal energy until the limit is reached. The earlier appearance of a second kind of crystal, or phase, would involve a greater storage of energy, and this might reside within the crystals of the second phase, which must themselves be saturated solid solutions, or it might reside (and in part must reside) in the interfacial energy that must exist at the junction of dissimilar crystal.

Were we able to express the relations just indicated in a quantitative manner, we should be able to calculate the limits of solid solubility and the eutectic or transition temperatures of all alloy systems. As yet the data are lacking, but measurements of the latent heat of fusion of solid solutions of different concentrations and of the specific heats of the corresponding alloys in both the solid and liquid state would furnish at least some of the data needed. For the moment we must content ourselves with the general conclusion that solid solutions do contain internally stored energy and that it is the amount of this energy which determines the limits of solid solubility in these bodies.

A slight local distortion of the lattice is more likely to occur in the harder and stronger metals of high melting point, as the lattice is much more rigid and the presence of a disturbing solute atom causes little local and not very much general distortion. This consideration connects strength and hardness, as well as high melting point, with the tendency to form long ranges of solid solutions. This connection, once pointed out, seems so obvious that I am surprised it has not been noticed before. The high-melting-point metals, such as iron, nickel, cobalt, chromium, copper, silver, gold, are known to form alloy systems mainly consisting of solid solutions—it is among them only that we find series showing an unbroken line of solid solutions between pairs of metals in all proportions. Metals having lower melting points, like zinc and aluminum, form decidedly more limited series of solid solutions; while the soft metals having a low melting point, like lead, tin, cadmium, etc., show alloy systems that tend to approach the entirely eutectiferous type arising from very slight solid solubility. In these two general and fundamental ways, therefore, the theory here advanced explains satisfactorily well-ascertained facts. The accordance is greatly strengthened by consideration of what, at first sight, appears to be an anomaly.

In the preceding regarding melting and freezing, the argument was based on the supposition that the space lattice of the solvent metal is, on the average, enlarged by the addition of the solute. It is equally important to determine how the matter stands when the lattice of the solid solution is slightly smaller in scale than that of the pure solvent. The preceding line of reasoning is applicable but in the inverse sense; the melting point of the alloy should be higher than that of the pure metal and during the freezing process the first portions to crystallize will be the richest in solute. This conclusion is in striking agreement with those known equilibrium diagrams of alloy systems in which it is quite certain, even before actual x-ray measurements have been made, that the introduction of the solute will lead to a slight contraction of the lattice. One example of this kind is furnished by the alloys of copper with nickel. In this system, there is a well authenticated series of solid solutions extending from pure copper to pure nickel. The lattice constant of nickel is slightly smaller (2.50) than that of copper (2.54), and as there is a perfectly continuous change in melting point and other properties from one end of the series to the other, the lattice constant must change gradually from the larger value for pure copper to the smaller value for pure nickel. This steady, though slight, contraction is accompanied by a continuous rise in melting temperature, while the shape of the equilibrium diagram shows that even near the copper end of the series the portions that solidify first have a higher nickel content than the liquid from which they form. A precisely similar case is presented by the alloys of molybdenum and tungsten, but such cases, where the melting point rises as the result of solid-solution formation, are not numerous.

Another point that arises from the picture of the melting and freezing
processes which we derive from our theory is that there must be an intimate relation between the extent to which the freezing point of a solvent metal is depressed by the addition of solute and the amount of lattice distortion that this addition brings about.

Considerations of this kind (those relating to the depression of the freezing temperature and the width of the range between liquidus and solidus) suggest the possibility of accurate quantitative calculation, provided that the atomic constants concerned were known. The normal lattice constant of an atom is but one of these factors and the actual strength of the interatomic bonds, their maximum effective range, and the work done in either breaking them or displacing them by a definite small amount enter into the problem. To attack the matter from this side, additional data regarding the atoms are needed. On the other hand, the phenomena of freezing, also others that have been referred to, lend themselves to accurate measurement and it may be possible to reverse the process, in a few of the simpler cases, in order to utilize the data obtained from solid solutions to determine the constants of the atoms. Such an attack would prove valuable to the physicist and the metallurgist. For the latter, it opens the prospect of being able, ultimately, to predict by calculation the entire properties of an alloy system.

Another phenomenon that requires explanation is the occurrence of diffusion within the crystals of a solid solution. All metallurgists know that the crystals formed during the freezing of a solid-solution alloy at any ordinary rate are by no means homogeneous in composition. When suitably etched, they show a well-marked cored structure sometimes so strongly marked that it is difficult to realize that one is not dealing with a duplex alloy. Yet, when an alloy containing such definitely cored crystals is suitably annealed, the coring vanishes and the structure resembles, in every way, that of a pure metal. This change implies a redistribution of the solute atoms that, at solidification, are considerably concentrated in the outer layers of each crystal. This redistribution occurs without anything like an approach to fusion; by what mechanism can it occur?

It is, first of all, important to realize that the diffusion whereby the crystals become homogeneous can, and does, take place without anything like a complete recrystallization of the alloy. It is known that, in favorable circumstances, if an alloy is heated to a suitable temperature for a sufficiently long time complete rearrangement of the crystals may occur. In order to test whether the tendency toward diffusion is enough to bring about such crystalline rearrangement, some experiments on a variety of alloys have been carried out, at my suggestion, by F. Adcock. He examined carefully marked regions on a polished and etched specimen and photographed the crystal outlines, as well as the dendritic cores, before and after annealing. He found that the crystal boundaries undergo very slight changes, of such a nature as to smooth out minor irregularities and sharp curvatures, but that there is, in cast solid solutions (mainly of alloys of copper) no tendency to general recrystallization. The small boundary changes are interesting, particularly as there is strong evidence that they occur during the first cooling of the alloy, long before the dendritic cores have begun to disappear. On the other hand, these observations make it clear that the mechanism of diffusion must exist within the organization of each crystal and that it is not dependent on complete rearrangement with its probable temporary passage through an amorphous condition.

