CLAYS OF NEW YOEK 533 



salts driven out by heat. The residue is dissolved in water, filtered 

 into a weighed platinum dish, evaporated, dried and weighed as IsTa 

 CI + K CI. If the last addition of ammonium carbonate failed to 

 produce a precipitate, the transfer to another dish may be dispensed 

 with and the^ ammonia salts driven off at once. 



Rational analysis 



It is a common custom of the manufacturers of porcelain, white 

 earthenware, fire brick, and other refractory goods — in fact of all 

 products made from high grades of clay — to use the rational 

 analysis as a guide in making up their mixtures and keeping them 

 constant. The advantage of this analytical method is that it re- 

 solves the clay intoi its mineral components, and enables us thereby 

 to get an insight into the physical character of the material used, 

 which is frequently a matter of far gTeater importance than its 

 chemical composition. 



The ordinary quantitative or ultimate chemical analysis regards 

 the clay as a mixture of oxids of the elements, though they may be 

 present in entirely diiferent combinations, such as silicates, carbon- 

 ates or hydrates, sulfates, etc. This condition of combination is 

 of importance, for it may make a vast difference whether a material 

 is present as a silicate or a carbonate. Silica if present as quartz 

 will decrease the shrinkage and up to certain temperatures increase 

 the refractoriness, but if present in the clay as a component of feld- 

 spar it serves t]ie purpose of a flux and somewhat increases the 

 plasticity. 



It is not intended, though, that the rational analysis should en- 

 tirely supplant the ultimate, for this is not possible, as each serves 

 its own purpose. The ultimate analysis may be used to supply in- 

 formation on the following points: 



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

 alumina, combined water, and fluxing impurities. 



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

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

 clay. ; 



