CHEMICAL PROPERIIES OF CLAYS. 11 



of the fluxing impurity. Eefractory clays, on the 

 other hand, should contain a low amount of fusible 

 substances:. Porcelain clays might have as high a per- 

 centage of fluxes as 5 or 6 per cent., provided they did 

 not exert a coloring action on the clay. 



ALKALIES IN CLAYS. 



The alkalies usually contained in clays are potash, 

 soda and ammonia. , , 



Ammonia is a very common consitituent of mpjst 

 clay and is absorbed by the latter with great avidity ; 

 indeed it is largely responsible for the qharacteristic 

 Oder of clay.* 



If the 'ammonia remained in the clay, it would act 

 as a strong flux, but its volatile nature renders it 

 harmless, for it passes off as a vapour at a temper- 

 ature considerably below dull redness, and in fact may 

 even volitilize with the moisture of the clay during the 

 early stages of burning. 



Potash and soda on the other hand, which volati- 

 lize only at a high temperature, are present in almost 

 every clay from the smallest amount up to 9 or 10 per 

 cent, and of these potash is by far the commoner of the 

 two. Their variable percentage may be caused by the 

 presence of more or less undecomposed feldspar, of 

 which f>fthoclase, the common species, has nearly 17 

 per cent, of potash while the other feldspars contain 

 varying amounts of soda. 



These alkalies may be present in the clay in the 

 form of either soluble or insoluble compounds*, the 

 latter being represented by feldspar, mica, or other 

 minerals, while the soluble ones are usually the result 

 of their decomposition. 



Soluble alkaline 'compounds may be found in almost 

 any clay, but they are rarely present in large amounts, 

 and their chief importance lies in 'the fact that they 

 are often responsible for the formation of an efflor- 

 escence or white coating on the surface of the ware, 

 they having become concentrated on the surface by the 



*F. Senft, Die Thon Substanzen, p. 29. 