With a crystal, however, we must think of the atoms as fixed upon or vibrating about fixed points on a space lattice, with only small interstices between the atoms or rather between their outer electron shells. In such a system, there can be no room for wandering atoms of the same order of dimensions as those upon the space lattice. Fortunately, our substitution theory at once suggests a simple and an adequate explanation.

At ordinary temperatures, it is possible to bring about slip within the crystals of most metals and solid solutions by the application of external stress. The intensity of the stress required to produce slip decreases very rapidly with rising temperature; i.e., metals become very soft when hot. But the stress required to bring about plastic deformation (slip) in a crystalline aggregate is larger than that which would be needed in a single crystal, as there is no doubt as to the stiffening and strengthening effect of the numerous crystal boundaries. Further, it is only a crystal here and there that is most favorably oriented in respect of the particular external stress applied. It follows that the stress required to produce slip within a single crystal at high temperatures is not very great.

The essence of the substitution theory of solid-solution structure is that the presence of a solute atom in a space lattice produces somewhat intense stresses in its neighborhood. At a high temperature, if such stress can relieve itself by the occurrence of internal slip, one would expect such slip to occur. But here, all that is necessary is a slip of one atomic step—a slip about 0.002 as large as those that occur during plastic yielding. Under external stresses, the atoms generally slip in layers or planes; under the internal stress caused by a solute, only a single row or line of atoms need move at one time. If the solute atoms are uniformly distributed through the crystal lattice, the resulting stress will be balanced and no slip need occur or the amount of slip occurring in different directions will be equal and opposite, so that the net result on the distribution of the solute atoms will be nil. On the other hand, if the concentration is not uniform, there will be more slipping in the direction leading toward the region of low concentration than in any other. Slips of one step at a time, taking place at intervals in the various principal planes of the lattice, will be sufficient to account completely for diffusion and it is possible by a suitable number of successive slips of this nature to carry a solute atom from any one position in the lattice to any other.

The question, “what happens to the atom at the end of a row when slip occurs?” opens the question of crystal boundaries, which I have avoided although it is readily treated on the lines followed in our conception of the crystal structure itself. I need only say that the end atom, which will be pushed out when slip occurs, probably passes into the more or less non-crystalline or amorphous layer present at the boundary; while at the other end of the row, room is made for the entry of an atom passing into the lattice from the intercrystal layer. As there is reason to suppose that, particularly at high temperatures, such an interchange of atoms between crystal lattice and intercrystal layer is constantly in progress, our view of the mechanism of diffusion within the crystal requires no fresh assumption.

It does not seem easy to find any ready means for testing the probability of this suggested mechanism for inter crystalline diffusion. At first sight, it would seem that solute atoms which cause the greatest amount of distortion in the solvent lattice should exhibit the most rapid rate of diffusion, for presumably the stresses in the lattice tending to cause slip would be greatest for such atoms. It is true that the resulting distortion of the lattice would tend to hinder general slip under externally applied stresses, but the extremely local slip required for diffusion would occur in such a way as to relieve distortion and would not be hindered in that way. It is, however, difficult to determine the relative rates of diffusion of different kinds of solute atoms in the same solvent lattice, partly because it is not practicable to determine the initial concentration gradient in a cored crystal nor the precise moment when that gradient has either disappeared or fallen to a known lower value. We can, therefore, form nothing more than vague estimates of the rapidity with which diffusion occurs in different solid-solution alloys. In this connection, we may recall the extremely slow rate of diffusion in the copper-nickel alloys, in which the lattice distortion is very slight. On the other hand, carbon in gamma iron is known to diffuse with great rapidity and its solid solubility is by no means high. However, we have the equally well established fact that phosphorus in solid solution in iron only diffuses with great difficulty, yet its solid solubility is by no means high and its hardening effect considerable.

A less well-defined case is that of zinc in copper, which is thought to diffuse more rapidly than tin in the same solvent; in all these cases, however, there are no quantitative data and it is by no means certain that lattice distortion is the only, or even the most powerful factor affecting diffusion by the mechanism suggested. The facility with which the interatomic bonds can undergo slight displacement, thus enabling an atom in a slipping layer, or row, to transfer its bonds from one atom to the next in the adjoining row, must play a most important part. It seems quite possible that such slipping may not be the simple geometrical process pictured, but that the transfer of interatomic bonds involves a certain degree of temporary dislocation of the whole lattice structure in its neighborhood. In the case of carbon and phosphorus, we are dealing with metalloid elements the interatomic attachment of which to the metal atoms may be of quite a different nature from that between two metal atoms. Sir William Bragg has definitely suggested that in the solid solution of carbon in gamma iron, the carbon atoms are situated at the body centers of the face-centered cubic lattice. The evidence for this view is by no means strong; indeed, the latest view of Westgren, to the effect that the gamma-iron lattice is not appreciably expanded by the introduction of carbon, seems to indicate that here we must be dealing with a substitution structure, for otherwise the density would increase much too rapidly with the addition of carbon. It is none the less possible that smaller atoms, such as those of carbon or phosphorus, may be able to find room in such lattice interstices as that indicated. In the case of phosphorus, although measurements are not available, one fact that points in that direction is the extreme slowness with which phosphorus diffuses in iron. If the phosphorus atom is located at the cube body centers, diffusion by slipping would require that the row of atoms in which a phosphorus atom is present would move in such a way that the phosphorus atom would pass from one body center to the next, and in doing so the phosphorus atom would have to pass through the normal position of one iron atom in the cube, while a number of iron atoms would have to pass through the body-center position, and this would be almost impossible. In such a case, diffusion within a stable crystal might be impossible and the process could occur only during a recrystallization process. We have here a point that might be tested experimentally, but the difficulties to be overcome are considerable.

Solid Solutions Containing Several Kinds of Solute Atoms

Solid solutions in which several kinds of solute atoms are present simultaneously are well known. If the substitution and lattice-distortion theory is correct, the presence of one kind of solute atom must affect the limiting solubility of the other. If the two kinds of atoms produce similar kinds of distortion in the solvent lattice, they will mutually tend to diminish solubility; the atom that causes the lesser degree of distortion will tend to throw the other out of solid solution. The limiting solubilities are a matter of the balance of energy content as between a more severely distorted solution lattice or a duplex system of two distinct phases.

