550 NEW YOKK STATE MUSEUM 



shrinkage of the clay is not only arrested, but the alkalis are also 

 volatilized, whereby the hard burned clay becomes more refractory. 

 This is the case in Belgium, where the clay used as chamotte is 

 burned twice. (Bischof. 2d ed. p. 265) 



In the case that cast off wares are used, it is necessary to see 

 that the pieces have not become impure by any slagging action 

 that may have taken place in the kiln. Powdered fire bricks are 

 sometimes used as chaiuotte. 



Graphite and coke. These are materials that are often added 

 to clay to increase the refractoriness, but they also serve the pur- 

 pose of imparting to the ware a greater heat conductivity and 

 making it more resistant to^ changes of temperature. It is this 

 last property that makes a. mixture of clay and graphite specially 

 adapted to the manufacture of crucibles. The graphite should con- 

 tain 90^ or more of carbon. It should also be intimately mixed 

 with the clay. 



Coke is sometimes used instead of graphite, but is less refrac- 

 tory, and works best in materials which in use are not in contact 

 with the air. 



Sawdust. This also acts as a diluent but, unlike the others, burns 

 when subjected to a high temperature, and leaves a cavity behind. 

 It is therefore necessary that the particles should not only be small, 

 but of even size. Sawdust and similar substances leave more or less 

 ash behind, whose mineral constituents may act as fluxes. As, 

 owing tO' the formation of these pores, the clay may be somewhat 

 weakened or loosened, it is necessary that only very plastic clays or 

 those of high tensile strength should be used. 



Fusihility 

 Change on heating. In the heating of a clay, or subjecting it to 

 a gradually rising teiuperature, it not only shrinks but begins to 

 harden. If raised only to a temperature sufficient to drive off all 

 moisture, the clay will still be soft enough to permit its being 

 scratched with the finger nail. If the temperature is raised still 



