The Mexican rastra has been improved some little in its construction and adaptation to our wants; and in many cases mule-power has been superseded by steam; but the principle remains about the same.
When the rastra is properly prepared, a "batch" of about five hundred pounds is generally emptied into one about ten feet in diameter; but the quantity is always regulated by the size of the machine. It is then ground very fine by means of the drag-stones attached to arms fixed in the perpendicular shaft, and which are generally given about eight revolutions per minute. At this rate it will require from three to four hours to grind the batch sufficiently; but this is somewhat regulated by the grit and weight of the drag-stones. About three quarters of an hour before the whole is thoroughly ground, a sufficient quantity of quicksilver is added; but the amount is regulated by the richness of the quartz in process of grinding. If, for instance, the five hundred pounds of tailings placed in the rastra is supposed to contain about three quarters of an ounce of gold, about one ounce of quicksilver is generally used--or about twenty-five per cent more of the latter than the former. Some judgment is required in this--too much quicksilver being a disadvantage, inasmuch as the amalgam should be kept hard to make it effectual in saving the gold.
Quicksilver should also be kept very free from grease, as it cannot be too clean, and should invariably be well retorted every time it is used.
About ten minutes before the grinding is finished, about sixteen buckets of water are poured into the rastra, to the quantity named, and the same motion continued, the whole appearing like muddy water. This is then bailed out, or run off quickly. Five hundred pounds more of the quartz are then added, and the process repeated, adding the same portion of quicksilver to every batch.
This is kept on for one, two, three, or even four weeks, according to the richness of the quartz, or the taste and wants of the owner. The larger the amount contained in the rastra, the more gold is there saved, in proportion, to the ton.
The amalgam is then taken out of the crevices in the bottom of the rastra, and carefully panned out, and as carefully retorted. After this, most business men melt the gold into bars or ingots, before sending it to the mint to be coined.
Before commencing to grind again, the crevices between the stones covering the floor of the rastra, about one and a half inches wide, are tightly packed and filled with clay, level with the stone.
This mill as used in Chili, and from whence its origin and name are derived, is nearly as simple in its construction as the rastra, with the walls a little higher, and more regular; and, instead of the "drag stones," a large stone wheel, attached to the horizontal shaft, is used for grinding the rock. Into this mill a small stream of water is constantly running, a portion of which is forced out at each revolution of the wheel. The gold is saved by means of quicksilver on the bottom of the mill, in the same manner as in the rastra.
To make this principle more subservient to the purposes of quartz mining, and better adapted the requirements of a faster age and people, the "improved Chili Mill" was invented. This consists of two heavy cast-iron wheels, from three to five feet in diameter, and from ten to fifteen inches in thickness: these revolve on an axle, moving steadily round in a circular iron basin about 3 foot in depth, into which the tailings from the blanket tables are conveyed, and ground to powder.
As these improved mills are generally worked by steam, the speed attained, and the work accomplished, of course very far exceeds the old process.
The methods of saving the gold which passes over the blankets in the tailings, are almost as numerous as the mills where the quartz is crushed. The principal, however, is to allow the tailings to run down a series of inclined tables, or sluices, at the end of each of which is often placed a wood trough, or iron pan, containing quicksilver, into which they flow, when the gold falls into the quicksilver on the bottom, and is there retained; while the light material floats over the edge of the trough or pan into another sluice, at the end of which is another pan, where the same process is repeated. Thee sluices, or inclined tables, are generally fitted up with "patent riffles" across the bottom, filled with quicksilver. After the tailings have passed through the whole series of sluices they are sometimes: worked through the improved Chili Mill, or other machine; but are oftener allowed to run into a large vat, from which the water flows off while the tailings settle at the bottom. These are then thrown into a heap and allowed to "rust," preparatory to other processes at some future time.