The flagging should be of tough, coarse rock; granite, basalt or compact quartz are all good. This flagging should be at the very least a foot thick. When the arms of a 10-foot arrastra are revolving 14 times a minute, the outer stone is traveling 400 feet a minute. Round holes closed by wooden plugs, or a side gate, lets the liquid mud out. Some mill men use chemicals in the arrastra; potassium cyanide, and wood ashes or lye are probably the most useful, as the latter cuts grease and the former gives life to the quicksilver. Rich silver ores are treated with blue stone and salt. When the pulp has been ground sufficiently, quicksilver is added, sometimes 250 pounds being put in a single charge. A 12-foot arrastra will never treat more than two tons a day, and often no more than one-half that. One man a shift can look after a couple of arrastras, and the owner, in case of one arrastra that is working on tailings, often does everything himself. Overshot wheels, or turbines, or hurdy-gurdies, furnish the power in many cases. A simple mule-power arrastra may be built for $150.
A side hill should be chosen for the site of a battery. Ample water power is necessary, though provision may be made for saving it in catch basins should such a course be desired. Moreover, there must be plenty of room below the mill for the tailings, as it may be desirable at some future time to put them through a second course of treatment.
STAMP BATTERY.
Automatic ore-feeders are always put in by good mill men. In cold climates the water that goes through the mill should be heated, and this may be done by the exhaust steam, but care is necessary that no grease get into it, as it would prevent the gold from amalgamating. The stamps for a light mill may be 3 or 5 in number, and weigh from 700 to 850 pounds. Tables must be water-tight, with half an inch to one inch drop to the foot, according to the fineness of the gold. Below them tables, having the same inclination and covered with blanketing, are used to retain specks of gold that have passed over the plates without amalgamating.
THREE STAMP BATTERY.
After the concentrated materials, always spoken of as the concentrates, have passed over the tables, they are often roasted to get rid of the sulphur, arsenic, etc., and afterwards treated with quicksilver in the pan, or tin, with chlorine or cyanide. These processes belong, however, to the domain of the professional chemist and metallurgist, and require the knowledge and experience of an expert to stand a chance of success.
The coarseness of the mortar screens is subject to infinite variety, according to individual preference. The number of holes to the square inch ranges between 60 and 800 in Australia, and between 900 and 10,000 in the United States. The holes, when round, agree in numbers with those of sewing-machine needles, from 0 to 10. When slots are preferred to holes, they are generally 3/8-inch in length and No. 6 diameter. Russia sheet iron, or sheet steel 1/32-inch thick is the material of which they are made. It should weigh one pound to the square foot, be very soft and tough, have a clean, smooth surface, and show no rust or flaws. In Australia 1/16 sheet copper is preferred. The holes in any case must be punched in the sheet so that the rough edges are turned, and thus any pulp that finds its way into one of the holes is certain to get out again and not clog. A battery may require 13 sets of screens a year; each screen having a surface of about 1½ square feet. Russia iron screens endure 15 to 40 days. As the work a stamp can do depends entirely upon how much pulp can escape through the screen in any given time, the latter is evidently a very important detail of a battery.
Prospecting stamp batteries differ from ordinary batteries, chiefly in being of light build and weight.