Clay imparts:

Clay imparts the essential tensile strength to the moulding sand in order that after ramming, the mould does not lose its shape. But, as the quantity of the clay is enhanced, the permeability of the mould is decreased.

Clay is represented by the American Foundrymen's Society (A.F.S.), as these particles of sand (under 20 microns in diameter) that fail to settle up at a rate of 25 mm per minute, while suspended in water. Clay contains two ingredients: true clay and fine silt. Fine silt is a sort of foreign matter of mineral deposit and contains no bonding power. True clay supplies the essential bond. Under high magnification, true clay is found to be made of very minute aggregates of crystalline particles, known clay minerals. These clay minerals are further composed of flake-shaped particles, approximate 2 microns in diameter that are seen to lie flat on one another.