The study of ternary alloy systems at the National Physical Laboratory by Doctor Hanson, Doctor Haughton, Miss Gayler, and Miss Bingham has brought to light a number of cases where the introduction of one metal has seriously diminished the solubility of another, magnesium and copper in aluminum being a good example. Magnesium and copper have smaller lattice constants than aluminum, but copper (3.60) is nearer aluminum (4.05) than is magnesium (3.22); while one would, on this basis, expect these two metals to interfere with the solubility of one another, copper would be expected to win and the solubility of magnesium to be reduced. Other complicating factors intervene, however, for the solubility of magnesium in aluminum (about 9 atomic per cent.) is much higher than that of copper (2 atomic per cent.), in accordance with which fact, magnesium reduces the solid solubility of copper in aluminum. This may be because the coefficient of thermal expansion of magnesium is larger in the ratio of 25 to 17 than that of copper, so that at the high temperatures where solid solutions are formed, the difference in lattice size between the two metals may be much less than at the ordinary temperature. Magnesium may be present in the solid solution in a form slightly different from that in which it forms its own hexagonal lattice, so that its effective lattice constant may be much closer to that of aluminum than the figures quoted would suggest.

Another interesting example of incompatibility in solid solution is that of carbon and phosphorus in gamma iron. There can be no doubt that carbon, which otherwise diffuses with great readiness through hot gamma iron, persistently refuses to enter the high-phosphorus regions that originate from the coring of the iron-phosphorus solid solution. Phosphorus is distinctly more soluble in gamma iron than is carbon, but the manner in which phosphorus enters the gamma-iron space lattice may prove the determining factor in this case.

One more interesting point, although its complete explanation is not plain, is the influence of dissolved impurities on the magnetic properties of iron. Most of such impurities tend to “harden” iron magnetically, particularly increasing the hysteresis loss. Ewing recently put forward a new model of atomic structure that serves to account for the observed facts of magnetism in a complete manner. In this model he assumes the atom is slightly unsymmetrical in certain respects. Any distortion of the lattice into which such an atom is built must bring with it a corresponding distortion of the atom itself, however slight. Such a slight further distortion of the atom, however, according to Ewing, would serve to account for the marked effect on magnetic properties of both added elements (impurities) and mechanical deformation. There are at least two apparent marked exceptions to the rule in regard to the effect of added elements, for the addition of silicon and aluminum to iron improves it magnetically in regard to hysteresis loss. The present view would readily explain this action if it were found that the impurities always present in ordinary iron tend to produce one kind of distortion in the lattice while the two elements named tend to produce an opposite, and therefore neutralizing, kind of distortion. This, however, opens a field of inquiry I do not propose to enter at this time.

Structure of Crystals

We must think, then, of a crystal as being built up of atoms fixed upon an imaginary, but none the less effective, framework in space, the average distance of the atoms from center to center being most definitely governed in this way. Under the influence of thermal agitation, the atoms may be regarded as oscillating about these equilibrium positions. Thermal expansion, which is the consequence of this thermal oscillation and the resulting tendency to drive the atoms farther apart, brings with it a corresponding expansion of the lattice. This expansion cannot be carried beyond a certain point. If a certain limiting interatomic distance is exceeded, the interatomic linkage that holds the atoms in place breaks down. The result is the formation of a new phase. If the extension of the lattice is caused by thermal expansion, when the limiting extension is passed, the lattice breaks down and disappears more or less completely: the crystal melts, the new phase being liquid. If the lattice is extended by mechanical stress, the phase formed is Beilby’s amorphous metal. In other cases, the new phase may be a crystalline solid having a different space lattice.

For the present discussion, the simplest plan would be to adopt the atom, suggested by the theory of Langmuir, consisting of a relatively large heavy positive nucleus with electrons placed about it in definite positions. Even if obliged to adopt the Bohr-Rutherford atom, with inner and outer electron shells rotating about the nucleus, we must realize that an atom which, in a state of equilibrium, comes to rest upon a definite space lattice must have certain strongly defined directional properties, which we may call “bonds.” In a cubic substance, these bonds must tend to lie in directions parallel with the cubic axes. That the outer electrons play an important part in this interatomic bonding is indisputable, even though there is divergence of opinion on the precise mechanism by which they bring it about. None the less, the approximate dimensions of the atoms are known and, if we regard the “size” of the atom as determined by the diameter of its outer electron shell, we find that, in most metals, the dimensions of the space lattice are such that the atoms almost or quite touch one another. It must be borne in mind, however, that the atom, looked at in this way, is not an extremely hard, rigid body, but that the outer electrons or their orbits can be deflected not only by their mutual interaction as between an atom and its neighbors, but even by the application of external stress. Whatever, therefore, tends to change the dimensions of the space lattice must, to a corresponding extent, bring about a distortion of the atoms or at least of their outer electron shells.

With this conception of the atoms and their arrangement on the space lattice in our minds, we can form an approximate picture of the internal structure of a crystal—or at least of such portions of a crystal, well away from boundaries or other disturbing causes, in which truly uniform orientation of the atoms reigns. We see a perfectly orderly regular arrangement of the atoms, each in a position of equilibrium so far as its attachments to its neighbors are concerned, oscillating about the fixed points of the space lattice and, together with the outer electron shells, occupying space quite completely. That is to say that in the case of the more closely packed types of lattice, at all events, there would be no interstitial space comparable in dimension with the space occupied by any single atom. It must, however, be borne in mind that the atoms at best only “occupy” the space defined by their outer electron shells with extreme “thinness.” The outer electrons may be moving great rapidity, but they are very minute compared with the radius of the atom. The atom, therefore, occupies its own region of space in much the same way as the solar system occupies the space included in the orbit of Neptune. Just, therefore, as comets are able to wander freely about the inter-planetary spaces, so free electrons such as those carrying electric current should be able to move about and, in a sense, through the atoms. Like the comets, however, they are apt to be seriously deflected if they approach the central nucleus.

