PHYSICAL PROPERTIES OF CLAYS. 29 



consequently the heat can be raised rapidly in this in- 

 terval, but above and below these two points it must 

 proceed slowly to prevent cracking or warping of the 

 ware. 



FUSIBILITY OF CLAYS. 



It can be said in general, that other things being 

 equal, the fusibility of a clay will increase with the 

 all the fuxing impurities do not act with the same in- 

 approximate statement however, for in the first place 

 all the fluxing impurities do not act with the same in- 

 tensity, and of two clays containing the same amount 

 and kind of fluxes, that one which has the finer grain 

 will usually fuse at the lower temperature, in addition 

 to this the condition of the fire, whether oxidizing or 

 reducing, also exerts an effect. 



White mica tends to increase the refractoriness of 

 a clay, and to exert very little fluxing action even at 

 moderatly high temperatures. 



As a clay is gradually heated, it not only shrinks, 

 but also begins to harden. At the temperature at 

 which the combined water begins to pass off, the im- 

 pure clays acquire such a degree of hardness that they 

 can no longer be scratched by a knife ; but 'n the case 

 of purer clays, the temperature must be raised much 

 higher to obtain this same degree of hardness. This 

 condition is brought about by the clay particles be- 

 ginning to soften under the action of the heat, in other 

 words it represents the very first stages of melting or 

 incipient fusion, and in this condition the clay parti- 

 cles stick to each other, and bind the whole together 

 into a solid mass. In clays which have been burned to 

 incipient fusion, th' particles are however still rec- 

 ognizable. If the temperature be increased, a vari- 

 able amount, depending upon the clay, the result is 



