[ appears in a crystalline form. This is I. n >adly defined as amorphous 

 substance. Only one kind of amorphous matter, however, is active in 

 producing plasticity. Glass is an amorphous, noncrystalline material, 

 hut powdered glass is no more plastic when wet than is sand. The 

 particles which are active in forming coherent masses are highly 

 hydrated, that is? they contain so-called water of combination, which 

 disappears into some sort of porous structure too fine to be seen even 

 under the most powerful microscope. Clays, even when thoroughly 

 dried in the sun and air so that they have every appearance of being 

 dry powders, will sometimes contain as much as 12 or 14 per cent of this 

 combined water. If the clay is heated to a sufficiently high temperature 

 to drive off this water all plasticity and clay likeness is gone. Thus 

 brick dust is no more plastic than sand or glass-powder. The particles 

 have not become crystalline by heating, but the active hydrated particles 

 have been changed into dead amorphous ones. These active particles 

 which become coherent when wet are called colloid (which means glue- 

 like), to distinguish them from the inactive crystalline and amorphous 

 grains which are also present in almost every clay. 



BINDING POWER. 



It is this property which provides strength in the air-dried clay so that 

 articles fashioned of it can be transported to the kiln without crumbling 

 and breaking. It is a very important quality and the purer clays and 

 kaolins are often deficient in it. The property is tested in the labora- 

 tory by molding briquettes of a special shape, which after drying out 

 are pulled apart in a machine designed for the purpose. The strength 

 of different kinds of clays, as determined in this manner, is given in 



the following table : 



Tensile strength of clays. 



Pounds per ,v</. in. 



Pure kaolins 5-20 



Common brick clays __ 30-100 



Pottery clays _.. _ 100-500 



Ball clays and other very plastic clays. _ _ 200-500 



It is a curious fact that though high binding power is usually asso- 

 ciated with high plasticity, this is not invariably the case. Some of 

 the plastic clays of New Jersey in use in the potteries are of compara- 

 tively low binding power. This can be explained, however, by the 

 colloid theory as outlined in the preceding paragraph better than by any 

 other theory as yet advanced. There are various kinds of particles 

 which assume to varying degrees the soft, almost viscous, condition 

 which leads both to plasticity and binding power. In order to illus- 

 trate what is meant let us suppose that an inert, non-plastic powder 

 were to be mixed with egg albumen on the one hand and with a glue 

 solution of equal viscosity on the other. Masses of some plasticity 

 would be obtained in both cases, but on drying out the binding power 



