t;so 



Report of the State Geologist. 



twenty or thirty per cent. Air drying expels most of the contained moisture 

 and at the same time a shrinkage of the material takes place. Sandy, coarse 

 grained clays show the least shrinkage, but fine grained ones may sometimes 

 show considerable diminution of volume on air drying. The larger the 

 quantity of water absorbed by a shale in tempering, the more will it shrink 

 in drying. If the clay is fine grained, rapid drying may cause it to split. 

 The last traces of moisture are generally driven off in the kiln during the 

 early stages of burning. 



Combined water. This is present in all shales. It usually varies from 

 three to eight or ten per cent, in shales, depending on the amount of clay 

 substance and perhaps other hydrated minerals present. Combined water is 

 driven off at a low red heat, or about 1,200° F., and with the passing off of it 

 there also begins a second shrinkage. While the amount of combined water 

 does not stand in any close relation to the plasticity, nevertheless when once 

 the combined water is driven off, the clay can no longer be rendered plastic. 



Physical Properties. 



These are considerations of as much importance as the chemical ones, for 

 they exert fully as much influence on the characters of clay or shale. The 

 important physical properties which should always be considered, are 

 fusibility or behavior under heat, plasticity, tensile strength or cohesion and 

 absorption, 



Elasticity. This is one of the two important properties which makes 

 clay of such use to man, for it permits of molding it into any desired form, 

 which is retained when the clay is dried. Plasticity, however, is a variable 

 property, some clays possessing it to only a slight degree, others having 

 it highly developed. The former are called " lean," the latter " fat." 

 Very fine and very coarse grained clays are generally lean. An excess of sand 

 also tends to diminish the plasticity, and consequently very siliceous shales, 

 those passing into shaly sandstones, should be avoided, or should not be used 

 unless mixed with more plastic material. Organic matter frequently increases 

 the plasticity of a day and makes it very fat, unless there is an excess of sand 

 present. If water be gradually added to dry clay and the mass thoroughly 

 kneaded, it will be found that its plasticity increases up to a certain point, 

 but if more water is added it begins to decrease until finally the clay runs 

 like soft mud. The amount of water absorbed by shales or clays depends in 

 general on their plasticity, very "fat" ones requiring the addition of a large 



