CLAYS OF NEW YORK 795 



One mixture of this type is as follows : 



Composition of porcelain mixture for the production of bodies of low 



shrinkage 



Parts by 



weight 



Quartz sand. 120 



Feldspar (IsTorwegian) 85 



Marble 3 



Zettlitz kaolin 60 to TO 



Porcelain sherds 20 to 60 



It has also been found that porcelains rich in fluxes are soft, 

 while those poor in these ingredients are hard ; neither do the most 

 plastic masses always show the greatest shrinkage. The shrink- 

 age of Soger's porcelain, which is rich in fluxes, occurs mostly in 

 drying, and the total linear shrinkage is lOfo ; it expands in firing 

 when a certain temperature is reached, owing to the high percent- 

 age (45;^) of quartz which it contains. Plastic clays give a very 

 smooth surface and are difficult to dry, and are not adapted to the 

 manufacture of large pieces. A mixture poor in fluxes, with a 

 high shrinkage, can be very lean when it contains no sedimentary 

 clay but kaolin as the plastic. element. Bodies of good plasticity, 

 but low in fluxes, are of comparatively recent introduction, and 

 are specially adapted to large objects and chemical stoneware.-^ 

 Owing to its high percentage of clay substance and low fluxes, 

 the mass acquires little translucency when burned at high tem- 

 peratures, but stands temperature changes very well. 



Hecht has recently published the results of some rather detailed 

 investigations on the composition of porcelains and white earthen- 

 ware bodies poor in lime. It has generally been considered that 

 these two classes of ware vary in composition only between fixed 

 limits, and that the predominance of feldspar in the mixture was 

 generally confined to porcelain. This, however, has proved to be 

 an error, and Hecht finds porcelains which are low in feldspar, 

 and earthenware bodies rich in it. The following examples are 

 given.^ 



' Chemiker zeitung. 1894. p. 821. 



^ Thonindustrie zeitung. 1897. no. 21, p. 714. 



