528 NEW YOKK STATE MUSEUM 



Organic matter may alsO' mask tlie presence of iron, sO' that the 

 clay, instead of burning white, would burn red at a temperature 

 above that at which the organic matter passes off. Below that 

 temperature, though, the vegetable matter would tend to keep the 

 iron reduced, and the color would be gray instead of red. 



In most chemical analyses the organic matter is seldom de^- 

 termined separately, but the amount of it can sometimes be judged 

 from the ratio between the loss on ignition and amounti of alumina 

 in the clay. 



Organic matter may increase the plasticity of a clay provided too 

 much sand is not present, in which case a highly carbonaceous clay 

 might be very lean, (see " Plasticity of clays ", p. 539) 



Water in clay 



All clays contain twO' kinds of water: 



1 Hygroscopic water, or moisture 



2 Chemically combined water 



Moisture, Clays contain two kinds of moisture: 



1 That which is held in the pores of the clay by capillary at- 

 traction, 



2 That which adheres to the surface of each clay grain as a thin 

 film. 



The latter is of little; importance practically. 



The former is of importance in connection with the shrinkage 

 and plasticity of clays. The amount of total moisture contained in. 

 clays varies within wide limits. In some air-dried clays it may be 

 as low as ,5^, while in those freshly taken from the bank it may 

 reach 30^ or 4:0 fc Capillary moisture is absorbed by clays only when 

 they are brought into actual contact with water, but that which 

 forms a film on the surface of the clay particles is readily absorbed 

 by the clay from the atmosphere, and to a certain extent given off 

 again as readily, so that some days a brick if left exposed to the 

 air would weigh more than on others. The amount of either kind 



