DEPARTMENT OF COMMERCE BUREAU OF STANDARDS George X. Burgess, Director TECHNOLOGIC PAPERS OF THE BUREAU OF STANDARDS, No. 348 [Part of Vol. 21] TARNISH RESISTING SILVER ALLOYS BY LOUIS JORDAN, Chemist Bureau of Standards L. H. GRENELL, Junior Metallurgist Bureau of Mines H. K. HERSCHMAN, Assistant Metallurgist Bureau of Standards REPORT OF A JOINT INVESTIGATION CONDUCTED BY THE BUREAU OF MINES AND THE BUREAU OF STANDARDS AT THE DIRECTION OF THE UNITED STATES SENATE COMMISSION ON GOLD AND SILVER INQUIRY JULY 23, 1927 PRICE 15 CENTS $1.25 Per volume on Subscription Sold only by the Superintendent of Documents, Government Printing Office Washington, D. C. UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON 1S27 T348 TARNISH RESISTING SILVER ALLOYS By Louis Jordan, L. H. Grenell, and H. K. Herschman ABSTRACT As one phase of the technical research on the uses of silver, undertaken at the direction of the United States Senate Commission on Gold and Silver In- quiry, the Bureau of Mines and the Bureau of Standards have determined some of the physical and mechanical properties of a wide variety of silver alloys, and have made a study of their resistance to tarnish in accelerated laboratory tests. In this investigation were prepared and tested alloys of silver with aluminum, cadmium, copper, magnesium, zinc, silicon, tin, bismuth, antimony, tellurium, manganese, nickel, molybdenum, titanium, chromium, germanium, beryllium, gold, and sodium, chiefly in binary alloys and to a lesser extent in ternary and quaternary alloys. For most of the alloys prepared determinations were made of the freezing point, density, tensile strength, and elongation of rolled and annealed sheets, and of the hardness of both the cast and the rolled and annealed alloys. Their comparative resistance to tarnishing was studied by exposure to a mixture of hydrogen sulphide and sulphur dioxide and by dipping in a sodium polysulphide solution. Alloys of silver with cadmium or zinc were the most tarnish resistant of the workable binary alloys, but were also of the lowest tensile strength. Antimony and tin, either singly or together, were rather effective in strengthen- ing zinc-silver alloys. These alloys also were readily workable and of marked tarnish resistance. Alloys of silver with 15 or 20 per cent of zinc or cadmium and 20 per cent of gold were very easily worked, possessed strength and hard- ness values more nearly comparable to copper-silver of sterling fineness than any of the other alloys prepared, and at the same time were very tarnish resistant. None of the alloys made and tested in this investigation can be described as " nontarnishing." All could be tarnished under sufficiently severe conditions. Many of them, however, were distinctly more resistant to laboratory sulphur- tarnish tests than ordinary sterling silver. CONTENTS Page I. Introduction 461 II. Qualities desired in a tarnish-resisting silver 461 III. Previous work on tarnish-resisting silver alloys 463 1. Development of alloys 463 2. Methods of testing tarnish resistance . 464 459 460 Technologic Papers of the Bureau of Standards [vol. 21 Page IV. Preparation of alloys 465 1. General 465 2. Melting procedure 465 3. Rolling of ingots and the preparation of test specimens 466 V. Testing of alloys 467 1. Mechanical and physical properties 467 2. Tarnish resistance 468 (a) Gas tarnish test 468 (&) Sodium polysulphide tarnish test 470 VI. Binary silver alloys 470 1. Preparation and physical properties 470 (a) Pure silver and standard sterling silver 472 (&) Aluminum silver 472 (c) Cadmium silver 473 (d) Magnesium silver 474 (e) Zinc silver 475 (f) Silicon silver 475 (g) Tin silver 476 (7i) Bismuth silver 476 (i) Antimony silver 476 (;') Tellurium silver 477 (k) Manganese silver 477 (I) Nickel silver 477 (m) Molybdenum silver and tantalum silver 477 (n) Titanium silver 477 (0) Chromium silver 478 (p) Germanium silver 478 (g) Beryllium silver 478 2. Tarnish resistance 478 3. Summary of properties of binary alloys 482 VII. Ternary silver alloys 483 1. Preparation 483 (a) Chromium alloys 484 (0) Nickel alloys 485 (c) Molybdenum alloys 485 (d) Tantalum alloys 485 (e) Ternary and quaternary alloys with a zinc-silver base 485 (f) Recently suggested tarnish-resistant silver com- positions 486 (g) Commercial tarnish resistant silvers 487 2. Mechanical properties 487 3. Tarnish resistance 487 4. Summary of properties of ternary alloys 489 VIII. Alloys containing gold 490 1. Preparation and physical properties 490 2. Tarnish resistance 492 IX. Electroplated silver alloys 493 X. Miscellaneous tarnish tests 493 XI. Summary and conclusions 494 eneW ' Tarnish Resisting Silver Alloys £T%?ctan ] 461 I. INTRODUCTION Early in 1923 there was created by a resolution of the United States Senate the Senate Commission of Gold and Silver Inquiry. Among the duties of this commission was the investigation of the causes of the continuing decrease in the production of gold and silver, the causes of the depressed