HOISTING-WORKS.
Upon the application of gradual but intense heat, the mercury separates rapidly from the silver, which from the retort-house is taken to the assay-office. All mining companies do not do their own melting and assaying. It is only a few of the leading companies that can afford to have assay-offices of their own.
The assay-office of the Consolidated Virginia Mining Company is a large and handsomely constructed building standing a short distance south of the main hoisting works. It is divided into a number of rooms, in which are the several departments of the business. In the melting-room are six furnaces ranged in a row in which are placed the melting-pots, which are made of plumbago. These pots are capable of holding 300 pounds of silver each, but the quantity melted is generally from 220 to 230 pounds, sufficient to make two large bars or “bricks,” as they are commonly called.
After the silver is thoroughly melted it is well stirred up, and the dross which rises to the surface is skimmed off. The pots are then lifted out of the furnace, and the molten silver is poured into iron moulds which form the bars, weighing a little over 100 pounds each.
When the pots of molten silver are lifted out of the furnace, a small quantity of the liquid mass is taken from the surface in a little ladle.
The silver thus taken out is thrown into water, when it scatters, and spreads out in a thousand fantastic shapes. Some of these sprays of silver resemble butterflies, flowers, or the leaves of plants—all are very bright and beautiful. They are called “granulations” and it is from these particles of silver that the assays are made by which the value of the bar is known.
As the molten silver is poured from the pot, in moulding the second and last bar, the little ladle is dipped quite down to the bottom of the pot and a small quantity of the liquid metal is taken out and thrown into cold water, as was the first. The resulting granulations are assayed, and the two assays must agree exactly, or all is to be done over again before the bars can be stamped with their value in silver and gold.[gold.] All of the Comstock bullion contains a considerable percentage of gold. This percentage varies in different mines. Thus in the Belcher bullion it is often as high as 50 per cent., while in the Consolidated Virginia bullion it is as low as 10 per cent.
On an average there are melted, moulded into bars and assayed at the Consolidated Virginia assay-office from 500 to 600 pounds of bullion per day.
In making an assay of the granulated silver, a French gramme in weight is taken. This is wrapped up in a thin sheet of pure lead—lead which contains no silver—when it is put into a cupel, made of bone ashes, and the whole is then placed in a muffle-furnace. In the great heat of this furnace both lead and silver are soon liquified, when the lead is absorbed by the cupel, carrying with it whatever base metal there may be in the gramme of bullion. The “button” left at the end of this process of cupellation is weighed, when is ascertained the weight in fine metal—gold and silver.
The bullion is now hammered out till it forms a thin sheet, when it is placed in an annealed glass flask, called a matrass, and strong nitric acid is poured over it. The flask is then placed in a sand-bath (a sort of oven, the bottom of which is covered to the depth of an inch or more with hot sand) and the flattened button is boiled in the acid until all the silver in it is dissolved. The gold which remains in the bottom of the flask in the form of a fine powder, is collected in an unglazed porcelain crucible. The crucible is placed in a warm place until the gold has dried; when it is put into a furnace and annealed—heated until the particles unite and form what is called “matte.”