Natural moulding sand contains variable amount of clay, which acts as bond between the sand grains. This sand, therefore, posse strength, plasticity and refractoriness to varying extend depending upon the clay minerals present. When it contains a greater amount of clay, it is blended with river sand, which is relatively clay free so as to get the optimum properties desired in the sand mixture.
Washed grains shall be mostly sub-angular to rounded shape. As far as possible, the sand shall be free from gravel. Natural moulding sand for use in foundries shall be of three main grades, namely, A, B and C with respect to clay content.