Structure of Crystals of Solid-Solution Alloys

But our present interest centers upon a type of crystal that is much more complex—that of a solid-solution alloy. Within the organization of the crystal we must find room for two kinds of atoms, which may be called the solvent and the solute atoms, respectively. That the general organization of the crystal remains much the same as that of a pure metal is evident from the fact that, although the properties of solid solutions differ in most important ways from those of the pure metal, the crystalline nature of the material remains strongly marked and closely resembles that of the solvent metal. Thus all the alpha solid solutions based on copper share the tendency of pure copper to undergo twinning. We must, then, find room for the solute atoms in such a way that their presence affects, but does not destroy, the crystalline organization. It is possible to think of the solute atoms introduced into the crystal in several ways. One of the earliest ideas was that the solid-solution crystals were not simple crystals but ultra-microscopically duplex—intimate mixtures of two kinds of crystals built up together as true “mixed” crystals. This view has been credited on metallurgical considerations alone, but recent determinations of the space lattices of a series of solid solutions, made by Owen and Preston at my suggestion, have disproved it, for the solid solution exhibits a single well-defined space lattice that differs only slightly in dimensions from that of the solvent metal.

The second possibility is that the solute atoms find their way into the interstices of the atomic spacing of the parent metal. Such an introduction of additional atoms could occur only in the case of extremely small atoms and then probably only in such lattices as the face-centered cube where there is a certain amount of interstitial space at the body center of the cube. Larger atoms, if thus introduced, would cause an enormous degree of distortion or distension of the parent lattice, and we should expect to find ready solubility in the solid state for the smallest atoms and a systematic decrease in solubility as the size of the solute atom increased. Actually, this is not the case and, in itself, serves to discredit the general correctness of this view, although it may still be possible, in special cases, that small atoms can find a place at such points as the cube centers of a face-centered lattice. Of course, in the case of extremely small atoms, like those of hydrogen, the position is entirely different and “solution” may mean simple penetration into or through the interstices of the space lattice.

In view of these difficulties attaching to the two possible schemes of solid-solution structure just outlined, I have adopted the third possible view: that the solute atoms are placed upon the actual space lattice of the solvent metal in simple substitution for the atoms of the solvent. This view, at first put forward in a somewhat speculative manner, has found extensive confirmation, not only by actual x-ray measurements of solid-solution space lattices, but by the easy and simple manner in which the assumption of such a structure serves to explain many of the most striking properties of solid solutions. With the aid of one or two simple and, in my view, natural conceptions, this view of the structure of metallic solid solutions leads to a somewhat general view of the atomic structure of solids, both vitreous (amorphous) and crystalline, that serves to connect and to explain a wide range of known facts in regard to their behavior and properties.

Inferences Drawn from Substitution Theory of Structure of Solid Solutions

The substitution of an atom of solute for an atom of solvent in the space lattice of a metallic crystal has great effect on the entire space lattice in the vicinity. If the solute atom differs appreciably in properties

from the solvent atom, a state of local strain or dissymmetry must result. This can be inferred from a very simple consideration. Figure 6 (a) shows the simplest possible distribution of atoms in one plane of a cubic crystal, where the atoms lie at the corners of squares. In a pure-metal crystal, where all the atoms are exactly alike, the arrangement will be perfectly uniform and symmetrical. Now, suppose that one of the atoms is replaced by a slightly different kind of atom, indicated in (b) by a circle, and further suppose that the natural space lattice upon which this atom would fit (i.e., the space lattice of a crystal consisting entirely of that kind of atom) is slightly larger in scale than that of the solvent metal. This would probably mean that the solute atom would tend to allow the adjacent atoms of the solvent to remain farther away than they are in their normal position; in other words, the forces acting upon the adjacent atoms of the solvent and emanating from the atom of solute would be smaller than those emanating from adjacent atoms of the solvent itself. The adjacent atoms of solvent, therefore, will no longer be in an equilibrium position in their normal space-lattice places, and some sort of distortion of the lattice is bound to follow. As a first effect, one might suppose some such distortion as that indicated by the dots in (b), but such purely local action is quite impossible—the effect must propagate itself through many layers of atoms, producing a slight expansion, tapering off as we pass outwards in all directions from the disturbing atom of solute. None the less, each such disturbing atom must affect a relatively large number of the atoms of the solvent. For instance, if we think of a plain cube-corner lattice and suppose that the disturbing effect is limited to a total range of five atoms on each side of the solute atom, a concentration of only one atom in eight thousand atoms of solvent would be suffi-

cient to affect the entire space lattice, bringing about not only a series of locally relatively intense distortions of the lattice, but also a slight but general expansion of the entire lattice. A concentration of one atomic per cent, might, therefore, well produce a measurable change of dimension in the lattice as a whole in addition to causing more numerous local disturbances.

It is important to note that the relative amounts of what we may term “local” and “general” extension of the lattice will depend on the nature of the lattice itself. If the lattice is comparatively soft or flexible, there is likely to be something of the sort of distortion shown in Fig. 6 (b), which is mainly, if not entirely, local. In a stiff lattice, on the other hand, local distortion will be resisted, and therefore diminished, while there will be a correspondingly increased general distortion, somewhat as indicated in Fig. 7, where the whole lattice is shown slightly expanded. This difference in the manner of distortion is of great importance in determining the behavior of various alloys. The cause of stiffness in a lattice is not obvious except that it must be caused by the nature and location of the interatomic bonds. Where these are not only strong but firmly held, as regards direction, the lattice will be stiff. We should expect to find this in the harder and stronger metals having a high elastic modulus.

The question is, how such disturbance of the lattice might be expected to make itself felt in the x-ray spectrum of the crystal? A slight general expansion of the lattice will be measurable when it attains the order of 1 per cent, of the normal lattice spacing; this need not be a uniform extension, the x-ray spectrum will lead to the determination of the average size of the lattice unit. If the local disturbance in the immediate vicinity of a solute atom is not considerably greater than the amplitude of thermal vibration, we could not expect to find any definite effect of such distortion on the sharpness of the spectrum lines obtained with the crystal, so that the only measurable effect would be a slight expansion, or in some cases contraction, of the average lattice spacing.

