552 NEW YORK STATE MUSEUM 



clays they may not be over 50° F. apart, wliile in refractory clays 

 they are separated sometimes by an interval of Y00° or 

 800° r. The glass pot clays approach the/ latter condition. The 

 majority of clays show a difference of 200°-400° between the 

 points of incipient fusion and viscosity. 



• The practical bearing of this will be easily seen, when one re- 

 members that in the manufacture of many kinds of clay products, 

 the body has to be vitrified. Consequently the greater the differ^ 

 ence between the temperature of vitrification and that of viscosity, 

 the easier will it be to bring a kiln of ware up to the one without 

 overfetepping it and reaching tihe other, for kilns can not be regu- 

 lated within a range of a few degrees of temperature. 



In many clays the point of vitrification is midway between in- 

 cipient fusion and viscosity, but in others it is not. 



Temperature of fusion. The fusibility of a clay depends on: 



1 The amount of fluxes 



2 The size of the grain of the refractory and nonrefractory con- 

 stituents 



3 The condition of the fire, whether oxidizing or reducing 



All the fluxing impurities do not act with the same intensity. 

 Eine-grained clays fuse at lower temperatures than coarse-grained 

 ones, other things being equal. 



In order to express the relative fusibility of clays numerically, 

 Bischof^, on the assumption that the refractoriness of a clay ig 

 directly as the square of the alumina and inversely as the silica and 

 fluxes, deduced the following formula, in which F. Q. stands for 

 the ''refractory quotient". 

 F Q = IM^Pi^' 



SiOgXRO 

 This has been found incorrect when there is a variation in the 

 fineness and density of the clay, and in order to recognize the effect 

 of these two features, "Wheeler^ has suggested the formula: F. F.= 



, in which F. F. is called the fusibility factor. 



D+D^+C 



'i- Die feuerfesten thone, p. 71. 1876. 

 2Eng. and min. jour. 10 Mar. 1894. 



