The function of riffles is to hold back the gold particles that have settles to the bottom of a flowing stream of water and gravel. Any dead space in the bottom of a sluice where there is not current, fills quickly with sand and thereupon losses most of its value as a gold recovery unit, unless the sand remains loose enough to permit gold to settle into it. Therefore, the shape of riffles is important, regardless of the fact that under some conditions, as with coarse gold and free washing gravel, all forms of riffles are almost equally efficient.
The riffles should be shaped so as to agitate the passing current and to produce a moderately strong eddy or boil in the space behind or below it, thus preventing sand from settling there and at the same time holding the gold from sliding farther down the sluice. In other words, riffles for maximum efficiency should provide a rough bottom that will disturb the even flow of sand and gravel, will retain the gold and will not become packed with sand. Where grade is lacking the riffles must be relatively smooth, so as not to retard the current unduly; under these conditions the sluice must be long enough to compensate for the loss in gold recovery of individual riffles.
Natural stream beds act as gold saving sluices, not because they are particularly efficient as such but because most gold is hard to lose and the streams are long. Riffles of course should be designed so as to save the gold under the existing conditions. They should also be cheap, durable and easy to place and remove. Not all these qualities are found in any one type. Sluice-box riffles may be classified roughly as transverse, longitudinal, block, blanket and miscellaneous roughly surfaced ones, or according to the material as wood block, pole, cast iron, rail, angle iron, fabric and miscellaneous. More than one type of riffle can be used. The only general rule observed is that the size of the riffles is roughly proportional to the size of the material handled and that for fine material, particularly the screened gravel washed in most of the equipments operated.
The top surfaces of the riffles may be plated with strap iron. Transverse riffles of this type may be slanted downstream and the top surfaces may be beveled to increase the boiling action. The effectiveness of this practice is acceptable. Where large quantities of gravel are put through sluices, iron or steel riffles generally are preferred. Their superior wearing quality as compared with that of other material permits longer runs without stopping to replace riffles. Their durability may compensate the cost. Steel rails and angle iron are common riffles used in various ways. Old angle steel iron is a good option. Iron or steel riffles should be used in units to be handled readily. When used as traverse riffles lengths of steel are set upright, the flanges almost touching or not more than 1 or 2 inches apart. Where grade is lacking and gold saving is not particularly difficult, longitudinal riffles make excellent paving for a sluice as they provide a smooth sliding bottom for the gravel.
For shallow sluice streams carrying only fine material various materials are used, including brussels carpet, coco matting, corduroy and burlap. These may be held down by cleats or by wire screen. Some manufacturers often use a combination of them with riffles to catch gold and hinder its being washed out of the riffle by eddies. A corduroy woven may be used to catch coarse gold. As such gold would be considered fine by most placer prospectors, it seems probable that such a fabric would be useful for treating finely screened placer sands. Heavy wire screen such as that used for screening gravel makes an excellent riffle for fine or medium size gravel in fairly shallow streams.