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The hopper should be made 1/2 to 3/4-inch narrower than the inside width of the rocker; this results in a bumping action as the rocker is operated and assists scouring and screening the gravel. The amount of open area in the screen plate is important. A screen with too much open area allows the fines to pass through too fast and where much sand is present this can cause overloading of the riffles with resultant gold loss. A plate with 1/4-inch holes drilled or punched in an alternate pattern at 1-inch intervals and on 1/2-inch centers will provide a screen with 10 percent open area. This has been found -to be about right for small prospecting rockers.
The apron consists of a simple wooden frame covered with loose-fitting canvas or similar material. The resultant sag or "belly" in the apron functions as a gold and black sand trap and when large amounts of material are put through a rocker, the apron's gold-holding ability permits longer runs between clean-ups. In normal sampling work where short runs are made, the apron is less important and is sometimes dispensed with.
It is noteworthy that most old-time prospectors employed by dredging companies wash their samples in rockers without riffles. Experience and practice enable these men to concentrate the gold and black sand directly on the smooth wooden bottom of the rocker and, as a final step, to tail out the black sand and bring the gold to a point in a manner somewhat similar to panning. This technique is particularly useful when applied to small-volume churn drill samples and is mentioned here to point out that a properly operated rocker is an excellent gold saver which unlike the sluice, does not rely on riffles for its effectiveness. But it should be pointed out that a rocker equipped with good riffles will forgive much mishandling and, for this reason, the average person should use them. Parenthetically, it should be noted that even the experienced rocker operator, in certain cases, may find it more expedient to use riffles than to explain to a layman why they are not essential. A simple riffle arrangement suitable for general use can be provided by covering the rocker bottom with heavy-gage expanded metal lath placed over a canvas mat and held down by several transverse wooden slats as shown in the plans.
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