It may be suggested that on the other view—that the solute atoms are located in the interstices of the solvent lattice—there would also be local distortion and general expansion of the lattice. This is true, but there must be a wide difference in the degree of the expansion of the lattice in the two cases, and a study of the densities of solid-solution alloys together with measurements of their lattice dimensions makes it possible to distinguish critically between the alternatives. It is known that the densities of solid solutions differ only slightly from the values found by taking the mean of the densities of the constituent metals in proportion to their concentration in the alloy. If the solute atoms are supposed to be “pushed in” as it were, without replacement of solvent atoms, the space lattice of the solvent metal must be very much enlarged to compensate for the additional density due to the interpolated solute atoms. If the substitution theory is correct, however, we should have an exact mean density if the parent lattice remained unaffected, and a slight departure from mean density fully accounted for by a slight change in the scale of the parent lattice. Such changes of lattice dimension are very small; for a time it was supposed that they did not exist at all. Recent measurements by Owen and Preston, however, have shown these small changes in the scale of at least one solid-solution lattice and that these correspond with a considerable degree of exactness with what must be expected by the substitution theory. For the alloys so far investigated, therefore, the truth of the substitution theory must be regarded as established, while its wide applicability to most intermetallic solid solutions is extremely probable. Where comparatively small atoms, such as carbon or phosphorus (i.e., metalloid atoms) are concerned, however, the possibility of interstitial location is not excluded and may even be regarded as probable in a few instances.

The data of x-ray analysis bearing upon this point are of such fundamental importance that it is desirable to quote the actual results obtained by Owen and Preston in their x-ray analysis of the alloys of copper and aluminum. In the alloys rich in copper, they found that the spectra of the alloys containing 2.4 and 8 per cent, of aluminum were completely examined; they showed the characteristic features of a face-centered cubic lattice, but the intensities of the reflections fell off markedly as the percentage of aluminum was increased. No new lines were observed in any of these spectra. The results found for the mean value of the side of the elementary cube, in angstroms, are:

There is, thus, a steady slight increase in the lattice dimensions. If the values of α thus measured are the mean values in a slightly distorted lattice, the densities of the series of alloys can be calculated, inserting a

value for the “mean atomic weight” by taking the means of the atomic weights of copper and aluminum in proportion to the atomic concentration of each alloy. The values thus obtained are stated in the column headed p’; the observed densities of the actual alloy specimens are given in the column headed p. The differences between observed and calculated values, given in the column headed difference, are small. It must be borne in mind that in the density calculated from x-ray data the errors of experimental measurement are trebled. The differences found are, therefore, readily accounted for by errors of ½ per cent, in the measurement of the x-ray angles. These results are shown graphically in Figs. 8 and 9, where the values of α, p, and p’ are plotted against aluminum concentration. The values of densities found with the foregoing data as to lattice size, but based on the assumption that the aluminum atoms are present not in substitution for but in addition to the atoms on the copper space lattice, are also plotted. The comparison shows that the observations entirely contradict the latter assumption while they are well within the limits of experimental error, in accordance with the substitution theory.

In the alloys rich in aluminum, similar measurements were made on alloys containing up to 2 per cent, of copper. In these, the lattice constant was found to be the same, within the errors of experiment, no definite evidence of any expansion of the lattice being obtained. The densities calculated from the value found, however, agree closely with the observed densities. The maximum atomic concentration, though, is of the order of 1 per cent., while at the other end of the series the maximum concentration was as much as 16 atomic per cent. It is not surprising, therefore, that the effects at the aluminum end should be much smaller and apparently beyond the limits of accuracy of the method employed.

If we accept the fundamental conception that in metallic solid solutions the solute atoms replace atoms of the solvent on the normal space lattice of the latter, and that a local distortion as well as a general change of average lattice scale accompanies this replacement, consider a number of interesting inferences which follow in a simple and direct manner. The first of these relates to the limit of solid solubility of one metal in another.

Storage of Energy Within Crystals

We have good reason for supposing that the energy content of a solid solution must be greater than that of the simple solvent metal and are in a position to see where the potential energy content of the solid solution is stored; it is obviously in the distortion and change of dimensions of the lattice. Where there is a general expansion of the lattice there must be an artificial increase in the normal interatomic distance; an increase that, on the average, may be slight but is none the less important locally. The strength of the interatomic bonds is great and a small displacement of atoms against the action of these interatomic forces implies the storage of a large amount of energy. Indeed, the distortion of a lattice must mean a corresponding, although slight, distortion of the atoms themselves. Whether we think of stationary electrons or of electron-orbits, their position must be intimately related to the interatomic bonds; and if these are displaced so must the electrons or their orbits be displaced. It is in this slight internal change of form of the atom itself that we must seek the real locus of energy storage. It follows, however, that there must be a limit to the amount of energy that can be stored in this way without disruption of the lattice.

There are several ways of arriving at this conclusion. We know that the amount of thermal energy that can be stored in a crystalline array of atoms is limited and that if this amount is exceeded, melting occurs. One way of looking at this fact is that when the atoms are forced apart beyond a certain definite distance, their mutual bonds cannot hold them together and the lattice breaks up; this way of regarding fusion is of considerable importance, but for the moment it suggests that the distortion of a lattice is limited in amount. Consequently, the number of solute atoms that can be introduced into the space lattice of a solvent must be limited, but this limit will depend on the amount of distortion that the introduction of each atom produces. We find, therefore, a ready explanation for the fact that the solid solubility of metals in one another varies widely: the more similar the atoms of the solute are to those of the solvent, the greater the degree of solid solubility; where the similarity is very great, there is an unbroken series of solid solutions in the alloys of the two metals, but with increasing dissimilarity there are correspondingly smaller degrees of solid solubility. The only difficulty is, with our present knowledge, to decide what are the criteria of “similarity” between atoms of different metals. One of the most important factors must, if our present view is correct, be the normal size and type of space lattice upon which pure crystals of each metal are built. Where these are closely alike in scale and arrangement, we shall expect to find large solubility; and vice versa. A glance at the known facts will confirm this conclusion. Copper normally forms a face-centered lattice, the constant (size of unit) of which is 3.60 A; the constant of nickel is 3.54; between these two metals there is an unbroken series of solid solutions. The same thing applies to gold which has a lattice constant of 4.08, and silver, which has a lattice constant of 4.06. On the other hand, copper (3.60) and silver (4.06) show only a limited range of solid solutions at either end of the series. Copper and zinc (2.67) show a more limited degree of solid solubility at one end of the system, where the smaller atom replaces the larger, and practically no solid solubility at all at the other end, where the replacement would occur in the opposite way. Copper and aluminum (4.05) show very limited solid solubility at both ends, while copper and tin (6.46) show still more limited solubility at the copper end of the system. It is clear, however, when the details of these systems are examined, that the lattice constant is by no means the only governing factor, nor would we anticipate that this should be the case. What one might term, tentatively, the “shape” of the atom and perhaps the facility with which its bonds can adapt themselves to a slightly different configuration must play a very important part. It is not possible to pursue this aspect of the matter further, and we must content ourselves with the fact that the amount of energy stored in the composite lattice determines the limit of solid solubility and that this is dependent on the amount of distortion which the introduction of a solute atom produces.

