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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