50 CLAYS AND CLAY INDUSTRY. 



red heat, they are often determined collectively and expressed 

 as "Loss oh ignition." If carbonaceous matter, such as lignite, 

 is present this also will burn off at redness. To separate these 

 four, special methods are necessary, but they are rarely applied, 

 and, in fact, are not very necessary, except in calcareous clays, 

 or black clays. The loss on ignition in the majority of dry 1 

 clays is chiefly chemically combined water. The ferric oxide, 

 lime, magnesia, potash and soda are termed the fluxing im- 

 purities, and their effects are discussed under the head of iron, 

 lime, magnesia, etc., and also under Fusibility, in Chapter IV. 



As can be seen from the experiments described in Chapter 

 IV, all clays contain a small but variable amount of moisture in 

 their pores, which can be driven off at ioo° C. (212 F.). In 

 order, therefore, to obtain results that can be easily compared, 

 it is desirable to make the analysis on a moisture-free sample, 

 which has been previously dried in a hot-air bath. This is 

 unfortunately not universally done. 



The facts obtainable from the ultimate analysis of a clay are 

 the following: 



1. The purity of the clay, showing the proportions of silica, 

 alumina, combined water and fluxing impurities. High-grade 

 clays show a percentage of silica, alumina and water, approach- 

 ing quite closely to those of kaolinite (pp. 46, 47). 



2. The refractoriness of the clay, for, other things being equal, 

 the greater the total sum of fluxing impurities, the more fusible 

 the clay. 



3. The color to which the clay burns. This may be judged 

 approximately, for clays with several per cent, or more of ferric 

 oxide will burn red, provided the iron is evenly and finely dis- 

 tributed in the clay, and there is no excess of lime. The above 

 conditions will be affected by a reducing atmosphere in burning, 

 or the presence of sulphur in the fire gases. 2 



4. The quantity of water. Clays with a large amount of 

 chemically combined water sometimes exhibit a tendency to 



1 This means dried at 100° C. until their weight is constant. See under 

 Moisture. 



2 See Lime in this chapter. 



