A long tom usually has a greater capacity than a rocker and does not require the labour of rocking. It consists essentially of a short receiving launder, an open washing box 6 to 12 feet long with the lower end a perforated plate or a screen set at an angle, and a short sluice with riffles. The component boxes are set on slopes ranging from 1 to 1-1/2 inches per foot. The drop between boxes aids in breaking up lumps of clay and freeing the contained gold.
A good supply of running water is required to operate a long tom successfully. The water is introduced into the receiving box with the gravel, and both pass into the washing box.
The sand and water pass through the screen's 1/2-inch openings and into the sluice. The oversize material is forked out. The gold is caught by the riffles. The riffle concentrates are removed and cleaned in a pan. Quicksilver may be used to amalgam gold from the concentrate if the gravel contains very fine gold. Please note, quicksilver (mercury) is a hazardous substance and should only be used under controlled conditions and not released into the environment.
The quantity of gravel that can be treated per day will vary with the nature of the gravel, the water supply, and the number of men employed to shovel stones into the tom and then fork them out. For example, two men, one shovelling into the tom and one working on it, might wash 6 cubic yards of ordinary gravel, or 3 to 4 cubic yards of cemented gravel, in 10 hours.
A tom may be operated by four men; two shovelling in, one forking out stones, and one shovelling fine tailings away. Where running water and a grade are available, a simple sluice is generally as effective as the long tom and requires less labour.
From: Bureau of Mines Information Circular 8517 by J.M. West, released in 1971.
Rafal Swiecki, geological engineer email contact