76 GEOLOGY OF OHIO. 



this point, the clay will be found to continue to shrink and grow more 

 hard and dense and more impervious to water until finally, if the heat be 

 stopped at the right point, the clay will have become practically imper- 

 vious and very hard, strong, and tough. Clay in this condition is a new 

 chemical compound composed of a mixture of all its bases combined with 

 all its acids, forming a mineral as nearly indestructible as any known to 

 man. Small bits of pottery made in the earliest times of prehistoric man 

 come down to us as fresh and unaltered by the centuries of exposure as 

 they were when made. Records made in hard-burnt clay are imperisha- 

 ble except to animate force. If the heat be continued above this most 

 favorable point the clay begins to deteriorate in some of its qualities. It 

 may grow harder but less strong, or it may become spongy and vesicular 

 like lava, or it ma} r melt into a fragile glass, but whatever the change, it 

 is a retrogression. What the temperatures are at which these various re- 

 sults are obtained varies entirely with the nature and composition of the 

 clay under treatment. Some clays require only a low heat to develop 

 their best qualities, others demand the highest heats attainable in metal- 

 lurgical practice. 



Refractoriness or infusibility is the last property of claj^s which 

 makes them of such great value to man. This subject naturally becomes 

 involved with a consideration of the causes and conditions which contribute 

 to permanence, as in both, the action of the clay under fire is the subject 

 of discussion. The qualities which a clay develops while burning are 

 obviously largely dependent on its chemical and mineralogical composi- 

 tion. As has been pointed out, the heat which is applied and the way it 

 is applied and the kind of atmosphere which surrounds the clay under 

 heat, have an important bearing on the qualities it shall develop. But 

 the composition is of first and foremost importance and not only the ulti- 

 mate composition but the method of combination of its various parts. 



Kaolin or the kaolinite base of clay is practically infusible in the 

 highest heat obtained in metallurgical practice. Quartz, its chief and 

 bulkiest impurity, is likewise infusible in practice. Both can be fused in 

 the flame of the oxyhydrogen blowpipe. But intimate mixtures of clay 

 and quartz, even in the absence of all other impurities, are not as infusible 

 as the ingredients are separately and the application of intense and long 

 continued heat tends to form a new chemical compound having all the 

 silica combined with all the alumina in a new relation of base to acid. 



Fusibility of a clay has been in the past considered as depending 

 mainly on the relative amount and character of its impurities other than 

 quartz. Both quartz and kaolinite being infusible the idea has been that 

 the metalic oxides present formed the entering wedge which destroyed a 

 clay infusibilit}''. There is much to be said in favor of this view, as it is 

 an open fact that clays do stand high temperatures substantially in the 

 order of their purity. 



However, the drift of opinion now seems to be toward the theory that 

 the fire qualities of a clay will depend on its total aggregate composition, 



