CLAY. 



BY JEFFERSON MIDDLETON. 



[Jefferson Middleton, statistician; is the recognized authority on clay and the methods 

 of its extraction and manufacture ; for the past fifteen years he has been the expert in 

 this Une for the United States geological survey, and has written the government re- 

 ports on the industry,; also is author of many articles for reviews and technical peri- 

 odicals.] 



Popularly, clay is an earthy substance which, if mixed 

 with water and molded, will retain its shape after drying, and 

 which upon subjection to high temperature loses its plasticity 

 and becomes hard and brittle. 



Technically, pure clay or kaolinite, which is the basis of 

 all clay, is a hydrated silicate of aluminum, expressed by the 

 formula Al2032Si02, 2H O. All clay in its natural state con- 

 tains more or less impurities, the kind and quantity of which 

 determine its character; from purest varieties, called kaolin, 

 clays range through all stages of impurity down to a point 

 where the material contains so little kaolinite that it can not 

 be classified as clay at all. 



Clay is ordinarily classified as kaolin, ball clay, fire clay, 

 vitrified ware clay, and brick clay; there are also slip clay, 

 and paper clay. Fire clay includes clays used for stoneware 

 and for terra cotta, which are reported separately by the 

 geological survey. Vitrified ware clay includes the pipe clay, 

 reported by the survey. 



In the following description of the physical and chemical 

 properties of the several kinds of clay, the chemical analysis 

 has been supplemented, wherever possible, by what is called 

 the rational analysis. The ordinary quantitative chemical 

 analysis treats clay as a mixture of oxides, although the ele- 

 ments may be present in entirely different combinations, such 

 as silicates, carbonates or hydrates, sulphates, etc. The 

 nature of these combinations is of importance. For example, 

 silica in the form of quartz, which is infusible, will decrease 

 the shrinkage and up to certain temperatures increase the 



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