The most usual is to precipitate it by the interposition of Copper, which hath a greater affinity than Silver with the Nitrous Acid[7]. For this purpose the solution is weakened by adding twice or thrice as much very pure rain water. The cucurbit containing the solution is set on a sand-bath gently heated, and very clean plates of copper put into it. The surfaces of these plates are soon covered with little white scales, which gradually fall to the bottom of the vessel, as they come to be collected in quantities. It is even proper to strike the cucurbit gently now and then, in order to shake the scales of Silver from the copper plates, and so make room for a new crop.
The aqua fortis parts with the Silver by degrees only, as it dissolves the Copper; and therefore the liquor gradually acquires a blueish green colour as the precipitation advances. This precipitation of the Silver is to be continued as long as any remains dissolved in the aqua fortis: you may be sure that your liquor contains no more Silver, if the surface of a fresh plate of Copper laid therein remain clean and free from ash-coloured or greyish particles: or if one drop of a solution of Sea-salt let fall into it produce no white or milky cloud.
The precipitation being finished, the liquor is to be gently poured off from the precipitated Silver, which must be rinsed in several waters, and even made to boil therewith, in order to free it wholly from the dissolved Copper. The Silver thus well washed must be thoroughly dried, mixed with a fourth part of its weight of a flux compounded of equal parts of Nitre and calcined Borax, and then melted in a crucible. On this occasion care must be taken to raise the fire gently and gradually, till the Silver be brought to fusion.
With what accuracy soever the precipitated Silver be washed, in order to free it from the solution of Copper, yet the Silver will always be found alloyed with a small portion of the Copper: but then this Copper is easily destroyed by the Nitre, with which the Silver is afterwards melted; so that the latter metal remains perfectly pure after the operation.
Though the Silver be not previously cupelled, but be alloyed with other metallic substances at the time it is thus dissolved, yet the dissolving, precipitating, and fusing it with Nitre, would be sufficient to separate it accurately from them all, and refine it to a degree of purity equal to that obtained by the cupel.
The Copper that remains dissolved in the aqua fortis, after the precipitation of the Silver, may in like manner be precipitated by Iron, and, as it retains a small portion of Silver, ought not to be neglected when these operations are performed on considerable quantities.
In the two next processes we shall shew two other methods of separating Silver from aqua fortis.
PROCESS V.
To separate Silver from the Nitrous Acid by Distillation. Crystals of Silver. The Infernal Stone.
Into a large, low, glass body put the solution of Silver, from which you intend to separate the Silver by distillation. To this body fit a tubulated head provided with its stopple. Set this alembic in a sand-bath, so that the body may be almost covered with sand: apply a receiver, and distil with a moderate heat, so that the drops may succeed each other at the distance of some seconds. If the receiver grow very hot, check the fire. When red vapours begin to appear, pour into the alembic, through the hole in its head, a fresh quantity of your solution of Silver, first made very hot. Continue distilling in this manner, and repeating the addition of fresh liquor, till all your solution be put into the alembic. When you have no more fresh solution to put in, and when, the phlegm being all come over, red vapours begin again to appear, convey into the alembic half a dram or a dram of tallow, and distil to dryness; which being done, increase your fire so as to make the vessel containing the sand-bath red-hot. In the alembic you will find a calx of Silver, which must be melted in a crucible with some soap and calcined wine-lees.