INDUSTRIAL AND ENGINEERING CHEMISTRY
JULY, 1935
methods and new inventions. At this price it compares favorably with other metals already well established for industrial purposes. Increase in rate of production from one pound to 20,000 pounds of metal per day per unit is a miracle. Reduction in price from $5 per pound in 1915 to 30 cents per pound less than two decades later is another miracle.
Conclusions Light Teight is the outstanding characteristic of both aluminum and magnesium, with Lhe odds in favor of magnesium. All other things being equal, the two are competitors in the industrial field. Rut each is endowed with qualities peculiar to itself, and the erstwhile antagonists complement each other in the field of chemical industries. Aluminum and its alloys are indicated where acid resistance is a factor. Magnesium and its alloys are called upon for equipment to resist the alkalies, many types of organic chemicals, and most oils. T o attempt to substitute one for the other is fatal in the chemical industry.
751
Expansion of the field of usefulness of aluminum from a baby rattle presented as recently as 1855 to the Prince Imperial of a now obsolete monarchy, to the magnificent structures that span our skylines, to airplanes, ocean vessels, bridges, and streamlined trains that utilize this metal today, is an achievement worthy of Aladdin and his wonderful lamp. It is a far cry from the magnesium compounds isolated in 1695 by Nehemiah Grew, to the high-strength, light-weight alloys of magnesium-industry’s lightest structural metal; and the important developments of today and tomorrow lie in the utilization of the ultralight-weight magnesium alloys in the structural field. The surest guide to future possibilities is the past, and the achievements of aluminum and magnesium, since their modest industrial beginnings, less than a century ago, would indicate that the future holds unfathomed possibilities for development of both metals in the fields to which they are adapted. RECEIVED April 26, 1935.
Precious Metals as Materials of Construction
T
HE term “precious metal” indicates, in addition to noble properties, scarcity; therefore the use of precious metals in construction can naturally be only on a relatively small scale. However, this use is in vital spots and for extremely severe conditions. Iron has good properties and is cheap; for many applications, however, nonferrous metals are better and would be preferred except that they cost more than iron. Jeffries (6) gives present pig-iron production as fourteen times that of all nonferrous metals combined but prophesies that the proportion of the latter will slowly increase; he states that “in selecting metals in the future, engineering suitability will gradually be given greater weight as compared with cost.” It is possible that, in turn, nonferrous base metals will not be considered satisfactory in many applications, and in such places precious metals will be adopted in spite of much higher prices. This might conceivably go on until a shortage loomed and the prices climbed out of the economic range. It is proposed to deal here more particularly with the plati-
As construction materials the precious metals and their alloys are perhaps often hurriedly dismissed as being too expensive for consideration. The paper points out that sometimes the longer life, the improvement in the product, and the high resale value of these materials may well compensate for the higher initial expense. Emphasis is given to properties and uses of the platinum metals, but short sections are devoted to gold and silver.
FRED E. CARTER Baker & Company, Inc., Newark, N. J.
num metals as construction materials although gold and silver will be discussed a t the end of the paper.
Platinum Metals The platinum group of metals consists of platinum, palladium, iridium, rhodium, osmium, and ruthenium. They comprise two subgroups-namely, platinum-iridium-osmium and palladium-rhodium-ruthenium. There are certain chemical and physical similarities among the members of each subgroup; for example, the specific gravities of the first subgroup are all approximately equal and have values almost twice as great as any member of the second subgroup. In addition there are some analogies between corresponding members of the two groups; for example, platinum and palladium, iridium and rhodium, and osmium and ruthenium may be considered as pairs. The earliest finds of platinum were in the rivers of northern South America, and the world’s supply of metal was strictly limited until the rich discoveries in the rivers of the Ural Mountains in Russia were made. The considerable quantities then made available caused an increased interest in platinum, and much work was carried on concerning its chemical and physical properties. Its usefulness in the chemical industry and laboratory was soon recognized, and in addition Russia used it for coinage purposes for many years. Russia produced almost all the platinum although a steady but comparatively small supply continued to come
V O L 27, 40, 7
INDUSTRIAL A S D E N G I S E E R I S G CHEhIISTRY
732
from Colombia. When the World iI7ar cut off Russian supplies, Colombia introduced mechanical dredges, etc., and was able to increase her production notably. Then valuable quantities of the platinum metals were discovered in the nickel and copper ores a t Sudbury, Canada, where the precious metala are recovered as a by-product in the slimes during the electrolytic refining of the base metals, and the quantity of platinum or palladium has so rapidly increased that today it exceeds even the amounts obtained from Russia. Still more recently platilium was discovered in South Africa. There it is mined like gold, and, although the costs of extraction are relatively high, the South African fields constitute a large potential source of platinum metals. There are also large quantities of reclaimed metal available. Most of the platinum used in jewelry and for laboratory ware comes into circulation again, and a good proportion of the platinum used in the chemical industry is reclaimed. Probably considerable amounts of the platinum inetals used in dental work are carried to the grave. It is impossible to give exact figures, but probably there is nearly as much reclaimed platinum as there is new metal in circulation today. The percentages of the platinum metals used in various industries in 1933 were, according to the U. S. Bureau of Mines, as follom : Jenelry Dental Chemical
46 25 14
Electrical Miscellaneous
9 6
TABLE I. BRISELLHARDNESS OF PLATIXEM ASD PALLADXCJI ALLOYS
Compn.
