of the glue would be much higher than that of the albumin. The 

 difference in such a case is attributable solely to the kind of cementing 

 material which is present and its individual character. In addition to 

 this it is undoubtedly true that the shape and size of the particles, and 

 the proportional amount of plastic and non-plastic particles present 

 have an important influence on these properties. 



SLAKING. 



The slaking of clays refers to the quality possessed by some of them 

 of rapidly disintegrating and falling to powder when lumps are thrown 

 into water. Such clays are usually less dense, that is to say bulk for 

 bulk they are less heavy than those which do not possess this property. 

 Clays that slake, if touched to the tongue will adhere to it, sometimes 

 so strongly that it is painful to draw the tongue suddenly away. Both 

 this effect and the slaking are caused by a vigorous indrawing of water 

 into the porous structure of the particles. In more dense clays this 

 porous structure is already " stuffed" sometimes with water and quite 

 often with other substances both of organic and inorganic origin. The 

 truth of this is readily shown by a very simple experiment. Glycerin 

 is not absorbed into the clay structure rapidly enough to produce slak- 

 ing ; if, therefore, a lump of slaking clay be soaked in glycerin over 

 night it can be transferred to water in which it will then be found to 

 behave as does any non-slaking clay. 



Clays that have been slaked and the powder molded into masses and 

 air dried do not again disintegrate when wet, as the porous structure is 

 then sufficiently saturated with water to decrease the action. The power 

 to slake readily is often a great advantage, as it enables hard clay masses 

 from the bank to be disintegrated by weathering and thus does away 

 with the necessity for elaborate grinding processes. 



AIR SHRINKAGE. 



Air shrinkage is that which takes place during the air-drying of a clay 

 mass and is caused by the evaporation of the water, both that which 

 has been used in mixing and a portion of that which the clay contains. 

 As has been explained, very plastic clays possess a greater number of 

 the active hydrated particles with a porous structure, so that we should 

 expect these clays to exhibit a higher shrinkage than lean clays, and 

 this is actually found to be the case. Everyone is familiar with the way 

 glue-like bodies shrink on drying. This shrinkage varies in different 

 clays from 1 to as high as 10 per cent. We now begin to see why the 

 skill of the potter is highly taxed even before his ware goes to the kiln. 

 If he selects a clay which from its plasticity and binding power is all 

 that could be desired, it is probable that its high air shrinkage will 

 cause it to deform and check (crack) on drying out. Sometimes by 

 very slow and careful drying the danger can be avoided, but usually 



