THE PRODUCTS OF CLAY AND FIRE 



small quantities of lime, magnesia, traces of iron, and 

 from four to six per cent, alkali. It is prepared for 

 use by grinding between millstones. 



Flint, which is classed as an "indifferent substance," 

 is an oxide of silicon (SiO 2 ) which contains certain 

 organic substances, and sometimes iron, in its natural 

 state. It is used in the ware to prevent contraction 

 and give whiteness. It is widely distributed in the 

 earth's crust, but the best flint for the potter's use is 

 obtained near Dieppe, in France. It is prepared by 

 calcining in furnaces, then crushed in a stone-crusher 

 or stamp-mill, and finally ground between millstones. 

 As in the case of all substances that are ground for use 

 in the potteries, this grinding process is a delicate one, 

 from the fact that impurities may be introduced. Thus, 

 if the millstones contain an excess of lime, or iron, 

 or coloring matter, the ground product may acquire 

 these substances, and thus be rendered unfit for use in 

 making fine pottery. And this might not be discovered 

 until the ware had been molded and fired, involving 

 great loss of time and material. 



The determination of the proper degree of fineness 

 in grinding is made by passing the substance through 

 silk or wire lawn, although an expert can tell the con- 

 dition with wonderful accuracy by testing it between 

 his teeth or nails. When no grit can be detected the 

 substance is fine enough for the potter's use. 



No matter how pure the natural clays may be, there 

 is always present a trace of the oxide of iron, which 

 would give the ware a yellowish tinge after firing if 

 not counteracted by the use of a "stain," as it is tech- 



[237] 



