794 NEW YOEK STATE MUSEUM 



Porcelain 



The same materials are used as in the manufacture of white 

 granite, but thei proportions are usually different; the war© is 

 burned to vitrification, bo that the body is transparent, and the 

 fracture of it would show a vitreous luster. 



Porcelain which is fluxed by feldspar is spoken of as spar china. 

 It shows a slightly yellowish color by transmitted light, while 

 porcelain fluxed by calcined bones in part, replacing the feldspar is 

 spoken of as bone china,. It shows a bluish white color by trans- 

 mitted light. 



The proportion of fluxes is greater in porcelain than in white 

 earthenware ; but still, taking porcelains as a whole, there is a wide 

 range in their composition, as will be seen from the following 

 figures representing the range of the ingredients used in the manu- 

 facture of hard porcelains. (Hecht.- Dammer, Chem. Tech. 

 1 :773 and following ) 



Per cent 



Clay substance 40-66 



Quartz , 12-40 



Feldspar 15-30' 



Carbonate of lime (at times) . 6 



The variation outside of these limits should be very small, for if 

 the clay substance gets below 40^, the refractoriness decreases con- 

 siderably, as does also' the ability of the ware to withstand sudden 

 changes of temperature. 



' As excessive shrinkage in burning tends to cause cracking and 

 warping, one aim of ceramic chemists has been to produce bodies of 

 low shrinkage; and experiments have indicated that the use of 

 porcelain sherds ground up gives a much more homogeneous mass 

 than can be obtained by the use of quartz. (Chemiher zeitung. 

 1895. p. 89) 



