10 GENERAL DISCUSSION OF CLAYS. 



Alumina, organic matter and water are practically 

 the only non-volatile constituents, which do not exert 

 a fluxing, action on the clay in burning, and the inten- 

 sity of this fluxing depends partly on the amount of 

 fluxes, and partly on the temperature at which the 

 clay is burned. 



It is the custom to divide the impurities of clay into 

 those wh^'ch are fluxing, and those which ar6 non-flux- 

 ing. .' 



Pure clay is very refractory. The kaolinite com- 

 posing it contains two molecules of silica and one 

 molecule of alumina. A higher percentage of sil'ca 

 tends to increase the fusibility up to a certain point, 

 provided it is in a flnely divided condition, above this 

 point the refractoriness of the clay increases steadily 

 with the addition of silica. 



Other substances are far more powerful fluxes than 

 the silica however, and these fluxes contain not only 

 elements but also definite chemical compounds or 

 mineral species. 



The influence of fluxes increases not only wifh the 

 amount present but also with the state of division, 

 they being more active, the more flnely they are divid- 

 ed. If the fluxing material is present in large grains, 

 these will only exert a fluxing action on their upper 

 surface, while the single grains alone will for a while 

 act more like quartz grains i. e. as diluents of the 

 shiinkage.. The minerals which may be prestnt and 

 serve as fusible impurities are commonly mica, feld- 

 spar, hornblende, pyroxene, garnet, quartz, calcite, 

 gypsum, iron oxide and manganese, and the elements 

 contained in these constituting the active fluxing ■ 

 agents are alkalies, iron oxide, lime and magnesia. 



Opinions differ somewhat in regard to the order of 

 their relative effectiveness, but it is probably given 

 above, the alkalies being the strongest. 



The amount and kind of fluxes which it is desirable 

 for a clay to contain depends on the use to which it is 

 to be put. If a vitrified ware is desired then the 

 fluxes should be present in appreciable amount, say 

 10 to 20 per cent., depending upon the relativestrength 



