Foundry sand is called “greensand”, because of the slight moisture content.
A combination of sand, clay and water.
Foundry Sand/Greensand
In commercial foundries the sand is used over and over thousands of times. Between uses, the sand is rejuvenated by adding water and mulling (mixing and smashing). Sand grit is determined just like sandpaper. 150 is very fine and 50 grit is coarse. Fine sand will give good detail, coarse sand will give a pebbly or rough texture. If you look at the sand through a microscope there are gaps and spaces like a sponge. Any moisture that turns to gas during pouring can escape out the pores (gaps).
Other materials like plaster do not have gaps for the gas to escape. The sand most often used in greensand is silica (common/ordinary) sand. You can use beach sand, desert sand…… as long as it is clean and fine. The sand is held together with bentonite, a powdered clay. Bentonite is also used for wine-making, well drilling, cosmetics, farm-feed additive and milkshake thickener. Bentonite is also sold in health food stores for abdominal problems. You can also purchase bentonite locally at well-drilling suppliers.
Mulling
Mulling coats each sand grain with bentonite, smashing and squishing with weight. In a foundry they use a large tub with 2 large wheels to smash, 2 augers to plow (mix upward), a muller. You do not need a muller.
Mixing greensand without muller:
1. On a clean floor, dump 100 lbs of fine silica sand and spread to about 1″ thick (about a 6′ circle).
2. Evenly dump 1 gallon of water over the sand with a garden style water can, mix thoroughly with a shovel and rake.
3. Spread to about 1″ thick and evenly sprinkle 12 lbs of powdered bentonite over the sand /water. Stir in bentonite with a rake and turn over with a shovel to mix thoroughly.
4. Smash the mixture all the way flat with your feet, then fluff it up with the shovel, tilling the mix. Repeat this step 2 more times.
Please watch this video…….
Mix sand and water FIRST, then add bentonite.
https://www.facebook.com/foundry101/videos/2036175736492532
Sand Use
1.Pack loose sand into the flask, packing it tight. After removing the pattern and forming gating system, you can torch the casting cavity (mold) to remove any excess moisture.
2.After pouring, cooling and shake-out (removing flask and casting from sand), sprinkle water over the sand. Mostly on the hard, crunchy sand that was touching the casting. About 2 cups water per 25lb mold. Do NOT add too much water!
3. Store sand in an airtight container when not in use, to retain moisture. If you keep the sand moist, you do not need to mull. If you let the sand dry out completely, you must re-mull.