How to Prepare Molding Sand
- 1). Sift one bag of builder's sand through a fine mesh screen to remove impurities and large particles, sifting into a bucket or bowl.
- 2). Weigh out a quantity of sand once you've sifted. Molding sand should be 92 percent sand, so for a 20 lb. batch, you would need 18.4 lb. of sand. Store any sand you're not using in a lidded container.
- 3). Sift bentonite clay in the same manner as you did the sand, then weigh out 8 percent clay. For the 20 lb. batch, you need 1.6 lb. of clay.
- 4). Pour the sand and the clay in a large tub and mix them together by hand, until you've mixed the clay evenly into the sand.
- 5). Measure out 4 percent water in a container. For the 20 lb. batch, you would need 0.8 lb. water.
- 6). Sprinkle about half of the water over your sand and clay mix and rub the liquid into the particles to moisten them. Once you've worked in half of the water add half of what's left and work it in and then add the last bit and work that in. Good molding sand will feel like a damp sponge, so let the consistency guide you more than the amount. On humid days, you may not need all the water; on drier days, you could need more.
- 7). Leave your sand alone for one hour, so the clay can absorb all the liquid.
- 8). Test your sand after the hour's up: squeeze a portion of the sand in your hand. If the sand sticks to your hand, it's too wet. If the sand holds a shape and only a few grains stick to your hand, you're all set. If the sand won't hold a shape, you need more water.
To correct wet sand, add 1 percent clay and test it again. Keep adding clay until you reach the right consistency.
Source...