![]() The patterns of the gold-saving tables and sluices differ widely. Quantities of fine material may be seen passing to the stack-piles. Anyone who has watched the action of either sort of screen for any length of time, particularly on boats where a sticky clay is treated, does not have to be told that neither of them is a thoroughly efficient machine for the work. ![]() The holes are reamed on the outer side to give a free discharge. at the upper part of the screen and from ½ to 5/8 in. The diameter of the holes is governed by the size of the gold as this is usually very fine throughout the Sacramento valley, the holes vary from 5/16 in. The first cost and repairs of the revolving screen are usually greater than those of the shaking type. From the modern standpoint, however, the last consideration does not count for much. On the contrary, those who advocate the shaking screen contend that the jets have a thinner surface to play upon, the actual screening is larger and the material is deposited on a wider surface. As to the choice between the revolving trommel and the shaking screen, those who use the former claim that when constructed, as it usually is, with flanges and rods across it, it turns over lumps and exposes them to the action of the water-jets on all sides. There are cases too where the double-lift dredge with the long sluice might be advantageously replaced by the single-lift pattern. It is held by some authorities that the old fashioned single-lift dredge, where everything passed from the upper tumbler over long sluice-boxes with riffles supported on a pontoon behind the dredge, is of equal gold-saving efficiency and more economical (when properly designed and managed) than the present style. ![]() The aim also, of course, is to prevent the larger gravel and boulders from being washed over the sluices. The essential duty of the screen is to classify the material prior to concentration, it also serves to disintegrate or break up the material passing over or through it, so that particles of gold may not be carried off in lumps of clay or cemented gravel, to be lost by passing out at the lower end over the stacker. ![]() ![]() The Metallurgy of Dredging: The gold-saving appliances of a dredge consist respectively of screens, tables, and sluices. ![]()
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