Dry Washing is the use of a "Dry Washer" to separate gold from sand and gravels without the use of water.
A dry washer begins with a "Grizzly" onto which gravels are shoveled. The grizzly is usually a piece of expanded metal mounted on an aluminum or wood tray. The grizzly is mounted at a fairly steep angle and above the dry washer's sluice box. Anything larger than the holes in the grizzly falls or is scraped away and falls behind the dry washer. Anything smaller than the holes in the grizzly falls onto the tray and into the top of the sluice box.
The dry washer's sluice box is designed similarly to that of a conventional sluice box that uses water with a few exceptions. The riffles in the dry washer's sluice box are pointed up towards the top of the box, instead of towards the bottom of the box. This allows gold to be trapped under the riffle. Holes drilled into the sluice box allow forced air to blow away anything that is lighter than gold (and black sand). Therefore, the gold and black sand remains in the box, trapped behind the riffles, and anything lighter is blown away.
The air is forced through the holes in the sluice box by one of two methods. The original way used by the old-timers, and still used today, is a bellows mounted underneath the sluice box. The bellows is powered by a hand crank mounted on the side of the dry washer, or a gas or electric motor using pulleys and belts. This is called a "puffer type dry washer."
The second way, which is more popular, is by means of a leaf blower (gas powered). The leaf blower blows air through a flexible hose similar to home dryer hose, which is connected to the bottom of the sluice box. Inside the sluice box, a fan is mounted that spins as air is blown into the box. Mounted on the fan is a weight that throws the fan off balance when spinning, and vibrates the entire box. This additional vibration assists in forcing flour gold to the bottom to be trapped. The air is blown up through holes drilled in the bottom plate of the sluice box. The holes are located directly behind the riffles. The plate is covered with a fine mesh material that keeps "fines" from leaking through. The "heavies" are trapped behind the riffle and the lighter material is blown off.
The grizzly and sluice box are mounted on a frame in such a way as to have the grizzly "feed" the top end of the sluice box. The sluice box is mounted on the frame at an angle, with the top of the sluice box mounted solidly to the frame, and the bottom end suspended by a small chain and springs to allow efficient vibration.
And there you have it.
Dry washing requires that the sand and gravels being dry washed be powder dry. If necessary, shovel the material onto a tarp to dry in the sun, then poured into the grizzly.
Dry washing is very efficient, but not quite as efficient as dredging. But, when water is scarce or non-existent, dry washing is the method of choice.