Platinum S34 55 Iridium Osmium 50
Palladium Rhodium Ruthenium
$24 50 0 2 50 39 50
The specific grarities are as follow: Platinum Iridium Osmium
21 4 22 4 22 5
Palladium Rhodium Ruthenium
12 0 12 4 12 I
Therefore the approximate prices per cc. are : Platinum Iridium Osmium
S23 40 36
Palladium Rhodium Ruthenium
S 9 21 15
Gold, a t about $22 per cc., stands high in the price list cornpared with the platinum metals. The melting points, in C. (" F.), of the iix metals are: Platinum Iridium Osmium
1773(3223) 2350(4262)
2700(4892)
Palladium Rhodium Ruthenium
1554(2829) 1966(3571) 2450(4442)
For other properties of the metals and some of their alloys, reference may be made to the National Metals Handbook (9A). Platinum is, of course, the most important member; it is a soft ductile metal which does not tarnish a t any temperature. For commercial use it generally has to be hardened by alloying other metals with it. Brinell hardness figures showing the hardening effect of additions of other metals to platinum and palladium are given in Table I. The figures are baby Brinell numberi on the fully annealed alloys, but it should be noted that several of the alloys in the two tables are age-hardenable. Composition is given in weight per cent. Pure annealed
20%
15%
Platinum Ir
105
80
117 .~
0 4
Pd Rh Ru All
-70
67 10.5
73 158 148 125 135 195
102 80 110
-4g
Cu Ki
175
17.5
65
138
73 "i l
7.5 80
170
172 146 270
142
236
Palladium Pt I1
58 68 65 72 105 55 63 70 72
OS
Rh Ru AU
6S:i 5
63 85 80 90 172 61 74 90 102
7R 152 127
70
112 100 105
1l d
12 90 122 200
66 83 109 160
Workable alloys are not obtained with this amount of added metal
TABLE11.
EFFECT O F CHEYICAL REAGENTS OX PL.arrxr;>i METALP
Corrosive Agent HB0, HCl, concd. HKOs, concd.
Platinum and palladium are the most abundant of the six, the other four generally occurring only to a small extent with the first two, from which they are extracted during the refining process. h notable exception is the naturally occurring osmiridiuin or iridosmine M hich is composed almost wholly of iridium and osmium. The present average price per ouiice of metal (Id) is a' folloTvs :
10%
5%
Aqua regia, concd. HF NaOH Nan02 Na2COs KHSOI Alkali nitrates Alkali cyanides
Condition
Au
Pt
Ir
Os
Pd
Rh
Cold Cold Hot
A A h
2.1
.I
A S .A
6
A
Hot Cold Hot Cold Hot Fused Fueed Fused Fused fused Fused
A
h
A D D A
.I D A A A D
A D D A B D B B
&
h -1
A
.I h
Ru .i .I X
2A k-1 c"D 2 ?.I X .I .I
A
D A D B A A C A B C D B A B A R B B C . i D C . 4 . A .I B A
S .
