CLAYS OF NEW YORK 787 



should also burn dense at so low a temperature tliat when grog is 

 added the heat will not need to be raised too much in getting the 

 required density. The addition of grog wiU raise this point to an 

 extent depending on the amount added. Thus the temperature 

 of densification of a mixture given below is the same as cone 5, 

 while that of the clay is cone 1. If now a clay is used as binding 

 material which sinters at high temperature, the temperature at 

 which the mixture becomes dense will be so high as to make its 

 burning difficult. 



In judging the tensile strength, the size of the grain of grog must 

 be considered, as also the relation in which the different sized 

 grains are mixed, but no fixed rule can be laid down for the last 

 point. In the grinding of grog both a powdery product and angu- 

 lar grains are obtained, and practice has shown that it is desirable 

 to add both of these to the clay, since, if the grains alone were 

 added, the mixture would show a tendency to crack. 



The following mixture is one given by E. Cramer (Thonindus- 

 trie zeitung. 1897. j). 47) : 100 parts by weight of clay and 120 

 parts grog. On a sieve of 10 meshes to the square cm the grog 

 left no residue, but 20^ remained on a 60 mesh sieve, and 12^ on 

 one of 120 mesh, 24^ on a 900 mesh, 30^ on a 5000 mesh, and 14:^ 

 went through. 



The investigation of glass-pot clay is confined to a determination 

 of plasticity, shrinkage, temperature at ' which the clay becomes 

 dense, fusion point, and chemical composition. 



Clays fulfilling all these conditions satisfactorily are rare in 

 the United States. 'They are thus far known in only a few regions, 

 being found in Missouri and in small quantities in Ohio and Penn- 

 sylvania. In Europe they occur at several localities, in Germany, 

 Belgium, Bohemia, Russia, England, Erance, and Scotland. 



Large quantities of the German and Belgian glass pot clays are 

 annually e-xported to the United States^. 



1 For information concerning the properties of some of these European- 

 glass pot clays, see Report on kaolins and fire clays of Europe. \Wi ann. rep't 

 U. S. geol. SUV. pt 6. 



