The A. of gold ores is conducted in a manner similar to that of silver. When the ore contains gold, lead, and copper only, it suffices to mix more lead with it, and heat in the cupel in the muffle furnace, when the lead and copper sink into the cupel, and the gold forms a globule on the upper surface. The proportion of lead required is regulated by the amount of copper present in the alloy.
When the gold is accompanied by silver as well as copper, iron, and lead, it is neces sary in the first place to subject the alloy to the A. process in the ordinary way, which gets rid of the copper, iron, and lead, but leaves the silver still incorporated with the gold. The weight of this residual button gives the combined weights of the silver and gold present in the alloy. The method of separating the silver from the gold is called parting, and consists essentially in acting on the alloy with hot nitric acid, which dissolves away the silver, forming the soluble nitrate of silver (AgONO,,), and leaves the gold undissolved. When the silver is present in small proportion, the gold assumes a pro tective influence, and keeps the nitric acid from acting on the silver; and to effect this separation satisfactorily, it is necessary that there should be about three parts of silver to one of gold. As that proportion does not occur naturally, or in any kind of manufac tured gold-plate, it is requisite to incorporate some silver with it. This is generally accomplished by taking the proper quantities of gold and silver, wrapping them up in a piece of lead-foil, and heating on a cupel. The lead, during its disappearance from the heating vessel, causes the most intimate amalgamation of the silver and gold, which are left on the cupel as a metallic button. The latter, on being allowed to cool, is beaten out on an anvil with a smooth hammer, and is then passed through steel rollers, which yield a ribbon of alloy about the thickness of au enameled address-card. The ribbon of metal being coiled up, is technically called a cornet, and when introduced into the flask with nitric acid, the entire solution of the silver is accomplished, whilst the gold is left as a brown-colored spongy mass, of the shape and size of the cornet. To give the metal
the appearance and compactness of ordinary gold, the very friable metallic ribbon is gently transferred from the parting glass to a crucible by inverting the former into the latter; and the liquid which runs in with the gold being poured off, the crucible and its contents are raised to a red heat in a furnace, when the gold recovers its beautiful yellow color and metallic luster, and at the same time becomes soft and flexible. The gold is now pure, and in a fit condition to be weighed, and the amount obtained indicates the proportion of pure gold in the original alloy. As the quantity of silver which is required to be present during this process, in order that the parting by nitric acid may readily take place, is three parts of silver to one of gold, it is customary to call this department of a gold A. quartation or inquartation.
During the A. of silver or of gold, it is necessary to guard against any sudden increase or decrease in temperature. Independently of the probable loss of metal through the fracture of the cupels, it is found that when the final buttons of pure metal are obtained on the red hot cupel, if great care be not taken to cool the whole very slowly, the bead of gold or silver spits, and little portions are thrown off.
The mode of assaying gold now described cannot always be followed out in the examination of jewelry and other manufactured articles, as, though only a few grains are required for the A., yet the removal of such might entail the destruction of the article, and in such circumstances the touchstone is resorted to. This stone was originally brought from Lydia in Asia Minor, and consisted of a cross-grained quartz saturated with bituminous matter, but black basalt and other stones are now employed for the same purpose. The manner of using the stone is to draw a streak upon it with the auriferous article; and from the color of the streak the richness of the gold can be very accurately determined by the practiced assayer. The subsequent action of nitric acid on the golden streak serves still further as a means of determining the purity of the metal, as the acid readily dissolves the copper and silver, and leaves the gold.