III. Stamping. Before describing the refined methods of washing the more valuable ores of copper, silver, lead, &c., it is proper to point out the means of reducing them into a powder of greater or less fineness, by stamping, so called from the name stamps of the pestles employed for that purpose. Its usefulness is not restricted to preparing the ores; for it is employed in almost every smelting house for pounding clays, charcoal, scoriæ, &c. A stamping mill or pounding machine, [fig. 670.], consists of several movable pillars of wood l l l, placed vertically, and supported in this position between frames of carpentry K K K. These pieces are each armed at their under end with a mass of iron m. An arbor or axle a a, moved by water, and turning horizontally, tosses up these wooden pestles, by means of wipers or cams, which lay hold of the shoulders of the pestles at l l l. These are raised in succession, and fall into an oblong trough below m m, scooped out in the ground, having its bottom covered either with plates of iron or hard stones. In this trough, beneath these pestles, the ore to be stamped is allowed to fall from a hopper above, which is kept constantly full.
The trough is closed in at the sides by two partitions, and includes three or four pestles; which the French miners call a battery. They are so disposed that their ascent and descent take place at equal intervals of time.
Usually a stamping machine is composed of several batteries (two, three, or four), and the arrangement of the wipers on the arbor of the hydraulic wheel is such that there is constantly a like number of pestles lifted at a time; a circumstance important for maintaining the uniform going of the machine.
The matters that are not to be exposed to subsequent washing are stamped dry, that is without leading water into the trough; and the same thing is sometimes done with the rich ores, whose lighter parts might otherwise be lost.
Most usually, especially for ores of lead, silver, copper, &c., the trough of the stamper is placed in the middle of a current of water, of greater or less force; which, sweeping off the pounded substances, deposits them at a greater or less distance onwards, in the order of the size and richness of the grain; constituting a first washing, as they escape from beneath the pestles.
In the dry stamping, the fineness of the powder depends on the weight of the pestles, the height of their fall, and the period of their action upon the ore; but in the stampers exposed to a stream of water, the retention of the matters in the trough is longer or shorter, according to the facility given for their escape. Sometimes these matters flow out of the chest over its edges, and the height of the line they must surmount has an influence on the size of the grain; at other times, the water and the pounded matter which it carries off, are made to pass through a grating, causing a kind of sifting at the same time. There are, however, some differences in the results of these two methods. Lastly, the quantity of water that traverses the trough, as well as its velocity, has an influence on the discharge of the pounded matters, and consequently on the products of the stampers.
The size of the particles of the pounded ore being different, according to the variable hardness of the matters which compose them, suggests the means of classing them, and distributing them nearly in the order of their size and specific gravity, by making the water, as it escapes from the stamping trough, circulate in a system of canals called a labyrinth, where it deposits successively, in proportion as it loses its velocity, the earthy and metallic matters it had floated along. These metalliferous portions, especially when they have a great specific gravity like galena, would be deposited in the first passages, were it not that from their hardness being inferior to that of the gangue, they are reduced to a much finer powder, or into thin plates, which seem to adhere to both the watery and earthy particles; whence they have to be sought for among the finest portions of the pulverised gangue, called slime, schlich, or schlamme.
There are several methods of conducting the stamps; in reference to the size of the grains wished to be obtained, and which is previously determined agreeably to the nature of the ore, and of the gangue; its richness, &c. The height of the slit that lets the pounded matters escape, or the diameters of the holes in the grating, their distance, the quantity of water flowing in, its velocity, &c., modify the result of the stamping operation.