CHEMICAL PROPERTIES OP CLAYS. 23 



The moisture in air dried clays may be as low as .5 

 per cent, and reach 30 to 40 per cent, in those freshly 

 taken from the bank. In the air dried specimens in 

 the Alabama samples tested, it varied from .12 per 

 cent, to 8.4 per cent. 



In air drying most of the moisture is expelled, and 

 this is accompanied by a shrinkage of the clay, which, 

 in the case of the Alabama samples, was usually from 

 2 to 7 per cent., but in one case it reached 14 per cent. 



The air-shrinkage of the clay ceases however before 

 all the moisture passes off, the reason for this being 

 that the shrinkage ceases when the clay particles have 

 come in contact with each other, but there may still 

 remain spaces between tliem which hold the water by 

 capillarity, and the brick will contiue to lose weight 

 but not in size, until all of this water has been driven 

 off. 



la practice it is tliis latter portion of the moisture 

 that evaporates during the first period of the burning 

 known as water smoking. 



The air shrinkage of a clay varies with the nature 

 of the material. Sandy clays usually show the least 

 shrinkage, and of this kind the coarse grained ones 

 diminish the least in size, while highly plastic clays 

 usually show a high contraction in volume. 



The amount of water, which a dry clay needs to 

 develop its maxium plasticity is a variable quantity. 

 Plastic clays absorb a large amount, but a lean clay 

 and fine grained one may do the same. As a very gen- 

 eral rule it may be stated that lean clays absorb from 

 twelve to twenty per cent, of water, while fat clays 

 anywhere from twenty to fifty per cent., and the more 

 water a clay absorbs the more it has to part with in 

 drying and the greater will be its shrinkage. 



If green ware is dried too rapidly it may split, not 

 only from differential shrinkage between the exterior 

 and the interior surface, but the rapid escape of steam 

 may, in the first stage of the burning, tend to burst 

 the ware. 



Highly aluminous clays do not always absorb the 

 most water, nor are they the most plastic, and some 



