270 CLAYS AND CLAY INDUSTRY. 



The soluble salts are undesirable because in drying they may 

 come out through the color slip. In order, therefore, to render 

 them insoluble, if necessary, the clay is usually treated with 

 barium chloride or carbonate. 



Properties. — To give a tabulated statement of the properties 

 of clay used for terra-cotta manufacture would involve listing 

 40 per cent, or 50 per cent, of all the clays mined in the Wood- 

 bridge district. It may be of interest, however, to give the prop- 

 erties of a terra-cotta mixture, the tests being made on a soft 

 green mass as tempered at the works. Its physical properties 

 were as follows : 



Air shrinkage, 4^2 per cent. Tensile strength, 97.5 pounds 

 per square inch. Its behavior in burning was as follows : 



Physical qualities of a terra-cotta mixture. 



Cone 01 Cone 5 Cone 10 



Fire shrinkage, 1.5% 4-8% 5% 



Hardness, not steel-hard, nearly steel-hard. 



Absorption, very absorbent, slightly absorbent, nearly impervious. 



Color, pale buff. grey buff. grey buff. 



In making terra cotta, the clay is not carried to the tempera- 

 ture last given, as there would be danger of its warping, but it is 

 fired between cone 6 and cone 8, at which point this danger is 

 greatly, if not entirely, diminished. 



In the following table are given the physical characters of some 

 of the New Jersey clays used for terra-cotta manufacture : 



