The basis for molding sand is clean silica sand. The type used to mix concrete is just fine. However it must be sifted through the finest window screen you can find because there are a lot of large pieces in it. The finer (smaller grain size) your sand the smoother the surface and better the detail of your castings.
Some sandblasting sand comes in very fine grade and will work for the molding sand. The place I bought this from didn't know what mesh size it was (the higher the number the finer the sand is) he said it was simply "medium" grade. So I decided to gamble and buy this bag (100 pounds for $12.50). When I got back home I opened it only to discover that is was more course than the sand I sifted through window screen! Waste if money... This sand is not typical sand. It is actually black in color and the package says; "contains slag and coal." Yes this sand is slag from an iron foundry. I paid 12 bucks for some ground up foundry slag!
In the top left corner is the sandblasting sand (which will NOT be used). In the lower left corner is the sifted silica sand. On the lower right corner is the "gravel" sifted out of the silica sand! And on the top right is some sifted fire clay. The clay requires sifting just as much as the sand because there are chunks in it as big as the gravel sifted from the sand.
After all the sifting I weigh out a quantity of sand and drop it into my mixing tray. In this case I happen to have 40-something pounds worth all sifted.
I moistened the sand slightly then sprinkled a percentage (the percentage depends on formula you use and type of clay you have) of clay onto the sand and began mixing it in. Once it's all mixed in, moisten the mix 'til it has about as much water as a rung out rag or sponge. Then perform the squeeze test (squeeze an handful into a cylinder and see if the sand retains the shape and shows details of your hand). Let the molding sand sit for an hour or so because the clay does not immediately absorb the water. Afterward perform the squeeze test again, the sand should feel dryer and it should perform better.
If the sand holds a cylinder shape after you squeeze it and not much sand sticks to your hand then it's good stuff. If a lot sticks to your hand then it is probably a little too wet. If it doesn't even hold it's shape then there is probably not enough clay. Add more clay 1 or 2 percent at a time, then retest.
Here you can see that the sand forms a clean cylinder when I squeeze it in my fist. It also breaks in half cleanly. This is good sand!
After all that I have 48 pounds worth of new molding sand! The small hand shovel (also shown in the above photo) is an aluminum casting of mine and was done with a technique called "coping down" detailed in the booklet below. If you’re a beginner and want more info on mixing greensand and making molds with it consider getting yourself a copy of my booklet How to cast metal in sand. Thanks