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Riffles and Sluice Boxes - How do they work?

What is a Sluice Box? Sluicing for gold

A sluice box is like a long tray which is open at both ends. Most will have riffles, spaced evenly along the length of the sluice, usually every few inches, perpendicular to the length of the sluice. Riffles cause small barriers to the water flow which creates eddies in the water, giving the heavier material (black sand and gold) a chance to drop to the bottom, behind the riffles. The "upstream" end of the sluice can also have a flare to aid in increased water flow.

The sluice is usually placed in a creek or river and can be held in place by larger rocks, packed in against the sides or one large flat rock on top. The slope or angle of the sluice can be adjusted, by arranging rocks under the sluice. Also the flow of water can be adjusted by placing a large rock in front of the intake end of the sluice, to divert some of the water around the box. You have to adjust these variables several times until you get the material moving through the box at the correct speed leaving just the black sand in the riffles.

When you're ready to stet processing material, drop in material at the upstream end of the sluice. As material piles up at the downstream end, you will have to clear it once in a while, or just move the sluice slowly upstream, as you work the material.

What you will notice when first starting is the riffles will appear to fill up with lighter material. But over time this lighter material will slowly migrate down the box and finally exit out the end of the sluice, washing out from behind one riffle, and then the next. During the flow, as heavier material comes through, it also settles behind the riffles, and as lighter material migrates, the heavier stuff settles slowly to the bottom. This is assuming you have the slope and flow rate adjusted correctly.

Eventually the riffles will fill up with heavy material like black sand. When you see lots of black sand showing in the riffles, you could be in a good spot for gold.

How do riffles work



The reason that riffles work is two fold. First, there is an eddy created behind each riffle, causing a temporary lull in the water flow. The material that is flowing is in a liquid state. This causes the heavies to be at the very bottom of the flow. As the flow passes over a riffle, the heavies will fall to the bottom behind a riffle.

Second, the riffles are spaced a couple of inches apart, and act as a series of small dams, stopping the creep of the heavy material down the sluice. Without them, there would be a slow, but sure creep of gold out the end of the sluice.

Normally most of the trapped gold will be behind the first couple of riffles. This is because the heavies fall fast. Small flour gold may extend several riffles further, and, hopefully, the last few riffles have ZERO gold. If you find gold in the end of your sluicebox you surely have lost mor eout the end.

The riffles are usually hinged at the upstream end, with a latch at the downstream end. So you can release the latch, and swing the riffles up. Usually there will be a 3/16th inch layer of material called miner's moss or carpet, that resembles in texture, a kitchen scouring pad, laying on the surface of the sluice. It is a loose weave matt with lots of air space. It traps and holds the smallest gold particles. On top of that can be a layer of expanded metal. This creates a criss-cross pattern of spaces, each with their own eddies. Then the riffles, that resemble a ladder, with each riffle a rung, when swung up, is lowered into place and latched down.

Each time you clean up, you raise the riffles, and remove the moss. You then wash down the sluice, wash out the moss, and put it back together, all inside a tub, of course. How often you do this depends on whether you want to know what you're getting, whether to move or not. Otherwise, you can probably run the sluice for quite a while, before cleaning (I used to think that you should clean it often, but I tend to think otherwise now). The gold is going to fall out early in it's travels down the sluice, and it would have to fill up with gold, before you started loosing anything.

The way to tell if you have things close to right or not, is to notice whether you are finding gold many riffles down from the front of the sluice. It should be very close to the front, in the first 1/2 of the sluice for sure. One way to always know is to make the miners moss two sections, butted together. All the gold should be in the first mat. There should be nearly nothing in the second. I have divided mine into 4 sections. I pan the last section to make sure that there is nothing in it. I pan the first section to see what I'm getting.


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