CHEMICAL PROPERTIES OF CLAYS. 21 



that as it increases the plasticity, tensile strength, and 

 air shrinkage tend to decrease. In fact silica es- 

 pecially if present abundantly in large grains, may 

 cause an expansion of the clay in burning. 



Quartz serves as a flux at very high temperatures, 

 but at lower ones it tends to increase the refractori- 

 ness of the clay, and this property is governed some 

 what by the size of the quartz grains and the amount 

 of fluxing material which will fuse at lower tempera- 

 tures. 



Sand acts as a diluent of the shrinkage in air drying 

 and also in burning up to a certain point depending 

 upon the fusiblity of the constituent grain. 



In the burning of low grade clay, the quartz grains 

 tend to act as a skeleton and preserve the form of the 

 mass, while the fluxing impurity by their fusion bind 

 the whole together. 



TITANIC ACID IN CLAYS. 



Titanium generally occurs in clays in the form of 

 the mineral rutile (titanic oxide). It has always 

 been looked upon as a rare element and a non-detri- 

 mental impurity, but the idea of its rarity has pro- 

 bably resulted from the fact that it is not commonly 

 determined or looked for in the ordinary quantitative 

 analysis. Its effect on the fusibility of clay has never 

 been thoroughly understood, although it has seemed 

 probable that its action was somewhat analogous to 

 that of silica. 



The experiments of Seger have indicated that when 

 a hundred parts of kaolin and 6.65 per cent, titanic 

 oxide were heated to above melting point of wrought 

 iron, the resulting mass was densely sintered, and 

 showed a dark blue fracture. 



13.3 per cent, added to a hundred parts of kaolin 

 gave a deep blue enamel at the same temperature, 

 while an equal amount of kaolin with the addition of 

 10 per cent, of sil'ca burned to a snow white mass at 

 the same temperature and did not fuse. From this it 

 will be seen that the actions of titanium and silica at 

 high temperatures are not exactly alike. 