Effect of Distortion on Hardness, Strength, and Ductility

The amount of distortion that a lattice undergoes, however, will determine the extent to which certain properties of the crystal are affected. Perhaps the most interesting of these is its hardness or resistance to plastic deformation. It is generally accepted that plastic deformation in a metallic crystal occurs by a process of slip, which takes place on some of the crystallographic planes of the crystal. This means that when external stress is applied to a crystal there is an elastic (and slight) distortion of the lattice; when this has reached the limit of the atomic bonds, slip occurs and layers of the crystal slide over one another. Such sliding will be most readily brought about in a perfectly regular and symmetrical crystal with perfectly uniform atomic spacings. The layers of atoms lying on the two sides of a slip plane may then, in a general way, be regarded as two perfectly smooth surfaces sliding over one another; at all events, for a given strength of atomic bond, the resistance to slip will be least in such an arrangement. If, however, the arrangement is distorted in the least degree, there will be a corresponding increase in resistance to slip. Thus a slightly distorted lattice must imply an increased degree of hardness and strength and also a diminished degree of ductility.

At this point, we arrive at an inference that is directly amenable to verification. If the power of a solute atom to cause distortion of the lattice of the solvent is closely related to the hardening effect it produces, and this same power of producing distortion also determines the limiting solid solubility of one metal in another, we must expect to find that the hardening effect of one metal on another in the form of a solid solution is closely and inversely related to its limiting solid solubility. The greater the range of solid solutions formed, the less should be the hardening effect of the added metal per atom added.

An examination of the well-studied alloy systems in which the limits of solid solubility have been reliably determined will show that this generalization holds good. If the copper alloys are taken, the hardening effect, within the range of solid solubility in each case, of alloying metals may be placed in the following order: nickel, manganese, zinc, aluminum, tin. The limits of solid solubility, in atomic per cent., for these metals are nickel, 100; manganese, 100; zinc, 36; aluminum, 14; tin, 67. In the case of nickel and manganese, although there is an unbroken series of solid solutions in both cases, the shape of the liquidus curve of the copper-manganese system indicates that the formation of a eutectic near the middle of the series is only just avoided; an alteration of pressure, for instance, might easily result in the formation of two separate phases— probably a eutectic. The solid solubility of nickel in copper may, therefore, be fairly regarded as being greater than that of manganese. A similar rule holds for the alloys of aluminum; zinc forms a much longer range of solid solutions with aluminum (up to 15 atomic per cent.) than does copper (up to 1.5 atomic per cent.), and their hardening effect is in the inverse order. In the alloys of iron, in so far as they have been studied apart from the effect of carbon, a similar law holds good.

The qualitative verification of the inference drawn from our theoretical conceptions, therefore, is good; so good that the inference may be regarded as a new general principle governing the properties of solid solutions, a principle that had not been recognized until the present considerations brought it to light. The law or principle is rather more than qualitative, as it serves to group alloy systems in a rough quantitative order. Any accurate quantitative verification is difficult at present. We do not possess data concerning even the average expansion or contraction of solid-solution space lattices except for the one system for which preliminary figures have been given. Even if these were available, we would not know the degree of local distortion produced by the solute atoms. Further, we are not justified in assuming that successive additions of solute atoms to a solid solution produce equal increments of hardness; in fact, the reverse is known to be the case. From the point of view of the substitution theory, this is to be expected as an increase in the number of solute atoms would result in a greater average change of the lattice size, but need not, and probably would not, increase the degree of local distortion in the lattice. It may be that, with a fuller knowledge of the atomic forces involved in these complex relations, a quantitative formula for the relation between internal energy, lattice distortion and hardness may be worked out. As yet we seem to be some way from that stage, so that the rough quantitative verification of our inference is as much as we can hope to attain.

Reduction of Melting Point

A property of solid solutions, even more typical, perhaps, than hardness, which also finds ready explanation on the basis of the “substitution” theory is the lowering of the melting point and the spreading of melting and freezing over a range of temperature. This difference is indicated in the equilibrium diagrams of Figs. 2, 3, and 4. To arrive at the explanation which our theory affords for these phenomena, let us consider a crystal of a pure metal in which the atoms are arranged in perfect regularity upon the normal space lattice. With thermal expansion there is, inevitably, a corresponding expansion of the lattice. This has been observed, although in certain cases the lattice does not expand equally in all crystallographic directions; in the case of graphite there is a slight contraction of the lattice in one direction compensated by a correspondingly greater expansion in another. In a cubic or close-packed hexagonal lattice, however, there is no reason to suspect this and the fact that Westgren has found the characteristic lattice of gamma iron at 1100° C. to be a face-centered cube, while delta iron at 1450° C. shows the typical diffraction lines of a body-centered cubic lattice, provides direct evidence that in this metal there is no serious distortion of the symmetry of a cubic lattice by very considerable thermal expansion. We may, then, assume that the cubic metals undergo a regular and symmetrical lattice expansion proportional to their mass-coefficient of expansion.