A A
6
A
.I
1 1% C
. I
i l
h
01 A indicates unaffected; B , slightly affected; C , considerably affected D, useless.
attack by acids and other chemical agents. The propert are summarized in Table I1 from a paper by Carter ( Alloys of platinum with iridium, osmium, rhodium, and ruthenium are solid solutions, and are, as would be expected. increasingly resistant to aqua regia as the amount of added metal is increased. Alloys of platinum and palladium dissolve more readily than platinum, but, although palladiuni dissolves easily in nitric acid, the platinum-palladium alloycontaining up to 25 per cent palladium by weight are practically unattacked. Useful alloys are also formed by additions of base metals, but naturally with higher proportions of the latter the alloys gradually become less resistant to acid and atmosphere attark. K e are hardly interested in the alloys of platinum metak nith lolv-fusing metals, since the melting points are low and the alloys are generally hard and brittle and unsuitable a>coilatruction materials.
Uses of Platinum >%etals Jemelry still accounts for about half the annual conqumptioii of the platinum metals, but the amounts used by thtd chemical and dental fields are becoming increasingly miportant. No further mention of the jewelry industry mill he made here. The uae of platinum in chemical equipment EQTIPMESI. need not be confined to very minute parts. It is quite feasible to make large vessels, tubing, etc., of the precious iiirtai The initial cost is naturally higher than if baqe metal a l l o y Art:
the peace of mind obtained by the assurance that the vessel will have a long and healthy life. CATALYSIS.A catalyst is supposed not to enter into the chemical reaction, and the catalytic material can therefore be rightly classed as equipment or construction material. The most important catalytic processes using platinum metals are the contact methods of producing sulfuric and nitric acids. I n the former, platinum is deposited on a carrier such as asbestos, magnesium sulfate, or silica gel; its very finely divided condition is resDonsible for the reaction :
it is difficult to obtain statistics on the life, loss of platinum, etc. The experience of Baker & Company has been very satisfactory, and several platinum-wound furnaces have been used. A few years ago, some muffle furnaces, 21 x 9 X 3.5 inches, were made and run a t 1300-1400' C. The platinum required was 21.735 ounces per furnace; each furnace used about 2500 watts and had a life of about 200 hours. Records were kept of a tubular furnace (7A), 24 X 4 inches in size, wound with 18.720 ounces of platinum ribbon 0.5 inch wide and 0.009 inch thick: it operated a t 1200' to 1300' C. (2192" to 2372' F.) for about 1000 2502 0 2 = 2s03 hours, and the platinum lost was 0.438 ounce, or about 0.0055 gram There is only a small loss of the per hour per sq. c m . s u r f a c e . material which may operate for More recently this company has 10 to 20 years; and, since a t the been making furnaces wound with platinum-rhodium w h i c h g i v e end of this time perhaps 90 per b e t t e r life a n d r e q u i r e l e s s cent of the platinum is recoverresistor and less power; one such able, the contact mass has a high tubular furnace is 24 X 3 inches salvage value. I n the nitric a c i d i n d u s t r y , i n size, u s e s 7.6 o u n c e s of ammonia a n d air are passed p 1a t i n u m - r h o d i u m wire 0.037 i n c h i n diameter, and runs a t through platinum or platinum1255' C. (2291' F.) w i t h 922 r h o d i u m g a u z e a t 700' t o watts; it shows no signs of burn1000" C. (1292' to 1832' F.). ing out after some hundreds of The heat of reaction is sufficient to keep the gauze a t this temhours. Small tubular furnaces of this perature. At this high temperatype are useful for l a b o r a t o r y ture the gauze slowlv deteriorates and loses its mechanical strength operations up to 1300' to 1400' C. RHODIUM-PLATED REFLECTOR when it must be replaced, but the -for example, the determination actual loss in material is not more than about 10 per cent. of carbon in the various new special steels. I n addition to these processes there are numerous others Figures on the life of such furnaces are not useful, since temperature and size of resistor and other factors have such wherein the platinum metals bring about an oxidizing or a reducing reaction. great influence. However, experience has been that with Precious metal alloys are practically reasonable diameters of resistors (e. g., 0.015 or 0.020 inch) RAYONINDUSTRY. indispensable for the spinnerets used in rayon manufacture. a life of some thousands of hours a t 1300' C. may be obtained. It is difficult to hazard a guess as to the higher limit of In the process the walls of the extremely fine holes are subjected to considerable abrasive and corrosive action; and in economical size of such platinum-wound furnaces for melting, clearing out the holes, which choke up eventually with hardannealing, etc.; the weight of platinum required mounts ened solution, etc., acids or heat have to be applied. Precious rapidly with the size, but against the higher initial cost can nietals withstand all treatments very satisfactorily. Many be placed the ease of control of temperature and atmosphere, spinnerets are made of platinum, but gold-platinum and goldconsiderable scrap value of the platinum, and other factors. palladium alloys are more generally used; the former of these ELECTRICAL CONTACTS.It is somewhat surprising to find alloys, frequently with 30 per cent platinum, is hard, withthat very little has been written concerning the operation of stands chemical reagents excellently, and is of fine crystalline electrical