530 NEW YOKK STATE MUSEUM 



Moisture may plaj another important and injurious role in the 

 working of a clay, in that it tends to dissolve soluble salts in the 

 clay, and bring them to' the surface in drying, giving rise to the 

 formation of efflorescence. It may also permit acids contained in 

 the fire gases of the kiln to act on the mineral ingredients of the 

 clay and thus form soluble compounds, specially sulfates and 

 chlorids. 



By the addition of water tO' an air-dried clay, it gradually passes 

 from a powdery or lumpy condition to a pasty mass, the tenacious- 

 ness of which increases till the point of maximum plasticity for the 

 given clay is reached. If the addition of water be continued, the 

 clay gradually passes into a soft mud. In some clays this change 

 takes place slowly, in others (specially many residual clays) very 

 rapidly. 



Combined water is present in every clay. In pure kaolin there is 

 nearly 14^, and amounts are found in different clays intermediate 

 between this and 3^ or 4^. 



The sources of co'mbined water in clays are either kaolinite, 

 limonite, or hydrated silicates; the quantity in different clays can 

 be seen from the table of analyses given at the end of the report. 



It is driven off at a low red heat; and when this occurs an addi- 

 tional shrinkage takes place, the extent depending on the quantity 

 of water present. The shrinkage varies commonly from 2j^-10^ or 

 even 14^. 



While the amount of combined water does not seem to stand in 

 direct relation to the plasticity of the clay, nevertheless, when it is 

 once driven off, the clay can no longer be rendered plastic. 



Methods of analyzing clay 



By H. T. Vult§ Ph.D. 



One grain of the dried and finely pulverized clay is fused in a 



platinum crucible with five to 10 times its weight of a mixture of 



11 parts of dry sodium carbonate and 14 parts of dry potassium 



carbonate, the amount of fusion mixture necessary depending on the 



