328 CLAYS AND CLAY INDUSTRY. 



The above table shows that there is considerable variation 

 both in the chemical composition and fusibility of the New Jersey 

 fire brick, and a careful study of the table together with some 

 supplementary experiments brings out some interesting facts in 

 the relationship between the two. 



If from these analyses, the ratio- of kaolinite to> free silica be 

 calculated, as can be done approximately by assuming that all 

 of the alumina is present as kaolinite, it will be possible to com- 

 pare the actual fusion point of the brick with that of the kaolin- 

 silica mixtures in Fig. 41. 



For example let us take No*. 1 of the above series. If we 

 assume that the 19.24 per cent, alumina was present in the clay 

 as kaolinite (and probably most of it was), then the amount of 

 silica necessary to combine with it is 22.38 per cent., because 

 in kaolinite the ratio of silica to alumina is as 46.3 to 39.8, 

 and the brick would consequently contain 41.62 per cent. 

 of dehydrated kaolinite and 55.86 per cent, free silica. If we 

 recalculate this on the basis of 100 per cent, and add in the 

 amount of water necessary to the kaolinite it gives us 53.61 per 

 cent, silica, and 46.39 per cent, kaolinite. Such a mixture ac- 

 cording to the upper curve in Fig. 41 should fuse at cone 28 if the 

 silica and kaolin are finely divided and thoroughly mixed. 



As shown by the table, however, brick No. 1 fused in the 

 Deville furnace at cone 33, six cones higher than we should 

 expect it to fuse, if we figured its refractoriness from the silica- 

 kaolinite ratio mentioned above, and it therefore becomes neces- 

 sary for us to find the reason for this discrepancy. The ex- 

 planation of these apparently contradictory results is to be found 

 in the texture of the brick. If we examine a piece of this brick 

 that has been fused in the Deville furnace, we find that scattered 

 through the fused scraps there are white grains that seem to 

 have resisted fusion, and an examination of the brick shows that 

 probably 15 per cent, or 20 per cent, of it is composed of angular 

 quartz grains, some of them T V or |- of an inch in diameter. 

 We have, therefore, an explanation of the disagreement between 

 the theoretical fusion point of the brick based on the kaolin, and 

 silica ratio, and the actual fusion point of the brick, for as was 



