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Two important characteristics for pots used in melting optical glasses are those of durability against molten glass and of spalling. Present paper deals with the latter property and its bearing on the composition, especially the influence of the grain size distribution of chamotte. Grains of chamotte studied may be classfied as to their sizes as coarse, medium, fine, and very fine. (ratio of their diameters is roughly 14:5:2.5:1). State of dense packing can be obtained for mixtures plenty of grobe and very fine grains and meagre of medium or fine grains in mixtures of three kinds of grains, coarse-medium-very fine and coarse-fine-very fine. Casting slip made of these mixtures with clay have features as follows:i. The fluidity of casting slip is best when the grain size distribution of the slips do not incline toward coarse or very fine.ii. Drying characteristics of bodies run parallel with surface factor calculated after Jackson and Purdy, and are influenced by the state of chamotte packing.iii. Spalling characteristics at temperatures of 850°C (at this temperature, pot is sprayed with water after melting) and 1350°C (at this temperatures batch is poured in) are good by bodies not inclined toward coarse or very fine.iv. Firing characteristics of bodies run parallel with surface factor but are not affected from the state of chamotte packing.