CHEMICAL PROPERTIES OF CLAYS. 19 



When in large amounts, gypsum discloses its presence 

 by the formation of transparent crystals or crystallne 

 masses, whose surface shows a pearly lustre; at ot'her 

 times it forms as parallel fibres which fill cavities or 

 cracks in the clay. Gypsum may prove to be a very 

 injurious impurity even when in small amounts, es- 

 pecially if the clay is not burned to vitrification. 



In the first place it serves as a fluxing impurity, 

 secondly, ,it is dissociated at high temperatures, 

 and the escape of the sulphuric acid causes blistering 

 of the ware, and thirdly, although nearly insoluble in 

 water, nevertheless small amount of it may be brought 

 to the surface of the ware in solution by the evapora- 

 tion of water and there left in the form of a white coat- 

 ing. 



Kaolins commonly have very little lime, but in many 

 common brick and stone ware clays, it frequently 

 ranges from one to three per cent. 



MAGNESIA IN CLAYS. 



Magnesia is a constituent of many minerals, and yet 

 it seldom occurs in large quantities, the amount in 

 most of them rarely exceeding two per cent. 



It may occur, in the same classes of compound as 

 lime i. e. silioates, such as mica, chlorite, hornblende 

 and pyroxene; in carbonates, such as dolomite and 

 magnesite; and in sulphates, such as epsom salts. 



The silicates are, no doubt, the most important 

 source of magnesia, for mica, chlorite, and hornblende 

 are all common constituents of the more impure clays. 

 They are scaly minerals of complex composition and 

 contain from 1 to 25 per cent, of magnesia, The 

 mica is frequently to be noticed in the sandy seams of 

 the clay, while the other portions of the deposit may 

 be quite free from it. Hornblende and pyroxene are 

 to be looked for mostly in clays derived from the dark 

 colored igneous rocks, and indeed the two latter min- 

 erals not only furnish magnesia, but by their decom- 

 position furnish also iron oxide to the clay. 



Dolomite, the double carbonate of lime and mag- 



