THE PHYSICAL PROPERTIES OF CLAY. 99 



it will be safer to bring the ware up to a condition of vitrification, 

 without the risk of reaching the temperature of viscosity and 

 melting all the wares in the kiln, because it is impossible to con- 

 trol the kiln temperature within a range of a few degrees. In 

 many clays the point of vitrification seems to be midway between 

 that of incipient fusion and viscosity, but in others it is not. 



Temperature of fusion. — The temperature at which a clay 

 fuses depends on: .1) the amount of fluxing impurities; 2) the 

 condition of the fluxes; 3) the size of the grains, and 4) the 

 condition of the kiln atmosphere, whether oxydizing or reducing. 



1. Other things being equal, the temperature of fusion of a 

 clay will fall with an increase in the percentage of total fluxes. 

 If we compare the analyses of a brick clay and a fire clay we 

 shall find that the analysis o>f the former shows perhaps 12 or 15 

 per cent, of fluxing or fusible ingredients, while that of the latter 

 may show only 2 or 3 per cent., and that their fusion points are 

 perhaps 1093 C. (2000 F.) and 1644 C. (3000 F.) respect- 

 ively. All fluxing impurities do not, however, act with equal 

 energy, some being more active than others. 



2. The condition of chemical combination may also' affect the 

 result. Thus lime, for example, will induce a fluxing action in 

 clay at a lower temperature if present in the form of carbonate 

 of lime than as silicate of lime. 



3. The size of the mineral grains in a clay undoubtedly exerts 

 more effect than some investigators have been willing to admit. 1 

 Other things being equal, a fine-grained clay will fuse at a lower 

 temperature than a coarse-grained one, 2 for the reason that when 

 the particles of a clay begin to fuse or flux with each other, this 

 action begins on the surface of the grains and works inward 

 towards the centre. If, therefore, the easily fusible grains are of 

 small size, they fuse more rapidly, and are more effective in their 

 fluxing action than if the grains were large. Since some of the 

 mineral grains in the clay are more refractory than others, the 

 clay in the earlier stages of fusion can be regarded as a mixture 



'H. O. Hofman, Trans. Amer. Inst. Min. Engrs. XXVIII, p. 440, 1898. 

 2 See Chapter XVI, The Fire Clays and Fire-Brick Industry ; also paper 

 by H. Ries, Trans. Amer. Inst. Min. Engrs., Feb., 1903. 



