50 GENERAL DISCUSSION OF CLAYS. 



sand." The difference between that portion of the 

 sample insoluble in sulphuric acid and sodium 

 hydroxide and the total represents the "clay sub- 

 stance/' The method of analysis used to detrmine 

 the mineralogical character of the clay is called the 

 rational method, and when carried out in its simplest 

 form, determines the amount of clay substance or 

 kaolinite, quartz, and feldspar present 'n the clay. If 

 carried out more completely, it enables us to calculate 

 the amount of calcite or limestone (calcium carbon- 

 ate)' iron oxide and even mica in the clay. 



THE RATIONAL ANALYSIS OF CLAY. 



The rational analysis of clay consists in resolving 

 the clay into its mineralogical elements, thus giving 

 a clue to its physical as well as its chemical properties. 

 It is often utilized by manufacturers of porcelain and 

 other high grades of ware as a guide in the compound- 

 ing of their mixtures. 



The ordinary quantitative or ultimate analysis 

 regards the clay as a mixture of oxide of the elements, 

 although they may be present in entirely different 

 combinations, such as silicates, carbonates, hydrates, 

 sulphates, etc. This condition of combination is im- 

 portant for it makes a difference in the behavior of 

 the clay. Thus for instance, if silica is present in the 

 form of quartz it will decrease the shrinkage and also 

 increase the refractoriness up to a certain point, but 

 if present as a component element of feldspar it serves 

 as a flux and also increases the plasticity somewhat. 



It is not intended though that the rational analysis 