condition of the gold and silver industry, and the production, sale, and uses of gold and silver. Silver pro- ducers at that time were apprehensive of the future of the silver market, particularly at such time as the purchase of silver by the United States Treasury under the Pittman Act should cease. Following the formation of the Senate commission, and at the direction of the Secretary of the Interior, the Bureau of Mines made a preliminary survey to determine in what way, if any, it appeared possible to increase the uses of silver. A report of this survey was made to the Senate commission. The commission then expressed the opinion that technical research on silver promised to be of great value and that such work by the Government was fully justified. Several lines of investigation were initiated, and the first report issued dealt with an investigation by the Bureau of Mines of the use of silver in detonators. 1 The preliminary survey by the Bureau of Mines had indicated that one of the promising possibilities of any attempts to increase the use of silver would be the development of a silver alloy which should be nontarnishing, or at least less tarnishing than the usual sterling silver (copper-silver). Some work had already been carried out at the Bureau of Stand- ards on methods of testing silver alloys for their resistance to tarnish- ing, and a few tests had been made in cooperation with sterling silver manufacturers on special tarnish -resistant alloys. The pro- posed investigation looking toward the possible development of new tarnish-resisting silver alloys was, therefore, undertaken as a coop- erative investigation by the Bureau of Mines and the Bureau of Standards. The present paper is the report on that joint research. II. QUALITIES DESIRED IN A TARNISH-RESISTING SILVER The opinion of the silver trade as to the possibilities of the com- mercial development of a tarnish-resisting silver is divided. Some manufacturers believe that the beauty of decorated silverware would be very largely lost if a silver alloy were used which was truly non- tarnishing and did not darken by tarnish in the recessed portions of 1 " C. A. Taylor and W. H. Rinkenbach, Silver azide : An initiator of detonation," Army Ordnance, 5, pp. 824-825 ; 1925. 462 Technologic Papers of the Bureau of Standards [vol. 21 the design and thus give contrast with the high lights of the pattern. The opinion of the majority, however, seems to be that a really non- tarnishing alloy of sterling quality would be a commercial possibility if it also possessed the same color as copper sterling and an equal degree of hardness and workability. They do not regard with favor an alloy which is only tarnish resisting ; that is, one which tarnishes less rapidly than sterling but does ultimately blacken, since it is feared that the purchasers of such a silver, even though it were called "tarnish resisting" as distinguished from " nontarnishing," would expect to secure a nontarnishing material and dissatisfaction would result when the alloy finally did tarnish. Many silversmiths see but little promise, except for special applications, in an alloy which is not of sterling fineness since, due to effective advertising, the public has a very high regard for the sterling mark.2 Another consideration very important to silver manufacturers is the behavior of any alloy on remelting. Copper sterling silver scrap melts down cleanly, with very little change in composition. Any new silver alloy more liable to oxidation of one of its con- stituents on remelting would increase the difficulty of control of the fineness of the alloy and increase the costs of production. Such considerations as the control of fineness of a silver alloy in large-scale preparation or in the remelting of scrap, the working qualities of the alloy in rolling, spinning, and decorating, and the distinguishing of slight differences in color of the untarnished alloys are all factors which can be determined most satisfactorily under plant conditions and by the manufacturers or fabricators of silver- ware. The present investigation is concerned chiefly with the labora- tory preparation of silver alloys, accelerated laboratory tests of their tarnish resistance, and the determination of their mechanical properties. Some information on the workability of the alloys prepared is, of course, furnished by the data from the mechanical tests. The laboratory tarnish tests of the present investigation were limited to tarnish by sulphur in one form or another. Such a limitation is justifiable in the case of silver alloys, since the tarnish which is objectionable to the users of silverware is that which blackens the metal, tarnish such as results from contact of silver with eggs or rubber, or the exposure of silver to sulphur-bearing gases from burning coal, oil, or gas. 2 The composition of silver alloys is generally expressed in the trade in terms of " fine- ness," and " sterling " silver is defined by law in the same terms.