ioo CLAYS AND CLAY INDUSTRY. 



of fused particles, with a skeleton of iinfused ones. If the pro- 

 portion of the former to the latter is very small there will he a 

 strong hardening- of the clay with little shrinkage, and the burned 

 clay will still be porous. With an increase of temperature, and 

 the fusion of more particles, the pores fill up more and more, and 

 the shrinkage goes on until, at the point of vitrification, the 

 spaces are completely filled. Above this point there is no longer 

 a sufficiently strong skeleton to hold the mass together, and the 

 clay begins to flow. The conditions which influence the differ- 

 ence in temperature between vitrification and viscosity still 

 remain to be satisfactorily explained, but it probably depends on 

 the relative amounts of fluxes and nonfluxes and the size of grain 

 of the latter. 



4. Finally, it is found that the same clay will fuse at a lower 

 temperature, if in burning it is deprived of oxygen, than it will 

 if burned in an atmosphere containing' plenty of the latter. 1 



Classification of clays based on fusibility. — The fact that dif- 

 ferent clays fuse at different temperatures makes it possible to 

 divide them into several different groups, the ' divisions being 

 based on the degree of refractoriness of the material. Such a 

 grouping however is more or less arbitrary, since no sharp natural 

 lines can be drawn between the different groups, and it is to be 

 expected that no grouping proposed will meet with universal 

 approval. The following classification has been adopted in this 

 report : 



1. Highly refractory clays, those whose fusing point is above 

 cone 33. Onlv the best of the so-called No. 1 fire clays belong 

 to this class. 



2. Refractory clays, those whose fusion point ranges from 

 cone 31-33 inclusive. This group includes some of the New 

 Jersey No. 1, as well as some No. 2 fire clays. 



3. Scmircfractory clays, those whose fusion point lies between 

 cone 27 and 30 inclusive. 



4. Clays of low refractoriness, those whose fusion point lies 

 between cone 20 and 26 inclusive. 



1 See also "Iron Oxide." Chap. III. pp. 58. 59. 



