218 CLAYS AND CLAY INDUSTRY. 



although for the sake of economy they are often used for front 

 walls. They are usually made without much regard to color, 

 smoothness of surface, or sharpness of edges. 



Face, front or pressed brick include those made with greater 

 care, and usually from a better grade of clay, much consideration 

 being given to their uniformity of color, even surface and straight- 

 ness of outline. Enamel brick are those which have a coating of 

 enamel on one or sometimes two sides. Glazed brick differ from 

 enamel brick in being coated with a transparent glaze instead of 

 an opaque enamel. 



Raw Materials. 



Clays for common brick. — The clays used for common brick are 

 usually of a low grade, and in most cases red-burning. The main 

 requisites are that they shall mold easily and burn hard at as low 

 a temperature as possible, with a minimum loss from cracking 

 and warping. Since most common clays when used alone show 

 a higher air and fire shrinkage than is desirable, it is customary 

 to decrease this by mixing some sand with the clay, or by mixing 

 a loamy or sandy clay with a more plastic one. 



Measurements made at 29 yards manufacturing common brick 

 showed, however, that in practice there is apparently no uni- 

 formity in the air shrinkage of the brick, the linear air shrinkage 

 ranging from 0.7 per cent, to 10.9 per cent., with an average 

 of 5.41 per cent. This variation in the air shrinkage is due to 

 the character of the clay and the process of manufacture. Com- 

 mon-brick clays are mostly nonrefractory, for this insures their 

 hardening at a low temperature, whereas when No. 2 fire clays 

 are employed, they must be burned at higher temperatures to pre- 

 vent the brick from being soft and porous. Some common-brick 

 clays of the Cohansey formation found in Monmouth county are 

 a good example of this, for they remain porous even when fired 

 up to cone 8, and for use these are mixed with more fusible clays. 

 The plasticity of common-brick clays is usually good, although 

 it is possible to employ some very lean ones, and the tensile 

 strength of those employed in New Jersey ranges from 60 lbs. 

 per square inch up to 300 lbs. or more. 



