52 CLAY«S AND CLAY INDUSTRY. 



5. A red-burning brick clay, Sayreville, N. J. 



6. A calcareous clay, Canandaigua, N. Y. 



Rational analysis. 1 — This method has for its object the deter- 

 mination of the percentage o>f the different mineral compounds 

 present, such as quartz, feldspar, kaolinite, etc., and gives us 

 a much better conception of the true character of the material. 

 Most kaolins and other high-grade clays consist chiefly of 

 kaolinite, quartz and feldspar, the kaolinite forming the finest 

 particles of the mass, while the balance is quartz, feldspar and 

 perhaps some mica. The finest particles are known as the clay 

 substance, which may be looked upon as having the properties 

 of kaolinite. Now, as each of these three compounds of the 

 kaolin — clay substance, quartz and feldspar — have character- 

 istic properties, the kaolin will vary in its behavior according as- 

 one or the other of these constituents predominates or tends to 

 increase. 



As to the characters of these three, quartz is nearly infusible, 

 nonplastic, has very little shrinkage, and is of low tensile 

 strength ; feldspar is easily fusible, and alone has little plasticity ; 

 kaolinite is plastic and quite refractory, but shrinks considerably 

 in burning. The mica, if extremely fine, may serve as a flux, and 

 even alone is not refractory. It is less plastic than the kaolinite,. 

 and, when the percentage of it does not exceed 1 or 2 per cent., 

 it can be neglected. To illustrate the value of a rational analysis,, 

 we can take the following example: Porcelain is made from a 

 mixture of kaolin, quartz and feldspar. Suppose, that a manu- 

 facturer of porcelain is using a kaolin of the following rational 

 composition : 



Clay substance, 67.82% 



Quartz, 30-93% 



Feldspar, *. 1.25% 



If now to 100 parts of this there are added 50 parts of feld- 

 spar, it would give a mixture whose composition is : 



1 The method is described in the Manual of Ceramic Calculations, issued by 

 the American Ceramic Society. See also Langenbeck, Chemistry of Pottery,. 

 1895, p. 8. 



