THE PRODUCTS OF CLAY AND FIRE 



side, the opening facing a " trial hole" in the oven. 

 As the firing proceeds the fireman removes a ring 

 through the trial hole from time to time by means of 

 an iron rod, cooling it at once by throwing it into cold 

 water. Knowing the amount of contraction caused 

 by certain temperatures, the coloring effects, the con- 

 dition of the fractured edge, etc., at the various stages, 

 he is thus able to gage the temperature of his oven. 

 And as the trials are placed in several different posi- 

 tions in the oven he can ascertain easily by comparison 

 whether his oven is being heated evenly. 



A more scientific method is to use specially prepared 

 substances that melt at known temperatures. These 

 are made in the form of cones for convenience, and 

 arranged in a graduated series, each of its several cones 

 requiring a different degree of heat for melting. The 

 melting point of the most resistant one represents the 

 necessary degree of heat for properly firing the ware. 

 Such cones are entirely practical, but are more expensive 

 than the trial rings, or trial pieces of clay of various 

 kinds, and for this reason are not in general use for 

 ordinary firing. 



In England a gage invented by Wedgwood some- 

 thing like a century ago is extensively used. In this 

 gage the property of heated clay to contract a definite 

 amount at certain temperatures is taken advantage of. 

 The gage is a piece of metal in which is a long groove, 

 tapering from one end to the other, and marked off in 

 degrees. Bits of clay that fit exactly into a definite 

 point in this groove before it is placed in the oven are 

 used. As the temperature is raised the clay bits slide 



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