CLAY WORKING INDUSTRIES. 131 



cleavage lines, but pursue their way in plane surfaces of larger or smaller 

 area, without any regular arrangement. It is impossible to convey ade- 

 quately by words the peculiarity of fracture which comes from vitrifica- 

 tion. But it is something which the natural sense of any one very 

 readily teaches him to detect by observation. 



The qualities which a clay must have in order to be profitably made 

 into vitrified ware are : 



1st. A combination of refractoriness and fusibility ; there must be 

 refractoriness to enable the ware to stand up and keep its shape unaltered 

 during the process of vitrification. In most clays this process begins 

 somewhere in the neighborhood of a good red heat, and increases in 

 perfection as the heat is increased until the clay melts or until the iron 

 changes its condition and the clay puffs and cinders without undergoing 

 fusion. 



In other words, in order to produce vitrified wares at a profit, there 

 must be a margin of heat-resisting power which shall enable the burner 

 to conclude his operations with confidence as to the degree of non-absorb- 

 tion he has produced without too great fear of having destroyed the shape 

 and condition of the articles under heat. 



Thus in the nature of the case, the burning of clay ware to a vitrified 

 condition is an operation of risk; the two qualities, refractoriness and 

 fusibility are, as it were, in contention in the clay. If the heat is not 

 high enough or maintained long enough, the vitrification will be imper- 

 fect. If it be too high or long continued, the ware will begin to sink out 

 of shape. It will therefore be readily seen that the clay which is valuable 

 to the makers of vitrified ware is one which matures slowly under the 

 action of heat ; one which will begin to vitrify and continue to progress 

 in that condition at a temperature considerably below the point where 

 softening and loss of shape begins. 



2d. Assuming that this proper balance between refractoriness and 

 fusibility is present, the next quality of value is the possession of suf- 

 ficient plasticity to make the working easy and perfect. Many clays fail 

 in this respect. They may have been plastic once, but they have in a large 

 measure lost this quality, and the use of expensive mechanical work is 

 necessary to bring back enough of the quality to make the working of 

 the clay profitable or the results merchantable. Too great plasticity is as 

 much of a drawback as too little, however, for the passage of a plastic 

 material through a die or constricted area is, in the nature of the case, 

 sure to produce a certain rearrangement of the structure of the material, 

 and this structure in the manufacture of clay ware is in every case a 

 detriment to the strength and solidity of the ware. 



Here again is a condition of balance between opposing influences. 

 This condition is, however, one in which the mechanical faults are con- 

 cerned rather than the chemical, and this subject is therefore less import- 

 ant in view of the fact that it is possible to greatly influence the mechanical 

 structure of the clay by varying the treatment to which it is subjected. 



