CLAYS OF NEW YOKK 797 



f 



that a great number hold, without crazing, on bodies having the 

 following composition, whether burned in a hard porcelain fire 

 or moderate white earthenware fire: 



Composition of bodies on which many earthenware and porcelain glazes 



do not craze 



Per cent 



80 ... . Clay substance . 30 



20 ... . Quartz and feldspar 70 



The degree of tenacity with which the glazes hold, depends on 

 the temperature at which the biscuit and glazed ware are burned,, 

 and to a greater or less extent on^ the relative amounts of kaolin 

 and plastic stoneware clay in the body. 



The practical value of the above observations is that it points 

 toward much greater possibilities in underglaze decoration, for 

 while in the past such work could only be done under hard fire 

 glazes, we can now paint the porcelain with underglaze colors 

 hitherto used only for white earthenware and cover them with 

 easily fusible muffle glazes. 



The effect of excessive grinding on the ingredients of a porcelain 

 mixture has recently been shown to be serious. (Sprechsaal. 

 1896. no. 29. p. 812) It was found that, when a mixture of 

 kaolin, quartz and feldspar was ground in a ball mill for 120 

 hours, the ware in burning became blistered and showed a finely 

 vesicular structure throughout. If on the other hand only the 

 quartz and feldspar were ground for 96 hours, and the kaolin 

 then added, the result was a strong, translucent porcelain of normal 

 color, free from blisters. The experiments suggest how porce- 

 lains which are only slightly transparent can be made more so. 



Delft ware. This name was originally applied to a white ware 

 made at Delft in Holland, which was ornamented with blue de- 

 signs representing Dutch scenery. It is extensively manufactured 

 at many localities at the present day, the body of the ware being 

 porcelain or white earthenware. The articles which are usually 

 decorated under the glaze include clocks, vases, jardinieres, toilet 

 articles, etc. 



