are more liable to become brittle and to break, unless carefully annealed at the proper stages. 8-carat gold may be made to withstand the acid-test by using more silver than that given in the table (p. 293), but the alloy is paler to the eye. This quality works up well if proper judgment is exercised in the manipulation. § 129. Gold, Silver, Copper, and Zinc.-Mention has already been made of the fact that zinc is sometimes employed in gold alloys used by jewellers. It is generally used in the form of brass, termed composition, which varies in the proportions of its constituents with different makers, and may be typically represented as containing 2 parts copper to 1 part zinc. The effect of zinc on gold is to harden it and make it brittle. An alloy of 11 parts gold to 1 part zinc resembles pale yellow brass in colour, and does not tarnish in air. Gee gives 17 per cent of zinc in gold as the maximum amount that can be safely worked. When silver, in ordinary jewellers' alloys, is partly replaced by composition, the alloy appears of a deeper colour, and may be made to resemble one containing a higher standard of gold, but it is more difficult to manipulate, and more liable to change colour, depending of course on the amount of composition used. (See also gold solders, p. 312.) § 130. Gold and Tin.-These metals appear to mix in all proportions forming, for the most part, brittle alloys. Guettier states that when the tin does not exceed 8 per cent, the alloys have a certain amount of ductility. The colour is yellow, pale, or white, according to the quantity of tin present. Like the alloys of gold and zinc, the union of the two metals produces contraction that is, their specific gravities are in excess of the mean of their constituents. § 131. Gold and Lead. These metals unite readily in all proportions, producing very brittle alloys, which are harder and more fusible than gold, and without any utility in the Arts. 2000 part of lead melted with standard gold, and the alloy cast into a bar, can be broken with a slight tap IX GOLD ALLOYS 297 with a hammer; the colour is also altered to orange-brown, and experiments have shown that the tenacity of the metal is reduced from 18 to 5 tons per square inch. All the alloys of gold and lead expand on alloying, and this is greatest when copper is present and the quantity of lead is small. The greatest expansion, according to Guettier, takes place when the lead is only 001 of the alloy. An alloy of 11 parts gold and 1 part lead has the colour of gold and the fragility of glass. (See also p. 71.) § 132. Gold and Bismuth. These metals alloy well together in various proportions forming bodies having the appearance of brass, when the gold is in excess. Bismuth is highly injurious to gold, making it hard and brittle. § 133. Gold and Antimony.-Antimony has a strong affinity for gold, and dissolves it rapidly. Melted gold dissolves the vapour of antimony, and when this metal is present in gold in very minute quantity it renders the gold brittle. An alloy of 9 parts gold to 1 part antimony is white, brittle, and has a granular fracture. 2000 of antimony in gold hardens it, and considerably impairs its malleability. Antimony may be largely removed from gold by heat. § 134. Gold and Arsenic.-Arsenic, like antimony, readily unites with gold, and is equally injurious when present in minute quantity. The alloys are grayish-white when much arsenic is present, hard, more fusible than gold, and very brittle. § 135. Gold and Iron.-Iron in small quantity is sometimes added to gold alloys for ornamental purposes, in order to impart a characteristic tint. Gold and iron combine in all proportions, the former increasing the fusibility of the latter. Gold in small quantity does not seem to impair the qualities of iron. Guettier states that an alloy of equal parts gold and iron is grayish-white, brittle, and slightly magnetic. The alloy containing iron is pale yellow, and becomes grayish-yellow when the iron is increased to. This is known to some jewellers as gray gold. Gee gives 18 parts gold to 6 parts iron as the proportions of blue gold. With regard to this alloy, he directs that the gold should be melted first, and then iron wire in small pieces introduced successively into the molten metal. When cast it must be hammered on the edge and annealed, in order to give a closer grain, and prevent cracking during the rolling. This process may be wisely repeated upon the surface, and the ingot again annealed. The alloy may then be safely wrought into wire or sheets. § 136. Gold and Platinum. These metals unite to form ductile and elastic alloys, but require a high temperature to effect their combination in consequence of the high melting point of platinum. This circumstance, combined with the effect the platinum possesses of making the colour of gold paler, considerably limits the application of these alloys for jewellery. Platinum, however, like gold, is not acted upon by nitric acid, or by the atmosphere. An alloy of 7 parts platinum and 3 parts gold is infusible in the strongest blastfurnace, but with a greater proportion of gold fusion takes place. 2 parts platinum and 1 part gold form a brittle. alloy. 1 part platinum and 1 part gold form a malleable alloy of a pale gold colour. Clarke states that an alloy of 9.6 parts gold and 1 part platinum has the colour of gold and the density of platinum. It is well known that in gold-platinum alloys certain portions of the constituents separate and become concentrated either in the centre or in the external portions of the solidified mass. Mr. Edward Matthey1 has investigated this subject; he cast gold containing platinum into a spherical iron mould, 3 inches in diameter, and cut the metal so obtained into halves. The shrinkage was so great that the spheres had to be cast several times in order to get them solid. Portions were then taken from different parts of the spheres and assayed. A. Composed of about 880 gold and 050 platinum. IX GOLD ALLOYS 299 In the sphere A the maximum difference between the gold percentage is a variation of 032, viz. 887 on the outside against 883.8 at the centre of the alloy; and in the platinum 047.5 on the outside against 052.5 at the centre, showing an extreme variation of 005. In the sphere B the maximum difference between the gold percentage is a variation of 041, viz. 732-4 on the outside against 694.1 at the centre of the alloy; and in the platinum 122 on the outside against 166 at the centre, giving an extreme variation of 044. These results show indisputably that the platinum in cooling liquates from the gold and becomes concentrated towards the centre of the alloy. The above experiments were made on gold-platinum alloys containing silver and copper. In order to prove whether a similar liquation takes place with alloys containing gold and platinum alone, 900 parts of fine gold were repeatedly melted with 100 parts of pure platinum, and then cast as before. The result showed that the exterior contained 900 gold and 098 platinum, against 845 gold and 146 platinum at the centre of the sphere. § 137. Gold and Palladium.-Several alloys of these metals have been formed, the combination taking place without incandescence. 1 part palladium and 1 part gold form a gray alloy, having the colour of wrought-iron, less ductile than either of the component metals, and of a coarse-grained fracture. 1 part palladium and 4 parts gold yield a white, hard, ductile alloy. 1 part palladium and 6 parts gold is almost white. Alloys of gold, silver, copper, and palladium. have been used for bearings of the arbors in good watches; the colour is brownish-red, they are as hard as iron, do not rust, and cause the minimum of friction. The following is a typical alloy for watches: gold 37.5, copper 27.1, silver 22.9, palladium 12.5. Berzelius analysed a native alloy from Porpez, containing 85-98 gold, 9.85 palladium, and 4-17 silver per cent. § 138. Gold and Aluminium. An alloy of gold, copper, and aluminium, known as Nürnberg gold, is used in the manufacture of cheap gold-ware. Its colour resembles that of gold, and it is said to remain unchanged in air. § 139. Coloured Golds.-Jewellers and goldsmiths use a variety of gold alloys for purposes of ornamentation, so as to produce a number of different shades of colour in the same article. For example, red and white are employed for flowers, green for leaves, and yellow for stems, sprays, etc. The following table gives the composition per cent of alloys most in use: Gold. Silver. Copper. Iron. Platinum. Cadmium.] 8.4 4.3 12.5 gold The necessary precautions for the preparation of gold alloys containing iron have been given (see p. 297). The alloys of gold, silver, and copper call for no special remarks here. The alloys containing cadmium given in the |