METALLURGY. 



431 



the process of alkalies or organic acids. A simple 

 means given by Mr. Pelatau of testing quantita- 

 tively for gold and silver in the solutions with po- 

 tassium cyanide as a solvent at each stage of treat- 

 ment, consists of heating 500 cubic centimetres of 

 solution nearly to boiling point, adding 2 grammes 

 each of sulphate of copper and sodium sulphate 

 and of hydrochloric acid, so as to have strong acid- 

 ity ; filtrating the black precipitate in which all the 

 gold and silver are retained ; and scarifying the pre- 

 cipitate with 50 grammes of litharge. The precious 

 metals are recovered from the lead by cupellation. 



To refine the precious metal at the United States 

 mint from small percentages of base metals they 

 may contain, the metal is melted with the addition, 

 while it is fluid, of saltpeter as an oxidizing flux. 

 The saltpeter decomposes and liberates oxygen gas; 

 the oxygen seizes the base metals, forming oxides ; 

 these rise to the surface and are dissolved in the 

 flux ; the flux, when sufficiently thick, is skimmed 

 otf, and the purified metal, consisting only of gold 

 and silver, is poured into cold water to form granu- 

 lations. The silver is removed by boiling in nitric 

 acid, when it dissolves, leaving the gold in a finely 

 divided state. 



Medals of silver with polished surface soon tar- 

 nish, and several processes have been applied for 

 giving them a frosted texture. In England the 

 flat surfaces are left smooth and mirrorlike, while 

 the parts in relief are frosted. In France un- 

 polished dies are used, and care is taken to impart 

 a dead or frosted surface to the medals struck by 

 rubbing them with fine pumice. More recently a 

 sand-blast process has been employed at the French 

 mint. Sometimes the surface is darkened by ex- 

 posing -it to a solution of chloride of platinum, 

 after which rubbing with a brush and very fine 

 pumice changes the color to a delicate gray, and, 

 with skill, graduated shadows may be left wherever 

 they are desired. A medal with a frosted platinized 

 surface has, according to Prof. Roberts-Austin, a 

 great advantage over one with a polished table, as 

 the platinized one is merely dampened in tone by 

 exposure to the atmosphere, and does not become 

 disfigured by blotches of tarnish. The frosted 

 platinized medal may be restored to almost its 

 original freshness by careful rubbing with a soft 

 leather, while the tarnish on a silver medal is often 

 too deep to be rubbed off. 



Aluminum. In a review of the present state of 

 the aluminum industry, published on the authority 

 of Prof. Roberts-Austin, of the British mint, the 

 current selling price of the metal is quoted as 33.2 

 cents per pound. The cost of production is estimated 

 to be 27.2 cents per pound, of which only 2.2 cents 

 is for electric energy consumed in the production 

 of aluminum, while the cost of the raw material is 

 placed at 12 cents. No further reduction need be 

 expected in the cost for power, which appears to be 

 extremely low, but there is a possibility of effect- 

 ing a reduction in the price of the raw material, 

 in which direction, as in that of new methods of 

 manufacture, further improvement must be sought. 

 It is proposed by M. Becker, formerly manager for 

 the French Aluminum Company at St. Michel, to 

 substitute calcined bauxite for the refined alumina 

 formerly used in the manufacture, whereby the 

 cost of aluminum might be reduced to nearly 10 

 cents a pound. The objections are offered to this 

 substitution that aluminum thus made would con- 

 tain silicon and iron as impurities, for the removal 

 of which some cheap process of refining would have 

 to be applied. 



According to a paper read by Mr. E. Ristori at 

 the summer meeting of the British Institute of 

 Mechanical Engineers, great improvements have 

 been made in the process of manufacture used by 



the British Aluminum Company. The finished 

 product is a very finely divided powder of alumina, 

 and is packed for shipment in hermetically sealed 

 drums. The oxide is reduced by the Herault pro- 

 cess and the metal is run out of the electrolytic 

 baths into ingot molds. In this form the alumi- 

 num is pure enough for certain purposes, and much 

 of it is therefore sold without further treatment. 

 For uses where a pure metal is required, the crude 

 ingots are refined to a purity of 99.6 per cent. 

 With certain limitations imposed by the chemical 

 and physical properties of the material, aluminum 

 can be worked much like the other industrial metals 

 handled at the present day. It can be forged hot 

 or cold ; in comparison with other metals it ranks 

 third in malleability and sixth in ductility. Sheets 

 have been hammered as thin as one forty-thousandth 

 of an inch. In turning, the edge of the tool soon 

 becomes blunt, and the cutting speed should be high. 

 In its purest form aluminum is very soft, and is not 

 of great service in those arts in which much rigidity 

 and strength are required. One casting alloy hav- 

 ing a specific gravity of 2.9 is largely used, but its 

 composition is still kept secret. It produces re- 

 markably clean castings, which require very little 

 machining to finish up, and takes a high polish. 

 The two alloys particularly recommended as among 

 the best yet made are ternary alloys, and contain 

 tungsten as the leading ingredient after aluminum. 

 One of them has besides a small proportion of cop- 

 per, and the other nickel, and both display high 

 qualities of strength and elongation. Aluminum 

 bronzes have superior strength, and are especially 

 suitable for marine engineering. 



A method of blackening aluminum, represented 

 to be satisfactory, consists of polishing the surface 

 to be blackened with fine emery paper, spreading 

 on it a thin film of olive oil. and heating slowly on 

 a spirit flame, when the oil boils and takes a golden 

 color. Then another coat of oil is laid on. and on 

 heating strongly the golden color becomes brown, 

 and soon darkens to an intense black. 



In the Wegner process for plating aluminum 

 goods, the article to be plated is first pickled in 

 a bath composed of acetate of copper dissolved in 

 vinegar, iron oxide, sulphur, and chloride of alumi- 

 num. After having been taken out of the bath, it 

 is brushed with a soft wire brush. A metallic coat 

 then develops on the surface of the aluminum, 

 closing its pores and rendering it smooth. After 

 rinsing it in clean water the article is put into 

 the galvanic bath and operated on in the usual 

 manner. The current is kept closed until on the 

 aluminum or its alloys a metallic coat of gold, 

 nickel, copper, or other metal of the desired thick- 

 ness has been formed. 



A successful method of plating aluminum with 

 copper by a welding process is claimed to have 

 been invented by Ilerr Wachnitz, a German engi- 

 neer. It is said that the plated sheets can easily be 

 soldered, grooved, tinned, and nickeled, and that the 

 plating may be of any thickness desired, there being 

 no separation even with the thinnest sheets when 

 rolled or drawn. 



A remarkable property of aluminum is described 

 by Hans Goldschmidt and Leon Frank, who find 

 that if a mixture of metallic aluminum and the 

 oxide of another metal be heated at one point to a 

 high temperature the oxygen leaves the other metal 

 and oxidizes the aluminum, generating an extremely 

 high temperature, which continues the operation 

 till the reaction is complete, and the oxide is re- 

 duced to a metal which is free from aluminum. 

 Instead of oxides, sulphides may be used, but the 

 heat developed is less than with oxides. The pro- 

 cess may be used for generating heat for the pro- 

 duction of metallic alloys for brazing, welding, 



