CLAY WORKING INDUSTRIES. 125 



glaze are made manifest in many ways; it may "craze" or even "shiver." 

 The first term is applied to the formation of fine cracks all through the 

 glaze which destroys its beauty and makes-it permeable to liquids. This 

 occurs when the composition of the body and the glaze are not suited to 

 each other, and they contract at different rates on cooling. If the lack 

 of adjustment is great, the glaze will craze at once; if the difference is 

 slight the crazing may begin months after the ware has been sold. If the 

 difference is excessive, and the glaze is a thick heavy one and the ware is 

 rather weak or porous in structure the contraction of the glaze is some- 

 times powerful enough to crush the piece, or chip pieces out of the sur- 

 face of the ware. This is called "shivering". 



Glazes may also devitrify, or become opaque or stony in nature 

 instead of clear and glossy. 



The adjustment of a glaze to a body would not be beyond the range 

 of even ignorant experimenting, if the body and glaze could be kept 

 always the same, but the composition of these elements is bound to con- 

 stantly vary; the purest kaolins vary just as poor clays do; and, above all, 

 the composition of the felspar, which is depended on to unite the 

 elements of the pottery into a fused or vitrified material, is subject to 

 considerable fluctuation. 



Consequently, the same body and glaze will work well or ill, even if 

 mixed with never failing accuracy in the compounding room, on account 

 of irregularities of the materials. 



It seems that the composition of these bodies could be regulated 

 with the most valuable results by the use of chemistry in the pottery. 

 It would be useful in keeping track of variations of the strength of 

 supplies and still more so, in keeping the body of one uniform composi- 

 tion. 



As before suggested, it is not a chemist that is wanted as such but 

 it is a manager who can understand and use the work of a chemist. 

 The compounding and manufacturing of white pottery is a chemical 

 industry; it deals with chemical material and is governed by chemical 

 laws, and yet the business has been groping along in the dark for decade 

 after decade using the most expensive "cut and try" methods for the 

 accomplishment of every improvement. 



White ware glazes are usually what is called "fret" glazes. The 

 constitutents have been weighed out, mixed and melted in a sagger into 

 a fluid, which solidifies into a solid cake when cool. 



The sagger is then broken off from the cake of glaze which is 

 broken up and ground with the addition of a certain amount of other 



materials. 



The grinding of the fret is^ done in mills lined with French buhr 

 stone, an intensely hard silicious rock. The reduction to the neces- 

 - sary fineness is a slow operation. 



