CLAYS OF NEW YOEK 549 



to quartz sand, both artificial and natural flint, which is the 

 amorphous form of quartz, often furnishes a grog of splendid 

 purity. Quartz when exposed to high temperature gradually passes 

 from a crystalline to an amorphous condition and in so doing ex- 

 pands. 



The more nearly round the grains of sand, the greater will be 

 the interstitial space. In the case of quartz sand this amounts to 

 35^ or 40^. In fine mica sand (glazing sand, for instance) it may 

 reach 50^; and the more mica that is mixed with quartz sand the 

 greater is the amount 'of interstitial space, and the lighter the 

 weight. 



Dtimmler gives the following figures for one liter of sand, the 

 material in each case being compacted by shaking and jarring. 



One liter common moist sand, 1.61 kilograms. 



One liter fine quartz sand, 1.57 kg.; porosity, 35/^ to 40/^. 



One liter fine chamotte flour from hard burned material, 1.43 kg.; 

 porosity, 45^. 



One liter mica sand (glazing sand), 1.30 kg.; porosity, about 

 50^. 



One liter of the finest gTOund quartz or feldspar flour, 1.16 kg.; 

 porosity, about 56^. 



Chamotte. This is the term applied to burned clay. It pos- 

 sesses all the advantages of quartz as a diluent of the shrinkage, 

 but has the advantage over it that it does not affect the fusibility of 

 the clay, or swell with an increase of temperature. Hence, it does 

 not tend to loosen the structure of the finished product. The clay 

 used for this purpose must be burned to such an extent that it will 

 not shrink on being farther subjected to heat. The degree to which 

 this burned clay is ground depends on the use to which it is put; for, 

 to produce a porous body, it is not ground as fine as it would be if a 

 dense one were to be made. The burned clay used for this purpose 

 can either be ground up bits of broken ware, or can be clay 

 specially burned for this use. Through the hard burning of clay, 

 or the repeated burning of some wares, as in the case of retorts, the 

 3 DeutseJier ziegler-kalencler, 1898, p. 81. 



