3 i2 CLAYS AND CLAY INDUSTRY. 



total percentage of certain fluxing impurities such as ferric oxide, 

 lime, magnesia and alkalies, is small. This is necessarily the 

 case, since, if the fluxing impurities were present in large quan- 

 tities, the clay would fuse at comparatively low temperatures and 

 could not be classed as refractory. 



Effect of silica, — It is found, however, that clays running low 

 in fluxes, but high in silica, may also show poor refractoriness. 

 If we compare two 1 fire clays of low-flux contents, but high silica 

 in one case, and low silica in the other, it is found that, other 

 things being" equal, the high silica clay is less refractory than the 

 other. This indicates that a high percentage of silica, as well as 

 a high percentage of the fluxes mentioned above, diminishes the 

 refractoriness of the clay. We might therefore term the iron 

 oxide, lime, magnesia and alkalies low-temperature fluxes and 

 the silica a high-temperature flux. 



In any fire clay, some of the silica is combined chemically with 

 the alumina in the form of the mineral kaolinite (p. 47), while 

 the balance is probably there in the form of quartz. 1 If kaolinite 

 alone is heated, its refractoriness is found to 1 be high, for its 

 fusion point is the same as cone 36 of the Seger series (see p. 

 102), and the refractoriness of quartz or silica alone is nearly as 

 high, but if these two> minerals are mixed together in varying pro- 

 portions, then the fusion point of the mixtures will in every case 

 be lower than that of either silica or kaolinite alone. 



This fact was pointed out some years ago by Herman Seger, 

 the German ceramic technologist, who made up a series of mix- 

 tures of alumina and silica, and kaolin and silica. In the former 

 series of mixtures the quantity of alumina in each case was the 

 same, but the amount of silica was increased. Starting with 1 

 part of alumina 2 to one of silica by volume (91.5 of alumina to 



1 There cannot be many silicate minerals such as feldspar, mica, etc., in a 

 fire clay, otherwise the percentage of alkalies, magnesia, lime and iron oxide 

 would be higher than it usually is, so that the balance of the silica must be 

 quartz. 



2 What is meant here is parts by volume, which would not be the same as 

 parts by weight, because the 2 substances have different specific weights, 

 hence 1 alumina to 1 silica per volume would be 91.5 per cent, alumina to 

 8.5 silica by weight. 