We return, now, to the conception that for every lattice there is a maximum distortion (in most cases an extension) that cannot be exceeded without causing the lattice to break down with the resultant formation of a new phase. We cannot, however, assume that the actual maximum extension that can occur is independent of the nature of the second phase which results from a breakdown of the lattice. Considerations to be discussed later, make it certain that the maximum lattice extension is considerably dependent on the nature of the alternative structure that arises when that maximum is exceeded. With this reservation in mind, it is possible to derive from the “substitution” theory of solid solutions a complete explanation of the behavior of these bodies on melting and freezing.

A crystal of a pure metal possesses a perfectly uniform lattice, which expands uniformly with rising temperature, at all events in the case of cubic lattices. The ground for the latter assertion is that a number of lattices of this kind have been studied at quite high temperatures; for instance, the lattices of gamma and delta iron at 1200° and 1450° C., respectively, by Westgren, which, at those high temperatures, have shown the characteristics of a cubic lattice—an observation that disposes of any serious degree of unequal expansion in different directions in the lattice. In the case of graphite, it has been found that the expansion in one direction is much greater than that to be anticipated from the mass-coefficient of expansion; while in another direction there is a small contraction. In view of the other observations, however, this seems to be a special case and not to be applicable to cubic lattices. The result of uniform thermal expansion on a simple regular lattice must, therefore, be that at a definite temperature the whole of the lattice attains the limit of its extensibility; a further rise of temperature leads to the breakdown of the lattice and the formation of liquid—the whole of the metal melts at one uniform temperature. This conception of melting is in accord with the well-known fact that a high melting point is definitely associated with a low coefficient of expansion, while the harder and stronger metals are, in general, those with high melting points.

On the basis of the present theory, a crystal of solid solution possesses a lattice that is slightly extended throughout and, in addition, in the neighborhood of solute atoms, there are regions of local and more marked extension. If such a crystal is heated, the locally distended regions around solute atoms will reach the limit of lattice extension first, and melting will begin at those places at a temperature at which the bulk of the lattice is still stable. We thus have local melting with the formation of a liquid whose content of solute is higher than the average of the whole crystal. As the temperature rises, other regions of less distorted lattice will melt, until the whole crystal has melted. The highest temperature required to bring this about will be lower than in the case of the pure metal, because the entire lattice is extended and therefore reaches its limiting extension after a smaller thermal expansion. The limiting extension, however, is not constant throughout a series of solid solutions, as it must depend on the concentration of the liquid formed. The mechanism of this dependence, which can be accounted for in terms of energy content of the two phases, is probably connected with the internal, or rather the osmotic, pressure of the liquid solution formed. This pressure may act by restraining the expansion of the lattice under thermal expansion, but that point cannot be easily tested. The reality of the effect, however, is undoubted.

From the preceding, it is possible to draw interesting inferences as to the behavior of solid solutions on melting and freezing, as the freezing operation is exactly the reverse of the melting process as described. In a solid solution, two lines in the equilibrium diagram, known as the solidus and liquidus, determine the beginning and the completion of melting. On the basis of the substitution theory, the beginning of melting (the position of the solidus) is determined by the maximum local extension produced in the lattice by the presence of solute atoms; while the completion of melting (the liquidus) depends essentially on the general expansion of the lattice. If this is correct, we should find that where local distortion is large and general extension small, the solidus would fall rapidly with increasing concentration of the solid solution, while the liquidus will be much less steeply sloped. Further, the slope of the solidus will be increasingly steep the lower the solid solubility of the solute in the solvent metal. This latter point is borne out by the well-established equilibrium diagrams. Wherever there is a long range of solid solutions, the solidus falls slowly and does not lie far below the liquidus; on the other hand where solubility is low the solidus maybe nearly vertical while the slope of the liquidus is often relatively gentle.

Testing of Steel Hoisting Rope

It is difficult to know just when a hoisting rope should be removed from service and a new one substituted. It is desirable to utilize the full life of a rope but, on the other hand, the damage and possible loss of life resulting from delay in replacing it may amount to many times the cost of a new rope.

In the present methods of inspection the inspector must rely upon accumulated experience and a certain sixth sense or intuition in addition to any rules that may be laid down regarding the number and distribu-

tion of broken wires. This condition does not necessarily arise from lack of interest among users or makers of rope, but is due rather to the lack of satisfactory or adequate methods for testing ropes to determine their exact condition.

The main difficulty in devising a physical test lies in the fact that a number of causes, all operating in different ways, contribute to the deterioration of a rope in service, and affect not only the external conditions but also the inner structure of the material of which the rope is constructed. Wear and broken wires on the outer part of the rope are not difficult to detect, but a test is needed which will clearly indicate the presence of broken wires in the interior portions and the change in strength due to alterations in the structure and properties of the material itself.

The urgent need for a solution of this problem demands careful investigation of any testing methods which give promise of success. For this reason the Bureau of Standards is glad to undertake an investigation which has now been made possible by a small Congressional appropriation available July 1, 1923. The purpose of presenting the subject at this time, so long before any solution can be expected, is to

stimulate discussion, to invite suggestions, and to be speak cooperation.

Criteria for a Satisfactory Test

Any satisfactory method of test must be: (1) reliable; (2) non-destructive; (3) economical; (4) simple; (5) rugged.

It goes almost without saying that reliability is the prime requisite for a satisfactory test. While it is essential that the test should indicate a dangerous condition without fail, it would, on the other hand, be extremely unsatisfactory if it should indicate danger when no danger exists.

It is also evident that a satisfactory test must be non-destructive. It is now possible by a simple tensile test to determine whether or not a rope that has already been removed has suffered an undue reduction in its factor of safety. It should be possible, however, to test each element of the length of a rope while still in place, without injuring its properties.

It is desirable that a test be economical. It is possible that it might be cheaper to renew ropes with unnecessary frequency than to employ a test which costs too much to apply.

Simplicity is also an essential qualification. It is undesirable to require the services of a trained laboratory man, and every effort should be made to reduce any method to its simplest terms so that non-technical men of average intelligence can be trained to use it.

