12 GENERAL DISCUSSION OF CLAYS. 



evaporation of the moisture of the clay. They may be 

 rendered insoluble by the addition of chemicals to the 

 clay. In addition to its unsightliness the efflores- 

 cence may interfere with the adhesion of a glaze ap- 

 plied to the surface of the ware. 



Soluble alkaline sulphates are powerful fluxes and 

 they also cause blistering of the ware, if the clay is 

 heated sufficiently high to decompose the compound 

 and permit the escape of sulphuric acid gases. 



In some clays containing sulphate of iron, this com- 

 pound may be decomposed by chemical reaction tak- 

 ing place in the clay; the sulphuric acid, which is 

 thus set free, is apt to attack the alumina of the clay 

 substance and if potash, soda, or ammonia 's present 

 there is formed an alum of potash, soda or ammonia, 

 which can often be detected by the taste which ic im- 

 parts to the clay. 



Insoluble alkaline compounds. Feldspar and mica 

 which are the commonest of rock forming minerals 

 are the two important sources of insoluble alkaline 

 salts in the clay. 



The feldspars are complex silicates of alumina and 

 potash, or alumina, lime and soda. Orthoclase is the 

 only species furnishing potash and contains about 17 

 per cent, of it while the lime-soda feldspars have from 

 4 to 14 per cent, of soda depending on the species. 



Orthoclase is the common feldspar, and next to it 

 come albite end oligoclase with 12 and 14 per cent, of 

 soda respectively. 



The micas are complex silicates of alumina with 

 either lime or magnesia or potash. Muscovite, the 

 common species, contains nearly 12 per cent, of pot- 

 ash, and may at times also contain soda, While the 

 potash feldspar fuses completely at about 2300 Fahr., 

 the potash mica alone is very refractory and unaf- 

 fected by a temperature of 2550 F^hr., and though 

 it probably serves as a flux, it is not definitely known 

 at just what temperature its action begins. 



The alkaline silicates on account of t'heir fluxing 

 properties are frequently at an advantage, especially 

 if in the form of feldspar, as they serve in burning to 