contacts since these may constitute a most important member of a large piece of equipment. This fact may, to structure so that extremely accurate-sized holes can be made in it. Rayon manufacturers can probably use the precious a considerable extent, be due to the difficulty of ascertaining metals to advantage for various other applications in their beforehand how a particular contact material will operate processes where exceeding purity of product and accuracy of under special electrical conditions. It is well known that dimensions are essential. slight changes in the characteristics (for example, induction FURNACE RESISTORS. The resiitance furnace is a useful effect) of the electrical circuit will cause very large changes in tool in the laboratory or plant because of the ease of temperathe contacts. We are therefore forced to generalities on the ture control, uniformity of temperature, and controllability oDeration of precious metal contacts: of atmosphere. Nichrome is &excellent resistor on account (1) Direct current has a much more severe effect than alterof its high electrical resistance and IOW temperature coefficient nating current of similar voltage and amperage. of resistance. Its use, however, is limited to about 1000" or (2) For low amperages, platinum metals are best. 1100" C. (1832' or 2012'F.) as anupper limit. Thistempera(3) For light pressures, platinum metals are best since no coatings of insulating compounds (oxides, sulfides, etc.) are ture is sufficient for most annealing and other operations, formed as is usually the case with silver or base metal contacts. hut there is considerable demand for furnaces which can be (4) Fine silver contacts are best for currents of high amperrun a t somewhat higher temperatures-for example, up to age. 1300' or 1400' C. (2372' or 2552' F.) or even higher. When As far as the determination of the relative life of the various such temperatures are required, platinum and platinumcontact materials with certain definite electrical circuits is rhodium alloys have proved satisfactory. The resistor has a concerned, a satisfactory method has been developed by low rate of deterioration so that the furnace has a long life. Stauss in the writerJs laboratory. Since the loss in weight is very slight and since the resale It has been established experimentally that the discharge value of platinum is high, the actual cost is small. between contacts that occurs upon interruption of electrical There are many platinum-wound furnaces in use today, but
+
v
INDUSTRIAL AND ENGINEERISG CHEMISTRY
754
circuits may be either spark or arc. The arc-like discharge has been found to be much more severe upon contacts than the spark. It is to be expected therefore that the transition from the spark to the arc is important in the life of the contacts. Below the transition point, losses will be small, above it much larger. I n order to determine this transition point between spark and arc, a noninductive, 220-volt, d. c. circuit was used, and contacts 0.150 inch in diameter were tested. The transition current, or minimum arcing current as it is called, was determined by setting 2 the current a t a fixed v a l u e a n d I opening the contact slightly. If a 2 i continuous arc did not pass, the current was increased and the operation repeated. h large number of minimum arcing currents have been determined, a n d t h e s e values should prove useHARDENSG EFFECTON PALLADIUM OF 10 _PERCENT BY WEIGHT ADDITIONS ful in the selection OF OTHER METALS of t h e c o n t a c t material, a t least for use with small currents. The following table gives the average minimum arcing current in amperes (d. c.) for some of the materials listed: 0
d
Pt-Os-Ir
Pt
80 Pt-20 I r 90 Pt-10 Ru
1.5-1.8 1.25 1.3 1 1 ~
Pd 80Pd-20Ag 60Pd-40Cu 60 Pd-40 Ag
1.1 1.0 0.95 0,75
Fine Ag
Sterling A g Coin Ag
Y O L 27,
KO*7
thermocouples have a high thermoelectromotive force but cannot be used satisfactorilymuch above 1000" (1832' F.), For higher temperatures platinum metals are used. The negative element is pure platinum and the positive element is practically always an alloy of platinum and rhodium. Platinum-iridium alloys haye been used to some extent but are not recommended, since, owing to the rather high rate of volatilization of iridium a t high temperatures, the electromotive force is not constant. The platinum-rhodium alloys, usually 10 or 13 per cent rhodium, are much to be preferred in this respect. ELECTROPLATING. Great improvements have been made in recent years on plating with the platinum metals, and there are probably many places where the chemical industry could make use of such protective coatings. So far, platinum, rhodium, and palladium electroplates have been used, chiefly for their good appearance, but there should be a wide field in chemical engineering for a finish that withstands acid attack so well. LABORATORY WARE. This ware does not come drictly under the heading of construction material, and it is unnecessary to remind chemists and engineers of the wide variety of apparatus made of platinum and its alloys now used in the laboratory. COMPOSITE METAL. It is perhaps somewhat surprising that the jeweler knows more than does the chemist or engineer about the availability of composite metal-that is, a cheap base metal to which has been affixed by welding or soldering a layer of precious metal. The welding or soldering is done when the base metal is in the form of a large block, and the combined material is then rolled down to any desired thickness. The ratio of the precious metal to the base metal may be as little as one per cent. Tubing is readily made from such composite sheet with the precious metal either on the inside
e.
