If you were asked to describe how to separate a sand and gravel mixture, you'd probably say to use a sieve, and you'd be right. It isn't rocket science. A sieve is simply a screen usually supported in a frame that has a small enough mesh to allow the sand to pass through but not the gravel.
If you want to fill your aquarium with sand and all you have on hand is a mixture of sand and rocks you got at the beach, you could use a handheld sieve, perhaps even a colander from the kitchen. If you're working on a larger project, you might need to make a larger sieve with a wooden or metal frame. Depending on the size of the screen mesh, you could even use it to separate coarser grades of sand from finer ones.
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Why Separate Sand and Gravel?
One reason you might want to separate sand and gravel is to make your own mortar, which is a mixture of Portland cement and sand. You need mortar if you're laying bricks or setting tiles, and any larger stones that are mixed in with the sand will render it unusable.
You might also need sand minus the stones if you're laying brick pavers. It's customary to lay pavers in a bed of sand to prevent them from moving around and allow water to drain, and large rocks in the mixture will make the pavers unstable.
A third reason you might want to separate sand and gravel is to make a sandbox for the kids. If you get your sand from a beach or another natural source, you'll want to separate out all the rocks, shells, and other objects that could potentially cut feet and hands.
A Simple Sand and Gravel Separation Method
All you need to make a sieve that is useful for most construction and landscaping purposes is some 2x4 lumber, metal screen with a fine enough mesh to prevent the gravel from passing through, and a wheelbarrow. Construct a frame with lumber that is roughly the same size as the wheelbarrow opening, staple the screen to the underside of the frame, and you're ready to go.
Position the sieve over the wheelbarrow, pour the sand and gravel mixture onto the screen, and shake the sieve. The sand will fall into the wheelbarrow, and the gravel will stay behind. You can discard the gravel when all the sand is gone or save it for another project, such as lining the bottom of a drainage ditch. If you want to separate the sand into coarse and fine grades, just change the screen with one that has a finer mesh.
A More Sophisticated Separation Method
A handheld sieve works well for most small construction and landscaping projects, but if you need to separate a lot of sand and gravel, you're better off constructing a sieve that you don't have to hold that is easier to shake. You can still use your wooden-framed sieve, but you'll also need to construct a stand, which can be made with 2x4 lumber.
Construct a frame slightly larger than the sieve and attach the sieve to it with short lengths of chain so that it hangs slightly below the frame. Attach legs to the larger frame that are long enough to allow you to get a wheelbarrow under the sieve. Now, you can pour the sand and gravel mixture into the sieve, and you only need one hand to shake it. If you frame the sieve on three sides instead of four, all you have to do is tilt it to get rid of the gravel once all the sand has sifted through.