538 NEW YORK STATE MUSEUM 



Whether it will be of practical advantage to carry out a rational 

 analysis to this extent still remains to be seen. In its simpler form, 

 however, when applied to high grade clays, the rational analysis has 

 been found to possess great practical value, owing to the fact that if 

 two clays have the same rational composition they will, other things 

 being equal, behave much alike when burned. This fact is made 

 use of by the potter, for example, in the preparation of his porcelain 

 or white earthenware mixture, also by manufacturers of encaustic 

 tiles, fire brick, etc. 



To illustrate this point we may take the manufacture of porce- 

 lain. Porcelain is made from a mixture of kaolin, quartz and 

 feldspar. Suppose that we are using for the manufacture of porce- 

 lain or fire brick a kaolin which has 67.82^ of clay substance, 

 30.93 of quartz, and 1.25 of feldspar, and that to 100 parts of this 

 are added 50 parts of feldspar. This would give us a mixture of 

 45.21^ of clay substance, 20.62 of quartz, and 34.17 of feldspar. 



If now for the clay we had been using we substituted one with 

 66.33^ of clay substance, 15.61 of quartz, and 18.91 of feldspar, 

 and made no other changes, the mixture would then contain 44.22^ 

 of clay substance, 10.41 of quartz, and 45.98 of feldspar. 



This last mixture shows such an increase in feldspar that it must 

 give much greater shrinkage and fusibility; but, knowing the 

 rational analysis of the new clay, it would be easy, by making a 

 simple calculation, to ascertain how much quartz or feldspar should 

 be added to bring the mixture back tO' its normal composition. 



Physical properties 



These properties are fully as important as the chemical, if not 

 more, plasticity for instance being one of the two' characters in 

 clay which make it of such inestimable value to^ man. Similarity 

 in chemical composition counts for little in the comparison of two 

 clays, if they do not agree in physical characters. 



The physical properties of the most importance from a practical 

 standpoint are plasticity, fusibility, shrinkage, tensile strength, 

 slaking, absorption and density. 



