Within the vat, and working on the top of the copper plates, there is a heavy copper stirrer or muller, B, Figs. 1 and 2, caused to revolve by the shafting, C, at the rate of 45 revolutions per minute.  At Huanchaca this stirrer has been made with four projecting radial arms, D D, Figs. 1 and 2; but at Guadalupe it is composed of one single bell-shaped piece, Figs. 3 and 4, without any arms, but with slabs like arms fixed on its underside; and this latter is claimed to be the most effective.  The stirrer can be lifted or depressed in the vat at will by means of a worm and screw at the top of the driving shaft, Fig. 3.

The bevel gearing is revolved by shafting connected with pulley wheels and belting, the wheels being 3 ft. and 11/2 ft. in diameter, and 6 in. broad.  The driving engine is placed at one end of the building.  Each vat requires from 21/2 to 3 horse-power, or in other words, an expenditure of 1 horse-power per ton of ore treated.

At the bottom of the vat, and in front of it, a large wooden stop-cock is fitted, through which the liquid amalgam is drawn off at the end of the process into another shallow-bottomed and smaller vat, Figs. 1 and 2.  Directly above this last vat there is a water hose, supplied with a flexible spout, through which a strong stream of water is directed upon the amalgam as it issues from the grinding vat, in order to wash off all impurities.

The following is the mode of working usually employed.  The grinding vat or tina is first charged to about one-fifth of its depth with water and from 6 cwt. to 7 cwt. of common salt.  The amount of salt required in the process depends naturally on the character of the ore to be treated, as ascertained by actual experiment, and averages from 150 lb. to 300 lb. per ton of ore.  Into this brine a jet of steam is then directed, and the stirrer is set to work for about half an hour, until the liquid is in a thoroughly boiling condition, in which state it must be kept until the end of the process.

As soon as the liquid reaches boiling point, the stamped and roasted ore is run into the vat, and at the end of another half-hour about 1 cwt. of mercury is added, further quantities being added as required at different stages of the process.  The stirring is kept up continuously for eight to twelve hours, according to the character and richness of the ores.  At the end of this time the amalgam is run out through the stop-cock at bottom of the vat, is washed, and is put into hydraulic presses, by means of which the mercury is squeezed out, leaving behind a thick, pulpy mass, composed mainly of silver, and locally termed a “pina,” from its resembling in shape the cone of a pine tree.  These pinas are then carefully weighed and put into a subliming furnace, Figs. 5 and 6, in order to drive off the rest of the mercury, the silver being subsequently run into bars.  About four ounces of mercury are lost for every pound of silver made.