CLAYS OF NEW YORK 513 



Ammonia. Olays possess a strong absorptive capacity for gases 

 and in consequence of this frequently contain an appreciable 

 amount of ammonia, to wbicb is largely attributable tbe character- 

 ietic odor of clay.^ 



While the presence of tliis compound may exert some action on 

 the plasticity and absorptive power of the clay, still it need not be 

 considered in burning, for it passes off as a vapor at a temperature 

 considerably below dull redness, or may even volatilize with the 

 moisture of the clay during the early stages of burning. 



Fixed alkalis. These include potash, soda and lithia, but the 

 latter is such a rare constituent that it need not be considered. 

 Potash and soda are present in nearly every clay, in amounts vaiy- 

 ing from a mere trace to 10/^, but the usual average is l^-d/c The 

 chief sources of potash and soda are the different species of feld- 

 spar; white mica ot muscovite may furnish potash. The variation 

 in amount might be accounted for by the presence of undecompoeed 

 feldspar in the clay, the common feldspar orthoclase containing 17 fo 

 of potash alone. 



AVhen either feldspar or mica decomposes, the alkalis are con- 

 verted wholly or in part into soluble compounds, and thus we get 

 both soluble and insoluble alkaline compounds. 



Soluble alkaline compounds. These may be present in any clay, 

 but they seldom occur in large quantities. They may influence the 

 plasticity of the clay, by causing a flocculation of the particles; but 

 their chief importance, or disadvantage, is in giving rise to the 

 formation of efflorescence on the surface of the ware, where they 

 become concentrated by the evaporation of the moisture in the clay, 

 unless previously rendered insoluble by the addition of proper 

 chemicals. This crust may interfere with the formation of salt 

 glaze, or the adhesion of a glaze applied to the ware before burning. 



Soluble alkaline sulfates are powerful fluxes. They cause 

 blistering of the ware if the clay is heated sufficiently high to de- 

 compose the sulfate and permit the escape of sulfuric acid gas. 



g[i F, Senft. Die Thonsubstanzen p. 29. 