0.5 0,45 0.45
ELECTRODES.Some important electrochemical processes use platinum or preferably platinum-iridium alloys for the anodes; details cannot be given here. Because of their immunity from attack, the necessarily Large sheets can be made quite thin with a resulting ease of handling. The value of such completely resistant anodes in the manufacture of a pure product is obvious. FUSEWIRES. Iridio-platinum wires are chiefly used as the fuse mire in detonating caps. Base metal alloys, usually copper-nickel, have been and are being used; but vith such materials there is always some danger of corrosion taking place, particularly with caps which have been stocked for a long time. The possible hazard of nonfiring is obvious. Iridio-platinum fuse wires, which are not subject to this objection, generally have a diameter of 0.001 to 0.002 inch. Very fine platinum wires are also used to protect delicate instruments since they are available to fuse even a t such small currents as 10 milliamperes, TEMPERATURE IKDICATORS. Chemical processes have usually to be carefully controlled as to temperature, and the platinum metals are much used in the indicating devices. I n resistance thermometers, temperature is obtained by measuring the resistance changes of the wire with the temperature. Pure platinum wire is generally used because of it? stability and of its high temperature coefficient of resistance. The material must be of the highest purity, in which condition the coefficient is about 0.0039 per " 6. Such thermometers are exceedingly accurate up to about 900" C. (1652" Fa)*Above this temperature thermocouples must be used. Base-metal
I" PRICES OF PL4TlXUM MET44LS PER CUBIC CESTIIYETER
or outside surface in whatever position desired. If necessary, it is just as easy to protect both surfaces in this r a y . The precious metal may be platinurn, gold, os Silver, or t,heir alloys. Any proportion of precious metal to base metal can be u5ed, even as small as 1 to 100, but something like 1 to 10 is perhaps more usual. It is common to give the ratio as the volume of the precious metal to the total volume-e. g., one-tenth ratio
JULY, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
platinum on copper means that the thickness of the platinum layer is one-tenth of the thickness of the copper plus platinum. In order to determine the initial ratio of precious to base metal thickness, it is well t o consider what thickness the precious metal will be when the composite metal is reduced mechanically by rolling or drawing to the desired final size; this final thickness of precious metal should not, in general, be less than 0.001 inch. Thinner metal can be made, but there is danger of its not being completely continuous, so that the desired protection is not obtained. Lapped joints are generally used when two pieces of sheet or tubing have to be connected.
