By this process gold and silver are separated from each other. These two metals equally resisting the action of fire and lead, must therefore be separated by other means. This is effected by different menstrua. Nitric acid, muriatic acid, and sulphur, which cannot attack gold, operate upon silver; and these are the principal agents employed in this process.
Parting by nitric acid is most convenient, consequently most used; indeed, it is the only one employed by goldsmiths. This is called simply parting.
That made by the muriatic acid is by cementation, and is called cemented parting; and parting by sulphur is made by fusion, and called dry parting
_Parting by Aqua-fortis._
This process cannot succeed unless we attend to some essential circumstances: 1st. The gold and silver must be in a proper proportion, viz. the silver ought to be three parts to one of gold; though a mass containing two parts of silver to one of gold may be parted. To judge of the quality of the metal to be parted, assayers make a comparison upon a touchstone, between it and certain needles composed of gold and silver, in graduated proportions, and properly marked; which are called proof needles. If this trial shows that the silver is not to the gold as three to one, the mass is improper for the operation, unless more silver be added. And 2dly, that the parting may be exact, the aqua-fortis must be very pure, especially free from any mixture of the sulphuric or muriatic acid. For if this were not attended to, a quantity of silver proportional to these two foreign acids would be separated during the solution; and this quantity of silver would remain mingled with the gold, which consequently would not be entirely purified by the operation.
The gold and silver to be parted ought previously to be granulated by melting it in a crucible, and pouring it into a vessel of water, giving the water at the same time a rapid circular motion, by quickly stirring it round with a stick. The vessels generally used in this operation are called parting glasses, which ought to be very well annealed, and chosen free from flaws; as one of the chief inconveniences attending the operation is, that the glasses are apt to crack by exposure to gold, or even when touched by the hand. Some operators secure the bottom of the glasses by a coating composed of a mixture of new-slaked lime, with beer and whites of eggs, spread on a cloth, and wrapped round the glasses at the bottom; over which they apply a composition of clay and hair. The parting glasses should be placed in vessels containing water supported by trivets, with a fire under them; because if a glass should break, the contents are caught in the vessel of water. If the heat communicated to the water be too great, it may be properly regulated by pouring cold water gradually and carefully down the side of the vessel into a parting glass 15 inches high and 10 or 12 inches wide at the bottom; placed in a copper pan 12 inches wide at bottom, 15 inches wide at top, and 10 inches high, there is usually put about 80 oz. of metal, with twice as much of aqua-fortis.
The nitric acid ought to be of 22 B., afterwards of 32 B. Little heat should be applied at first, as the liquor is apt to swell and rise over the vessel; but when the acid is nearly saturated, the heat may safely be increased. When the solution ceases, which is known by the effervescence discontinuing, the liquor is to be poured off; if any grains appear entire, more aqua-fortis must be added, till the silver is all dissolved. If the operation has been performed slowly, the remaining gold will have the form of distinct masses. The gold appears black after parting; its parts have no adhesion together, because the silver dissolved from it has left many interstices. To give them more solidity, and improve their color, they are put into a test under a muffle, and made red-hot, after which they contract and become more solid, and the gold resumes its color and lustre. It is then called grain gold. If the operation has been performed hastily, the gold will have the appearance of black mud or powder, which, after well washing, must be melted.
The silver is usually recovered by precipitating it from the aqua-fortis by means of pure copper or by precipitation by muriatic acid and reduction. If the solution be perfectly saturated, no precipitation can take place till a few drops of aqua-fortis are added to the liquor. The precipitate of silver must be well washed with boiling water, and may be fused with nitre, or tested off with lead.
_Parting by Cementation._
A cement is prepared, composed of 4 parts of bricks powdered and sifted; of 1 part of green vitriol calcined till it becomes red; and of 1 part of common salt. This is to be made into a firm paste with a little water. It is called the cement royal.
The gold to be cemented is reduced into plates as thin as money. At the bottom of the crucible or cementing pot, a stratum of cement, of the thickness of a finger, is put, which is covered with plates of gold; and so the strata are placed alternately. The whole is covered with a lid, which is luted with a mixture of clay and sand. This pot must be placed in a furnace or oven, heated gradually till it becomes red-hot, in which it must be continued during 24 hours. The heat must not melt the gold. The pot or crucible is then suffered to cool; and the gold carefully separated from the cement, and boiled at different times in a large quantity of pure water. It is then assayed upon a touch-stone, or otherwise; and if it be not sufficiently pure, it is cemented a second time. In this process the sulphuric acid of the calcined vitriol decomposes the common salt during the cementation, by uniting to its alkaline base, while the muriatic acid becomes concentrated by the heat and dissolves the silver alloyed with the gold.
This is a very troublesome process, though it succeeds when the portion of silver is so small that it would be defended from the action of aqua-fortis by the superabundant gold; but is little used, except to extract silver, or base metals, from the surface of gold, and thus giving to an alloyed metal the color and appearance of pure gold.
For separating silver from lead ores, enables us to reduce profitably ores containing but 1 oz. of silver to the ton. It depends upon the fact that an alloy of lead and silver when cooled, with occasional stirring, to near the point of solidification, crystallizes in part, and these crystals are found to contain much less lead than the original fused mass. Eight or ten cast-iron pots are arranged in line and heated. Into the centre one a charge, say 5 tons, of the original alloy is put; as the crystals form they are removed by means of a perforated ladle, and put in the pot to the right until about four-fifths have been removed; the remaining enriched lead is transferred to the pot to the left. This process is continued with the remaining pots, thus gradually enriching to the left and becoming poorer to the right. The rich alloy, termed lead riches, is then cupelled.
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