T92 NEW YOEK STATE MUSEUM 



not he less tlian 125 pounds a square incli, tliougli 150 is pref- 

 erable. The clay slionld not shrink excessively in burning, and 

 should give a vitrified body at not over 2100° E. if possible, for the 

 lower the temperature of vitrification the greater economy in fuel. 

 The clay should however be sufficiently refractory to hold its form 

 at the temperature required to melt the glaze, and not do more 

 itself than vitrify at that temperature. The fusible impurities in a 

 stoneware clay should be sufficiently high tO' cause vitrification. 

 !Ferric oxid forms a desirable coloring ingredient, the same being 

 true of lime if not in excess of 2^-3^. iSulfur in any combination 

 is undesirable, as its escape at high temperatures causes blistering 

 of the ware. 



In mixing two clays, the one is generally used for supplying stiff- 

 ness to the body in burning, and the other, fluxing qualities. 



For common earthenware, almost any plastic clay, one which is 

 not too coarse, suffices. 



If the ware is to be glazed, the clay should be sufficiently re- 

 fractory, so that at the temperature required to melt the glaze, it 

 will not bum to more than incipient fusion. 



Analyses of stoneware clays are given in the table at the end of 

 this report. In addition, are given here the average of 10 stoneware 

 clays now in use. (E. Orton jr. Clays of Ohio, Ohio geol. sur. 

 V. Y, pt 1, p. 95) 



Clay base 56 . 65 



Sand 37.45 



Fluxes 4 . 44 



Moisture 1.57 



100.11 

 Total silica 65 . 09 



