CHAPTER XLVII.

ASSAYS OF THE SILVER BULLION.

The water and pulp discharged from the settlers runs through sluices to the lowest part of the building, where, some eight or ten feet below the level of the floor of the amalgamating-room, stand the agitators, four in number. These are huge tubs, having in them revolving rakes or “stirrers,” and here is caught whatever valuable matter may have passed through the settlers.

Twice in twenty-four hours, the heavy matter collected in the bottom of the agitators is cleaned out and placed in four small pans and two settlers that stand in the same room to be re-worked. Finally, the pulp leaves the agitators and, carried by a quantity of water to float it, passes out of the mill in a trough or flume through which it flows eastward to a considerable distance from the mill, when it reaches what are called the “blanket sluices,” the working of which will be described further on. In speaking of the pans and settlers, I have described but one row or set. The two rows, one on the north and the other on the south side of the large room, are exactly alike. Each row of pans has its row of settling tanks, settlers and amalgam strainers. To these strainers, in which we left the amalgam and quicksilver, a few minutes since, we now return.

While in the strainers a great quantity of the superfluous quicksilver mingled with the amalgam drains off, and flowing through pipes, is conducted to a large receiving-tank under the floor of the room. After it has thus drained till no more quicksilver will flow from it, the amalgam is removed from the ordinary strainers and is taken to the hydraulic strainer.

It is now a pasty mass of fine particles of silver, held together by quicksilver, and when pressed between the fingers gives out a peculiar squeaking sound. Although we may be unable to start a single globule of quicksilver from a lump of this amalgam by pressing it beneath our fingers, yet it is far from being as dry as it may be made by pressure. In this state it is placed in the hydraulic strainer, a heavy cylindrical cast-iron vessel, a good deal resembling a mortar—such as bombs are fired from. Over the “muzzle” of the “mortar” is fastened, by means of bolts and screws, a lid of iron through which enters an iron pipe. This pipe is then connected with a water-pipe, and water under several hundred feet of pressure is turned into the strainer. The pressure exerted upon the amalgam in this strainer amounts to 150 pounds to the square inch.

When taken out the amalgam has changed color and looks much less bright than before; one would think that but little quicksilver now remained in it, yet three-fourths of the mass is still quicksilver. Though strained and pressed as thoroughly as possible by ordinary methods, amalgam yields but one-sixth or one-seventh in silver bullion when retorted, whereas by the hydraulic strainer the yield is one-fourth.

The quicksilver pressed out by the hydraulic strainer is also conducted to the large receiving tank under the floor of the room. From this tank it is pumped up by powerful patent machinery—a pump having valves which are india-rubber balls [Toy balls of india-rubber, such as children play with may be used when those furnished with the pump are not at hand]—and goes to the distributing tanks. There are two of these tanks, one standing above each row of pans. Each distributing tank feeds eight quicksilver bowls, and each bowl supplies two pans, all by means of pipes. Thus, it will be seen, the quicksilver is in constant circulation. It passes through the pans, settlers, and strainers to the main receiving tank, from which it is pumped up into the distributing tanks, from these flows into the quicksilver bowls, thence passing into the pans again. So it goes on constantly circulating until it is worn out and lost.

The loss in quicksilver by grinding the “life” out of it in the pans is very great. In the eight mills of the Consolidated Virginia Mining Company—mostly mills of from ten to twenty stamps each—the loss in quicksilver amounts to between $60,000 and $70,000 per month. Much of this loss is occasioned by grinding quicksilver in the pans five hours, when it should only be subjected to this destructive process two and a half hours. The intention is to have quicksilver in the pans but the length of time last mentioned, but in drawing off their contents into the settlers a considerable quantity remains behind in the interstices of the dies in the bottom of the pans, and is thus subjected to the two and a half hours of grinding given the first charge of pulp, previous to the putting in of the usual dose of 300 pounds of quicksilver. Many millmen and amalgamators are experimenting for the purpose of, if possible, devising means by which this extra grinding of quicksilver may be obviated.

Through the whole length of the amalgamating-room, between the two rows of strainers, a car-track is laid upon the floor and on this runs the amalgam car, made wholly of iron, and capable of holding two tons of amalgam. When told that this car, so insignificant in size, holds two tons, we get some idea of the great weight of the amalgam. The car takes the amalgam from the hydraulic strainer and conveys it to the retort-house, standing about 30 feet from the main mill building.

The floor of the amalgamating-room is eight or ten feet above the level of that of the retort-house, and when the car, with its load, has reached the end of the car-track in the amalgamating room, it is run upon a hydraulic elevator by means of which it is quickly lowered to the level of the track running to the retorts.

The retort-house is built of brick and is 24×60 feet in size. It contains six retorts, capable of retorting five tons of amalgam per day, but the amount retorted daily is but from two to two and a half tons. The retorts are cast-iron cylinders about six feet in length and eighteen inches in diameter, placed horizontally in brickwork, each having under it a small furnace. The row of retorts closely resembles a row of little steam boilers.

In charging the retorts they are about half filled with the amalgam, which looks more like grey mud than silver or any other metal. It is very cheap-looking stuff. Although one cannot see a single globule of quicksilver in it, yet it is about three-fourths quicksilver. You can squeeze no quicksilver out.

[Illustration: 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. 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.”

It is then removed from the crucible and carefully weighed. The weight of this matter shows the gold contained in the button, and the loss in the weight—that which was dissolved out of the original button by the action of the nitric acid—represents the silver. The bars being next accurately weighed, their value is determined from the amount of gold and silver found in the sample of one gramme taken from the silver of which the bars were moulded. The calculations here required are much facilitated by the use of very comprehensive tables of values for all degrees of fineness of silver and gold—a species of logarithms. Thus, for instance, when silver is 900 fine, an ounce of such silver is worth $1,16,36, and when gold is 900 fine an ounce of it is worth $18,60½. This is seen at a glance by referring to the tables; and the same is the case no matter what the degree of fineness of the metal may be.

The scales used in assaying are wonderfully delicate and sensitive. The smaller ones will weigh a piece of hair only an inch in length, from the human head. There is a separate room in which the weighing is done and the calculations made. All in this room is as neat and clean as in the finest parlor. In another room are the muffle-furnace and sand-bath, and in still another the furnace where the assays are made, also a still for distilling water. In ore assays, 200 grains of finely powered ore are placed in a small earthen crucible; a proper quantity of flux is added, and the whole is then placed in the furnace and melted. After the mass has remained in the molten state a sufficient length of time, the crucible is taken out and allowed to cool. When cold it is broken by a blow with a hammer, and the button deposited by the ore is found at its bottom. This button is then assayed in the same way as the granulations taken from the melting-pot, and from the result the value per ton of the ore is calculated.

In the Consolidated Virginia assay-office from sixty to eighty assays of ore, tailings, and slimes are daily made. The finished bars of silver have stamped upon them their weight, fineness of gold and silver, value in gold and in silver, and the total value of the bars. They are then ready to be sent to one of the United States’ Mints to be coined, or to be shipped to Europe, China, or Japan, and sold. The total cost of the Consolidated Virginia reduction works was $350,000.

[Illustration]