§ 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 :

Colour.

Gold. Silver. Copper. Iron. Platinum. Cadmium.

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

above table are malleable and ductile. It should be borne in mind that cadmium is a volatile metal like zinc, and should therefore be added after the other metals are melted under a layer of charcoal. Much depends upon the rapidity with which it is added, and the perfect incorporation of this metal in the alloy will also depend on the vigour with which the metals are quickly stirred, preferably with a charcoal stick. In any case, some of the cadmium volatilises, and a little more than the stated amount should be added, to compensate for this loss. The term coloured golds in the above heading does not refer to the chemical process of colouring, which will be referred to hereafter.

§ 140. Standard Gold.--In most countries there are gold alloys of a certain degree of fineness, fixed by law, and used as the national standard of value for coinage. They nearly all consist of gold and copper, the latter metal being necessary to enable the gold to resist the wear to which it is subjected in commerce. Notwithstanding the hardening effect of the base metal, coins wear considerably when frequently used, causing them to become light in weight. An English sovereign weighs 123-27,447 grains, and remains a legal tender till it is reduced below 122.5 grains, the difference between these two weights being the remedy allowed by English law for abrasion or loss by wear.

Besides the standard fineness for coins, there is also a legal weight, fixed according to the regulations of the Royal Mint. 1 lb. troy of standard gold is worth £46:14:11, and if a person were to take 1 lb. troy of standard gold to the Mint to be coined, he would receive 46 sovereigns, 1 halfsovereign, and nearly 5 shillings in return without being charged anything for the coinage of the gold. Hence, standard gold contains nominally and intrinsically its full value of gold. The coinage is conducted with great exactness by the officers of the Mint with respect to weight, and the extreme accuracy with which they are compelled to work is seen by the following table :

:

The remedy, of and of a grain respectively, is the dif18 ference allowed to the Deputy-Master of the Mint, between the standard and real weight of the manufactured coins ; and he invariably confines himself well within these limits.

The British standard for gold coins is 22 carat or 916-666 parts of gold and 83.333 parts of copper per thousand, the remedy being 002 or of a grain. The old guinea is of the same standard as the sovereign, but contains a little silver and less copper.

The standards of different countries are given in the following table :

Previous to the reign of Charles II all the coin of this realm was made by hand-by forging pieces of gold to the proper thickness required for the coins, then cutting them into squares a little larger than required for the different sizes. The corners were afterwards removed from the squares, and the pieces rounded to the proper size, when they were adjusted to the desired weight for circulation. These round blanks were then placed consecutively between steel dies, containing the pattern of the intended coin; the upper die was then struck with a hammer to produce the impression.

The different alloys used by jewellers in this country have been already discussed. In France 18, 20, and 22 carat gold are assay-marked. In Germany 18, 14, 8, and a 6-carat alloy, termed joujou gold, used for electro- gilding, are recognised. Austria has three legal standards: No I 326, No. II 545, No. III 767 degrees of fineness per thousand.

§ 141. Since the introduction of the decimal system, the method of expressing the fineness of gold alloys in thousandths has been gradually gaining ground. Its simplicity, over the old system of carats and grains, is its great recommendation. The carat consists of 4 carat-grains. The following table shows the equivalents of carat-grains and carats in thousandths :

PREPARATION OF GOLD ALLOYS

$142. The gold and silver, as purchased from the refiner, is sufficiently pure, in most cases, for the purposes of the manufacturer. Various kinds of copper are used, but in all cases it is required to be of excellent quality. The impurities most likely to be present are: iron, arsenic, antimony, and sulphur, the three latter being specially hurtful. Persons using copper for alloying should specify that these elements be absent, or only present in very minute quantities. Arsenic and sulphur are much more commonly present than antimony. Dr. A. S. Taylor states that he found arsenic in not fewer than forty samples of copper, employed in the form of wire, sheet, gauze, etc. He further states that he found arsenic in two out of five specimens of electrotype copper. He does not appear to have examined any specimens of best-selected copper. It should be stated that the better qualities of commercial copper are now superior in purity to those manufactured forty years ago.

Gold and its alloys should be melted in good plumbago crucibles, which, if carefully annealed in an inverted position before use, will last a great number of times. The crucible should be covered with a lid. A new pot should be rubbed inside with charcoal powder, to prevent any particles of metal adhering to it. Some jewellers lay stress on the manner in which the metals are placed in the crucible, and it is a good plan to have the gold on the top, as it does not oxidise, and to some extent protects the copper, especially when covered with a layer of charcoal. In making alloys of which zinc is a constituent, this metal must be added after the

1 Research on Arsenic, Antimony, etc., p. 28.