THE PRODUCTS OF CLAY AND FIRE 



The essential qualities of the body mixture, as stated 

 by Sandeman, are as follows: 



"It must be sufficiently plastic to be easily work- 

 able. It must be sufficiently infusible to prevent col- 

 lapse in the ovens, but sufficiently fusible to become 

 dense and sonorous. It must have sufficient stability 

 to resist excessive contraction and must not become 

 crooked. It must be sufficiently free from coloring 

 matters to become clean and white after firing." 



The exact proportions in which the various pre- 

 pared materials are mixed, and the method of mixing 

 them, vary, of course, with the results desired, as well 

 as with the individual preferences of the manufacturer. 

 Every manufacturer has formulas which he considers 

 either better, or more expedient for his purpose. 

 Roughly speaking, however, some formula like the 

 following is used in most factories, the quality of 

 materials making a little difference in their relative 

 proportions : 



Blue clay 10-15 parts. 



Kaolin 8-9 " 



Flint 4-5 " 



Stone 2-3 " 



Stain (sufficient to neutralize yellow eolor) 



Each of these substances must first be brought into 

 a state of suspension in water, so that the mixture 

 represents a definite weight to the ounce, and is uniform 

 throughout, after which all the substances are mixed to- 

 gether thoroughly, and sufficient water drained, or 

 pressed out, to leave a plastic mass of the proper con- 

 sistency for molding and working. The mixing process 

 is called "blunging." Formerly it was done by hand, 



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