534 NEW YOKE STATE MUSEUM 



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

 approximately, for the greater the amount of iron present the deeper 

 red will the clay bum, provided the iron is evenly and finely dis- 

 tributed and an excess of lime is not contained in the clay. If the 

 proportion of iron to lime is as 1 to 3, then a buff product results, 

 provided the clay is heated to incipient fusion or vitrification. The 

 above conditions will be affected by a reducing atmosphere in burn- 

 ing or the presence of sulfur in the fire gases. 



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

 of combined water sometimes exhibit a tendency tO' crack in burn- 

 ing. This combined water would be shown in the chemical 

 analysis. ; 



5 Excess of silica. A large excess of silica would indicate a sandy 

 clay. 



These are practically all the points which the ultimate analysis 

 explains, and they are mostly of a chemical nature. As regards the 

 rational analysis, it may be carried out in a simple way or an 

 elaborate one. 



Most kaolins and other high grade clays consist only 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 on as having the properties of kaolinite,, for the 

 latter is present in it in such a large excess. 'Now as each of these 

 three components of the kaolin — clay substance, quartz and feld- 

 spar — have characteristic properties, the kaolin will vary in its 

 behavior according as one or the other of these constituents pre- 

 dominates 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 of low plasticity by itself; kaolinite is 

 plastic and quite refractory, but shrinks considerably in burning. 



In Europe, specially Germany, the custom has been to disregard 

 mica and figure it in as clay substance, partly because there was so 



