BRWK OLAYB. 183 



clay suffices, in fact so little attention is applied to 

 material used for this purpose, that the product is often 

 soft and porous. Clays for common brick should not be 

 excessively sandy, otherwise the brick will be weak and 

 porous. They should possess sufficient plasticity to mould 

 without cracking, but not be so plastic as to warp, due 

 to excessive shrinkage. Most brick clays burn red. Fer- 

 ruginous clays can be more safely burned to a hard pro- 

 duct than clacareous ones, which burn buff or cream 

 colored. 



The methods used for moulding common brick are the 

 4>oft mud, by which the soft plastic mass is forced into the 

 mo'ild; and the stiff mud, in which the clay is forced from 

 a die of rectangular cross section and then cut up into 

 bricks. The latter method gives greater capacity, but 

 the bricks unless thoroughly burned will not stand the 

 weather as well. Very plastic clays and very lean 

 ones are adapted to the stiff mud process, the former be- 

 cause they are not tenacious enough, the latter because 

 owing to their pastiness and the structure of the machine 

 a laminated structure is developed in the brick. 



Brick clays should have a tensile strength not less than 

 50 lbs. per square inch. They are not required to stand 

 a high degree of heat, a few common brick kilns attain a 

 temperature of over 1800 or 1900 degrees Fahr. 



The more rapidly the clay slakes the easier will it be 

 to temper it. 



Clays for front <}r pressed brick. For this purpose a 

 lighter grade of clay is required, and the material must 

 not only burn to a hard body but also to a uniform 

 color, for on the latter depends much of the beauty of the 

 structure. In no branch of the clay working industry is 

 the range of colors producible from natural clay mixtures 

 more carefully considered than in the manufacture of 

 pressed brick. 