Finally, the apparatus should be designed and constructed with due regard to the conditions under which it must operate. Too delicate and fragile an apparatus would be more aggravating than advantageous under ordinary operating conditions.

An ideal testing method would be one capable of indicating: (1) localized sources of danger; (2) general deterioration; (3) relative quality of a new rope.

Although such an ideal method may seem at present almost too much to hope for, it is well to have these things in mind and strive for their attainment.

Possible Methods

It appears that the proper procedure would be to consider in some detail all the possible methods that might be applied, in the light of the requirements just mentioned, and to give first attention to the investigation and development of the method or methods which appear to offer greatest promise. Among the possible methods may be included the following: (1) mechanical tests; (2) electrical resistance tests; (3) x-ray examination; (4) magnetic analysis.

There seem to be some possibilities in the direction of mechanical methods, especially along the line of recent developments in remote-reading strain gages, that should not be overlooked. It seems doubtful, however, that any purely mechanical means will be capable of indicating the strength of a rope which has suffered deterioration from bending, corrosion, or any other influence which would alter its internal structure.

At first thought, an electrical resistance test might seem to offer considerable promise. When it is considered, however, that the apparent resistance of the rope may be altered by slight changes in the contact between the individual wires, it would seem that such a method would be likely to fail in reliability, it is also understood that the electrical conductivity of steel varies only very slightly, even when stressed to the breaking point.

The use of x-rays can be dismissed very briefly. Photographic methods are necessary; installation and operation are expensive; and general deterioration could not be detected, but only flaws and broken wires. The item of expense alone would probably rule out this method.

Magnetic analysis, then, seems to be the best and perhaps our only hope. It is non-destructive and economical, and apparatus can be constructed that is simple and rugged. While, in its present state of development, the prime requisite, reliability, has not been attained, this does not seem impossible of achievement. For this reason it seems best to give this method of investigation our first attention.

Magnetic Analysis

The possibility of utilizing magnetic tests on iron and steel for the estimation of their mechanical properties is based upon the fact that the same factors which determine their mechanical properties also determine their magnetic properties. Any influence which operates to alter the mechanical properties produces a corresponding change in the magnetic properties. The change, however, is not directly proportional and herein lies the difficulty in the way of immediate application of the method. A considerable amount of work has already been done and a number of investigations are now in progress for the accumulation of the data necessary for determining the relationships that undoubtedly exist.

The degree of magnetization of steel, Fig. 1, is a complex function of the strength of the magnetizing force for which no mathematical expression has been developed. As the magnetizing force is progressively increased, the magnetic induction increases, first slowly, then more rapidly, and afterwards at a slower rate. After magnetization to a certain point, if the magnetizing force is then decreased the magnetization does not follow the same path by which it increased; when the magnetizing force has been reduced to zero a certain amount of residual induction remains, to remove which then requires a magnetizing force in the reversed direction, known as the coercive force. It is evident

then that there are many combinations of data observed in the determination of magnetic properties which may possibly be used for the estimation of physical properties.

It is not safe to draw conclusions from the value of induction corresponding to a single value of magnetizing force. It has generally been considered that materials which are soft in the mechanical sense are also soft magnetically. It has been found, however, that in materials which have been hardened and subsequently tempered there are cases where the higher magnetic permeability corresponds to the harder material. In the study of the magnetic properties as indications of the mechanical properties of steel, it is therefore necessary to take into account more than one of the magnetic characteristics.

Reference has been made to the special methods which have been developed for the detection of flaws by magnetic methods. The first apparatus for the detection of flaws consisted of an electromagnet between the poles of which the specimen was magnetized, Fig. 2. By means of properly arranged test coils and a ballistic galvanometer, it was possible to measure the leakage of magnetic lines of force along the length of the specimen. Since a flaw in the material would cause an apparent change in the magnetic properties, and consequently a disturbance of the normal distribution of flux, it was possible to detect and locate flaws. By such a method it has been possible to locate pipes and blow holes in material of which rifle barrels are made.

A subsequent development of this method consisted of magnetizing the specimen by means of a solenoid which surrounds it and moving this solenoid at a con-stand speed along the length of the specimen, Fig. 3. By means of test coils mounted on the same carriage as the magnetizing solenoid, variations in the rate of

magnetic leakage could be indicated. This method has been applied to the examination of rails, Fig. 4, and to the raw material of which rifle barrels are manufactured, Fig. 5. In the latest form of this apparatus, long bars are driven by rollers through the magnetizing and test coils, which remain stationary. By means of this apparatus a large number of steel bars have been examined which were intended for experimental determinations of the effect of heat treatment, and from which it was desired to eliminate any parts which by reason of chemical segregation or flaws might vitiate the results of the experiments. Apparatus has also been constructed for the testing of rifle barrels.

One of the difficulties in this line of investigation lies in the fact that the intensity of the magnetic disturbance is not proportional to the degree of the mechanical defect. Specimens have been examined by this method which showed relatively large variations in magnetic properties along their length, but upon subsequent examination revealed no mechanical weakness or flaws. On the other hand, some samples known to contain serious mechanical defects have given relatively small magnetic indications, Fig. 6. For this reason the application of this method in its present form for commercial testing would seem to be more or less limited. It may be possible by some modification to make the determinations in such a way as to differentiate between the various sources of magnetic inhomogeneity.

Some time ago an investigation was started at the Bureau of Standards in cooperation with the New Jersey Zinc Co. for the purpose of developing suitable magnetic methods of testing steel hoisting ropes. Unfortunately this work was interrupted by the war before any definite results had been attained. It will be necessary, therefore, to start again practically at the beginning in the development of methods suitable for this purpose. After the necessary preliminary study of the conditions under which hoisting ropes are used and of the causes of failure, it is proposed to prosecute the investigation along three general lines:

  1. Laboratory study of the effect on the magnetic properties of the various factors which operate to cause deterioration of a hoisting rope in service.
  2. Development of methods, instruments and apparatus for the application of magnetic tests under service conditions.
  3. Actual field tests to determine the possibility of practical application.

This investigation is being undertaken with a thorough appreciation of the difficulties to be overcome) but in the firm belief that there is no other method which offers so great possibilities.