562 NEW YORK STATE MUSEUM 



character of tlie clay, tliOTigli tlie primary changes are of a chemi- 

 cal nature. In kaolins this change simply amounts to the loss 

 by the kaolinite or clay substance of its combined water. In 

 impure clays many other changes may occur, viz: 



The burning off of organic matter, 



Limonite losing its water and becoming hematite. 



Pyrite (FeS^) becoming oxidized to ferric sulfates, which in 

 turn are broken up by the expulsion of their sulfur, leaving hema- 

 tite or ferric oxid. Both lime and magnesium carbonates if pres- 

 ent will part with their carbon dioxid. 



The general effect of these changes is first to make the clay more 

 porous, but subsequently to increase its shrinkage. The color of 

 the clay is also changed. A chemical interaction between the com- 

 ponent minerals of the clay has not taken place up to this point. 



It is held by Seger^ that the more plastic a clay is when wet, the 

 harder it will be after light burning. Such lightly burned wares 

 will not, however, withstand weathering or pressure, and are very 

 porous; resistance to weathering is attained only when certain por- 

 tions of the clay fuse, and unite the whole into a stony mass. 



The shrinkage and decrease in porosity will be the greater, the 

 larger the number of particles taking part in the fusion of the 

 mass. 



The process of fusion involves two separate processes, one physi-. 

 cal, causing change in volume, and one chemical, giving rise to the 

 formation of new compounds in the mass. These have a lower 

 fusing point than the substances through whose interaction they 

 were formed. In some cases however it is probable that solution 

 takes place. 



Trom the foregoing it would appear that the fusion of a clay 

 is influenced not only by the melting point of the most easily fusible 

 component of the clay, but also by the relative amount of infusible 

 ingredients, and the relative size of the fluxing and nonfluxing 

 particles. In the earlier stages of fusion we must therefore look 



1 Seger. Oes Schrift. p. 380. 



