IX GOLD ALLOYS 291 The price of fine gold from refiners is a little higher than the mint price, by about 1s. per ounce for a single ounce, and proportionately lower for larger quantities. From this it will be seen that the cost of material to the manufacturer cannot be calculated merely on the amount of gold the alloy contains, and allowance must also be made for the quantity of alloy added, which is a consideration when much silver is used. Take 18-carat gold, for example. The price given in table is £3: 3:9, the refiner's price would be £3 : 4:3, and assuming that the six parts of alloy consists of half silver and half copper, its cost will be 9d.; so that the 18-carat alloy costs £3: 5s. per ounce before it is manufactured into articles. § 126. Gold and Copper. These metals alloy well together in all proportions, and when the copper does not exceed 10 to 12 per cent the malleability is little alteredthat is to say, 21-carat gold and upwards are practically as malleable as pure gold. Many of the alloys have a density less than the mean of that of the separate metals. The gold coinage of this country is made from 22-carat gold, or 916.6 parts gold to 83.3 parts copper, the alloying metal being copper. This is termed standard gold. Standard gold is very largely used for the manufacture of wedding-rings. The makers formerly had to pay a duty of 17s. per ounce, one-sixth being remitted for loss in finishing. Wedding-rings of this quality must be Hall-marked, and this is required to be done when the rings are in an unfinished state, so that for every 6 ounces the maker paid for 5 ounces. Mourning-rings, watch-cases, etc., are also occasionally made of the above quality. The alloy of 20 parts gold and 4 parts copper is termed brown-gold, and is sometimes used by jewellers in decorative designs. The gold-copper alloys used for coinage are generally accepted as being homogeneous, but doubts were raised upon the truth of this some years since by Mr. Booth of the United States Mint. Much evidence was afforded on this subject during the preparation of the Standard Gold Trial Plate, made in the Royal Mint in 1873. A mass of standard gold weighing 72 ounces was cast and rolled into a plate 37 inches long and 6.5 inches wide; portions were cut from different parts of it and assayed. The greatest variation between any two assays was 10000, there being no evidence of concentration of the precious metal anywhere. Professor Roberts-Austen1 has conducted the following experiments to obtain evidence on the liquation of gold alloys. An ingot consisting of 984-7 parts gold and 15.3 parts silver was melted and cast into a spherical mould. From this sphere of gold a disc of an inch thick was cut, which weighed 31 ounces. It was then rolled in two directions at right angles and portions cut off for assaying. The result of 82 assays afforded no clear evidence of systematic rearrangement, for although there appeared to be an enrichment towards the upper part to the extent of 100000, such small differences as existed in the assays made on metal taken from the same horizontal planes could not be regarded as being due to any definite redistribution of the metal. It may be taken as proved that gold of high standard does not, like silver, show any marked tendency to reject on solidification the baser metal with which it is associated. M. Peligot 2 has published the results of an investigation which point to the same conclusion. 8 § 127. Gold and Silver.-These metals unite in all proportions, but do not generally appear to form true chemical combinations. Levol regards the union of these metals in equivalent proportions as being incapable of separating by gradual cooling. The alloys of gold and silver are harder, tougher, and more fusible than gold, and more sonorous and elastic than either metal taken singly. One-twentieth of silver is sufficient to modify the colour of gold, and is employed by jewellers to impart different shades of colour to 1 Annual Report of the Deputy-Master of the Mint, 1888. IX GOLD ALLOYS 293 gold. 27 to 30 per cent silver and 73 to 70 per cent gold form a green alloy. When the amount of silver exceeds 50 per cent the alloys are nearly white. The greenish-yellow cast of the sovereigns manufactured by the Sydney Mint, Australia, is due to the fact that the alloying metal used is silver, and not copper, as in the English sovereign. The Australian gold coins, however, are of the same standard in fineness, weight, and value as the English coins. Gold-silver alloys do not oxidise on exposure to air. § 128. Gold, Silver, and Copper. These three metals. are largely used by jewellers to form alloys more tough, malleable, and ductile than by using copper alone as the alloying metal. The alloy added to gold for manufacturing the old English guineas consisted of equal parts of copper and silver, which accounts for their yellow appearance. The guinea is of the same fineness as the sovereign, but differs in weight. It weighs 5 dwts. 9 grains; and a sovereign 5 dwts. and a little over 3 grains; of which 4 dwts. 223 grains, and 4 dwts. 17 grains respectively are fine gold. Guineas have not been coined for circulation during the present reign. 