Gold Although most emphasis is given in this paper to the platinum metals, it is necessary to add something about gold and silver in order to justify the title. Actually, quite sizable amounts of these metals (particularly of silver) are used in industry on account of their special physical or chemical properties. Gold has a fairly high melting point (1063’ C. or 1945’ F.) and does not oxidize at any temperature. It is completely resistant to the mineral acids but dissolves readily in aqua regia either hot or cold. The alkali cyanides, in aqueous solutions or particularly when fused, attack it quickly. For certain chemical processes the complete apparatus is made of fine gold. The use of the pure metal may sometimes be necessary, but it has the disadvantage of being quite soft and easily distorted. In order to increase its resistance to distortion, it is necessary to alloy the gold with other materials. The addition of platinum metals to the gold rapidly increases the hardness and resistance to chemical attack, Copper, nickel, etc., may be used as the added metal, provided the percentage is not too high. If certain amounts of the base metal are exceeded, the resulting alloys are attacked-for example, by nitric acid. Tammann (8), who has done so much work on the reaction limits of chemicals on alloys, has studied gold alloys in detail. He has shown that, when one component of an alloy is unattacked by chemical reagents, this nobler component has a protective influence on the less noble component up to certain limits; these reaction limits may be quite sharp, and Tammann found that they occur, for different reagents on gold alloys, a t definite points where the atomic percentage of gold is 12.5, 25, 50, etc. It is recommended that those who are interested in the use of gold alloys study these researches. His work is mentioned here t o show that, as a resistant construction material, it is not always necessary to use pure gold. It is quite feasible to remove the base metal from the surface layers of low-carat gold alloys by suitable acids, etc. (called “stripping” by jewelers) so that these surface layers become richer in gold. By subsequent mechanical work this gold layer is made continuous and acts as a coating which will protect the alloy layers underneath. Gold plating of base metal or alloys is useful as a protection against atmospheric corrosion but is not completely protective against attack by acids.
Silver Silver is used to a large extent as a construction material. Several papers have recently been published on the subject (1, 2, 4, 6, 7), and an investigation on the whole subject is a t present being carried on a t the National Bureau of Standards. The writer is merely suggesting that thought be given to the use of precious metals by operators of chemical processes who are faced with corrosion problems, These metals should not be dismissed summarily as being too expensive because quite often the longer, more satisfactory life obtained and the resale value of the materials may well render their use economical.
755
Silver melts a t 961O C. (1762 O F.). It is dissolved by nitric and hot sulfuric acids. It has the highest thermal conductivity of any metal and in addition has a low specific heat. It is very soft and ductile but is rapidly hardened by the addition of other metals. The best hardener is copper; wellknown examples of such alloys are coin silver (90 silver-10 copper) and sterling silver (92.5 silver-7.5 copper). __ Silver is useful for resisting chlorine. Actually a thin layer of silv e r chloride i s promptly formed and this protects t h e m e t a l from further action. Considerable amounts are used a s s i l v e r tubing, etc., in conveying chlorine in chlorination plants for water purification. Large amounts of silver would be used by a recently proposed plan of water sterilization where the w a t e r flows between elecHARDENING EFFECTON PLATINUM OF 10 trodes of silver bePER CENT BY WEIGHT ADDITIONSOF tween which a OTHER MET.4LS small c u r r e n t is passing. Silver-lined vessels of quite large dimensions are used in the food industry. Silver is not corroded by the acids in food and is a safe material for evaporating pans, etc., in this important industry. Strong alkali solutions do not attack silver, and even the fused caustic alkalies have practically no effect on silver vessels. The metal also withstands acetic acid perfectly. Very strong solders are usually silver solders in which the silver ranges from 5 to 90 per cent. They are called “hard” solders in contrast to the so-called “soft” solders which are lead alloys and which melt a t much lower temperatures. They need a red heat for working, which is sometimes a disadvantage. When it is desired to solder a t a somewhat lower temperature, silver alloys containing phosphorus are used as the soldering material.
Literature Cited (1) Anonymous, Can. Chem. Met., 18, 65 (1934). (1A) Anonymous, Metal & ;Mineral Markets, 6, No. 10, 4; No. 13, 6 (1935). (2) Anonymous, Natl. Bur. Standards, Tech. News Bull. 210, 212 (1934). (3) Carter, F. E., Chem. & Mel. Eng., 36, 554 (1929). (3A) Zbid., “Properties of Platinum and Platinum Metals,” p. 1363, Am. Soo. Steel Treating, 1933. (4) Forstner, H. M., Oberjlachentechnik, 12, 91 (1935). (5) Jeffries, Z., Metal Progress, 27, 19 (1935). (6) Rogers, B. A., Chem. & Met. Eng., 41, 631 (1934). (7) Schoonover, I. C., Ibkl., 41, 545 (1934). (7A) Swanger, W. H., Am. Soc. Mech. Engrs. and Am. SOC.Testing Materiala Symposium on Effect of Temp. on Properties of Metals (preprint), 1931, 630. (8) Tammann, G . , “Textbook of Metallography,” New York, Chemical Catalog Co., 1925. RECEIVEII May 2, 1935. Presented before the meeting of the American Institute of Chemical Engineers, Wilmington, Del., May 13 t o 15, 1933.
