CHEMICAL PROPERTIES OF CLAY. 75 



It has not been determined definitely whether finely divided 

 organic matter exerts any influence on the plasticity or tensile 

 strength, but it, no doubt, increases the absorptive power of the 

 clay for water. 



In most chemical analyses the organic matter is rarely deter- 

 mined separately, but is included in the "loss on ignition," for, 

 in fact, the amount is commonly so small as not to require 

 separate determination. 



Soluble Salts. 



Origin. — It has been pointed out in Chapter I (Origin of 

 Clay) that in the decomposition of mineral grains in clay, solu- 

 ble compounds are often formed. During the drying of the clay 

 the moisture brings these to the surface and leaves them there 

 when it evaporates, thus forming a scum on the air-dried ware 

 and sometimes a white coating on the clay after it is burned. 

 Those found in the clay are commonly sulphates of lime, iron 

 or alkalies, and their formation is generally due to the decom- 

 position of the iron pyrite frequently contained in the clay. A 

 much greater quantity of soluble sulphates will be formed if 

 the pyrite is in a finely divided condition and evenly distributed 

 through the clay, but these compounds may also 1 be formed with- 

 out the aid of pyrite, as when the carbonates are set free by the 

 decomposition of silicates such as feldspar. When the soluble 

 compounds have formed in the green clay their presence can often 

 be detected by spreading the dug clay out to weather, which will 

 result in their forming a crust on the surface of the mass. 



Their formation does not cease, however, when they are 

 removed from the ground, for in some cases fresh pyrite grains 

 remain in the clay after mixing, and if the clay is stored in a 

 moist place these may decompose, yielding an additional amount 

 of soluble material. One means of preventing this would seem 

 to be to use the clay as soon as possible after mixing. 



In some cases soluble sulphates may be even introduced into 

 the clay by the water used for tempering, for distilled water is 

 the only kind that is free from soluble salts. All well and spring 

 waters contain some at least, and if these flow or drain from clays 