1 Gee gives in the following table the proportions of silver, copper, and gold used in jewellers' alloys With regard to 18-carat gold, Gee states that, "if properly cast, it is malleable and tenacious. It is also exceedingly ductile. A hardness is imparted to this quality of gold which admirably adapts it to the manufacture of jewellery of the highest order. There is perhaps a difficulty in preparing 18-carat gold not experienced in some other alloys." The general opinion is that the occasional want of cohesion is due to the copper employed, as by using a purer variety of copper the difficulty in working is diminished. The best alloy appears to be that given in the preceding table, for by increasing the amount of silver the colour of the alloy would suffer. 15-carat gold is also largely used for articles required to be made of coloured gold, as it is technically termed. The colouring is effected by dissolving out the copper from the exterior by suitable solvents, and leaving the surface with a colour like that of pure gold. It can thus be made to look equal to fine gold. It is easy to work, and the 9 parts of alloy give to the articles the requisite strength and hardness necessary to resist wear, and to retain their shape when subjected to various uses. This alloy can be Hall-marked as a guarantee of its proper quality. 1 Gee cautions purchasers of 15-carat gold against an inferior quality of gold introduced into the trade, and called 15 carat, bearing a stamp similar to the Hall-mark; however, this is not the Hall-mark, but the private mark of the manufacturer. 121 to 13-carat gold is very extensively manufactured into all kinds of jewellery. This is the lowest quality that can be properly subjected to the colouring process, and retain a rich and uniform appearance without showing irregularities on the surface. The articles of the so-called 15-carat coloured gold, referred to above, were formerly made of this quality. 12-carat gold is known in the jewellery trade as the best of the bright golds—that is, qualities which cannot be properly coloured, and therefore show the true colours of 1 Goldsmiths' Handbook, p. 46. IX GOLD ALLOYS 295 the alloys of which they are composed, and not a surface of superior metal, as is the case in coloured gold articles, which have a much richer appearance than bright gold, and consequently are in much greater demand. The 12-carat alloy, using the proportions given in the preceding table, is malleable and ductile, and tolerably soft, so that it possesses good working qualities. It may be Hall-marked as a guarantee of its purity. 10-carat gold is similar in physical properties to the 12carat alloy, but has a different shade of colour, owing to the different proportions of the constituent metals. This quality is not Hall-marked. 9-carat gold is used for manufacturing articles of almost every description of jewellery, and when up to standard. fineness may be Hall-marked. The quality most extensively employed is somewhat below the standard, this being the extreme limit that will stand the test of nitric acid without exhibiting signs of corrosion. 1 Gee states that "9-carat gold of the mixture, given in the preceding table, p. 293, will stand more than ordinary treatment from the hands of the workman, and may be touched and removed from the annealing-pan while still red-hot, without injury to any subsequent manipulation of it; it may also be quenched at any degree of heat in pickle and water, if any advantage is likely to accrue from it; but we strongly object to the continuous quenching of gold alloys at every subsequent process of annealing-partly because, every time the metal is quenched in sulphuric acid pickle, a portion of the base metal in these low qualities is dissolved." 9-carat alloys are sometimes alloyed with zinc, or spelter, as it is generally termed in the trade, in small quantity; but it must be very sparingly used, or the alloys will be hard, brittle, and difficult to work, and, moreover, more readily acted upon by acids. 8-carat gold and qualities inferior to this are harder, and require more careful working than the higher alloys. They 1 Goldsmiths' Handbook, p. 48. |