As in the cupellation of silver, it is necessary that the due proportion of the metals with which the alloy is to be mixed, be employed. This is ascertained in different ways. A tolerably accurate idea of the quan tity of gold in an alloy is acquired by the use of touch needles. Of these there arc four different kinds : 1st, gold alloyed with silver ; 2d, gold and copper ; :Id, gold, with two of silver and one of copper ; 4th, gold, with two of copper and one of silver. These have different quantities of gold alloyed with them ; they are not, how ever, much employed, more particularly those contain ing copper, as the colour communicated by this metal to gold does not differ much from that of the different kinds of copper.
Another method by which the quantity of gold is estimated, is by the streak which the alloy gives on the darker sorts of basalt, or on black pottery. The streak is compared with those given by touch needles, which is, in general, made more distinct by the stone being previously wetted. Occasionally a drop of ni tric acid is put on the streak, and by the effects pro duced on it the quantity of gold is estimated. By the different colours communicated, the workman acquires, by experience, an idea of the quantity of gold present. When the proportion of gold is large, it is necessary to add a little muriatic acid to the nitric acid, otherwise the streak is not affected ; the gold seemingly protect:ng the other metals from the action of the acid. Accord ing to Vauquclin, the best acid is composed of 98 nitric acid, of specific gravity 1340, 2 of muriatic acid 1173, and 25 of water. When this acid is used, it must not remain long on the streak.
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The quantity of lead necessary increasing with the sil ver. Others recommend a smaller quantity of lead. The proportion of lead added to gold alloys is nearly the same.
After the cupellation, the product must be subject ed to the process of parting, which is nearly the same as has been described, when explaining the methods practised in the purification of gold. For this purpose, the coil of metal is put into a pear-shaped glass vessel, with two or three times its weight of slightly diluted nitric acid, and heat is applied, by which the acid acts on the silver, and leaves the gold. When the action has ceased, which is known by the cessation of the red fumes, the coil is left corroded, but unbroken ; the fluid is then poured off, another quantity of acid is boiled on it for a few minutes, and again decanted off; the vessel is then filled with water, and inverted, by which the coil falls out entire, The metal is afterwards subjected to a high temperature, by which it shrinks, and at last forms a button of resplendent gold ; after which it is accurately weighed. The silver in solution
is obtained by the immersion of a piece of copper, or by the addition of common salt, and fusion with pearl ashes.
It is of great consequence to be able, by easy methods, to separate gold from metals with which it is alloyed, when these are in large proportion, as in gold lace, and in gilded goods.
In separating gold from gold lace, the metal is fused and granulated, by pouring it into cold water. It is then treated nearly in the same way as has been explained for separating gold and silver, when the former is in small proportion.
For this purpose, a quantity of the granulated metal is mixed with one-eighth of flowers of sulphur, and kept in a state of fusion for about an hour, or till a kind of flashing appears on its surface. A quantity of the granulated metal, equal to about one-sixteenth part of the weight of that fused with the sulphur, is mixed with one-half its weight of litharge, and one-eighth of san diver. The one-half of this is then added in successive portions to the fused mixture, which is well stirred on each addition. After this is adder', the mix tire is kept in the fused state for ahem ten minutes, and the upper part of which is silver in um= witu suiphur, is removed ; the remainder is then poured into greased Cones.
The portion removed is brought to a state of fusion, and the remaining half of the mixture of alloy, litharge and sandiver, is added in the same way as above described, and the product is poured into a cone.
At the bottom of the cones a metallic matter, com posed of silver, with almost the whole of the gold, is collected. The substance above this still, however, re tains a little gold ; it is therefore again fused, and a small quantity of an alloy of equal parts of copper and lead is mixed with it, by which the whole of the gold is sepa rated, or collected at the bottom of the cone into which the mixture is poured.
The different portions of metal procured are mixed and fused with one-eighth of lead, and treated in the same way as hefore, with sulphur, the mixture of alloy, litharge, and sandiver. What is collected is then fused with one-sixteenth part of sulphur, and kept in that state for half an hour. It is then poured into the cone, the silver and sulphur collect at the top, and the gold falls to the bottom. The sulphuret of silver must again and again be fused with sulphur, till the whole of the gold is separated. The gold collected is then fused with one•sixteenth of copper and granulated ; the same quantity of sulphur is now mixed with it, and the mix ture is fused for about a quarter of an hour, and then poured into a cone. The gold found at the bottom of this, is afterwards purified by sulphur and antimony, as already described.