When rocking is periodically stopped to discharge rocks from the hopper, the material passing over the riffles will settle and tend to pack, particularly if much black sand is present. If washing is resumed without first loosening this material, any fine gold that has not penetrated the packed sand will be lost with the tailings. To guard against this, the material behind each transverse riffle should be loosened and scraped back toward the head-end of the rocker with -a flat, straight-edged scoop. This loosening procedure is the key to effective rocking. The loosened material is subsequently washed down and re-concentrated with the next run and if the rocker is kept free of packed sand in this manner, it- will effectively recover flour gold particles running 1,000 or more to the cent.

After the entire sample has been washed, the concentrate remaining behind the transverse riffles is picked up with the scoop and placed in the upper end of the rocker and then carefully re-washed once or twice with clear water to remove surplus sand and further reduce the concentrate volume. The apron, riffles and canvas mat are then removed and washed out in a pan or tub of water and the combined concentrate panned to a finished product.

Although reference books state that from I to 3 cubic yards can be washed in a rocker per man-shift, it should be noted that these figures apply to production-type work and give little indication of what should be expected when processing samples. Experience has shown that one man using a 12-inch x 4-foot rocker similar to the one illustrated in the how-to plan page can wash the equivalent of eight 80-pound samples per day where conditions are favorable. This includes the time for clean-ups, panning, and logging sample data. The total amount will, of course, vary with the size or number of individual samples and with the character of material being washed. Material containing much clay can halve this figure whereas loose, free-wash type material may double it. Dredging companies occasionally use large, engine-powered rockers in which the total gravel excavated from a shaft can be washed rapidly and economically.

Printable Placer Rocker Plans

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