This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
See also Casting and Matrix.
A high grade of molding sand should be fat, i. e., strongly mixed with clay. Naturally the molds of this sand should be employed only in a perfectly dry state. The fat molding sand is prepared artificially from quartz sand (fine sprinkling sand), fat clay, free
from lime and ferric oxide (red ocher). The molding sand is fixed by breaking up the loose pieces in which it is partly dug; next it is passed through a fine sieve and mixed up to one-third of its volume with charcoal dust, or, better still, with lampblack, which, owing to its looseness and fatness, does not detract so much from the binding qualities of the sand. The utility of the sand may be tested by pressing the finger into it, whereupon the fine lines of the skin should appear sharply defined; its binding power is ascertained by dropping a lump pressed together with the hand from a height, which is increased until it breaks.