CLAYS OF NEW YORK 529 



of moisture present in a clay depends on tlie number and size of the 

 spaces between the clay grains, tbe size of tbe clay particles, and 

 tbe amount of organic matter present. 



Air drying usually causes tbe evaporation of most of tbe water in 

 a clay, accompanied by a skrinkage of tlie mass, wbich. ceases how- 

 ever before all the moisture has passed off. The reason for this is 

 that the shrinkage: of the clay ceases when the particles come in 

 contact, which may happen and still leave interstices. These of 

 course still contain moisture, and consequently the brick will keep 

 on losing weight till not only this interstitial water, but also the 

 surface moisture of the particles, is driven off. In practice, it is 

 this that evaporates during the first period of the burning known as 

 " water-smoking." The shrinkage of the clay attendant on drying 

 varies, with the nature of the material, from 2^ or 3^ to 10^ or even 

 16fo. It is governed largely by the causes influencing the absorption 

 of the clay. 



Sandy clays usually show the least shrinkage, and of this kind 

 the coarser grained diminish in size the least. Highly plastic clays 

 generally show the highest shrinkage. 



The amount of water which a dry clay needs in order to develop 

 its maximum plasticity is a variable quantity. Plastic clays absorb 

 large quantities of water, but a lean clay if fine-grained may do 

 the same. As a very general rule it may be stated that lean clays 

 absorb from 12fo-20fc, while fat clays require anywhere from 

 25^-50^; and the more water a clay absorbs, the more it has to part 

 with in drying and the greater will be its shrinkage. 



Highly aluminous clays do not always absorb the most water, 

 nor are they the most plastic. Some clays low in alumina and high 

 in organic matter are not only highly plastic but also absorb a high 

 amount of water. 



Owing to the high shrinkage of most clays mth high absorptive 

 power, there is frequently danger of their cracking, if rapidly dried, 

 on account of the active disengagement of water vapor. 



