THE PRODUCTS OF CLAY AND FIRE 



them into insoluble substances before using. This 

 is done in the process of "fritting/' which will be re- 

 ferred to in a moment. 



Flint and china-clay give hardness, transparency, 

 and depth of tone to the glaze. Carbonate of lime, 

 in the form of chalk, makes the glaze harder and im- 

 proves the color. It does not promote fusibility as 

 readily as some of the other substances, borax (bi- 

 borate of soda) heading the list in this respect. Indeed, 

 borax is an absolutely indispensable substance to the 

 potter, and in recent years the cost of its production 

 has been greatly lessened, thanks to the work of prac- 

 tical chemists. Borax not only facilitates the making 

 of the glaze, but gives great brilliancy to the finished 

 product. 



The fritting process, by which the soluble substances 

 of the glaze are vitrified, is done by subjecting the sub- 

 stances to direct flames in a kiln. This kiln is a tank 

 made of fire-brick, so arranged that the flames coming 

 from the fire-box are reverberated down over the ma- 

 terials to be fritted. This reduces it to a mass of molten 

 glass which is then drawn off into a tank of cold water. 

 The plunge into the cold water breaks the stream of 

 molten glass into small particles, which are more easily 

 pulverized by the subsequent grinding process. The 

 deeper and colder the water into which the plunge is 

 made, the finer will be the frit particles. 



The frit is ground between specially prepared mill- 

 stones, or in a more modern machine, the Alsing 

 cylinder. The particles are moistened and ground 

 until every trace of grit has disappeared, the finished 



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