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As a real rough ball park number, I'd say that 20 mesh is about the number you are looking for. However, if there is not loads of clay in the material, Ive seen them recover a decent percentage (not close to 100 percent by any means) of stuff as small as 50 mesh by folks dry washing old hard rock mine dumps. The problem with clay is that small flakes will stick to little rocks by the dry clay and then roll right out of the dry washer attached to a rock. The more the clay and the more the clods, the lower your recovery is going to be. Dry with heavy clay will still give you a recovery that is not so good. You can see some of the gold in my photo below is pretty small.

dw_gold56b_small.jpg

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A patch I dry washed a couple years ago, I detected the piles and got a signal which seemed to come from a rock - not a piece of quartz, just a piece of country rock. I looked at the rock and it seemed clean, but it was the source of the target sound. I looked real close and at one end there was a tiny crack in the rock and a tiny pinch of dirt in the crack. I cleaned out the crack with my fingernail and sure enough, there was a little flake of gold in there that my SDC was sounding off on. That kind of thing can hardly be avoided when dry washing even if you smash all the clods. That is why its always good to go over your piles - both over size and the stuff that goes across the dry washer - with a metal detector to make sure you didn't miss anything big enough to detect.

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1 hour ago, Hard Prospector said:

The clay clods can be easily broken up by pounding it with a concrete tamp. Also using a cement mixer really breaks down the clods, just throw in some big chunks of iron stone or granite with the material.

I was wondering if some kind of pre treatment of the dirt would help to dislodge the clay, maybe some kind of vibration chamber or trommel with some rollers inside.

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Most seasoned desert drywashers, including myself,  own a cement mixer for this very reason. I use it whenever I can "practically" get it to the site as it makes small work of caleche, clods or whatever conglomerate crap needs reducing. I use the hand crank model w/wheels as its more portable when I have to drag it to the spot.

Once reduced to my satisfaction, i will just run the material right into the drywasher. If the area is mostly known for fine gold, I will classify to at least 1/4" then run it.

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