]84 DETAILED REPORT ON ALABAMA CLAYS. 



Many shades are obtained either by mixing two or more 

 clays, or by adding artificial coloring agents to the raw 

 materials. 



Clays for front brick should shrink evenly in burning, 

 and not warp nor crack. Straightness of outline and 

 evenness of size are essential to close fitting when set in 

 the wall. 



Many front brick are moulded by the dry-press process, 

 in which the clay is forced into the mould in the form of a 

 dry powder. Such bricks have straight edges and smooth 

 surfaces, but unless burned good and hard they chip 

 easily. At many localities the clay is moulded in soft mud 

 or stiff mud machines, and the brick, while still soft, re- 

 pressed in a second machine whereby the surfaces are 

 smoothed even and the edges straightened. These lat- 

 ter brick do not tend to exhibit the same brittleness along 

 the edges as the dry press brick are apt to. 



Front brick sell from $15.00 to $70.00 per 1000, de- 

 pending on the color and shape. 



Clays for paving brick. The nature of these must be 

 such that they can be burned to vitrification. To do this 

 economically and on a large scale the points of vitrifica- 

 tion and viscosity should be at least 125 F. apart and 

 preferably 200 F. If they were not it would be impos- 

 sible to bring a kiln full of bricks to vitrification without 

 running them up to the temperature of viscosity. For 

 this reason calcareous clays are not well adapted to pav- 

 ing brick manufacture. 



Paving brick clays should possess moderate or good 

 plasticity so that they can be moulded by the stiff mud 

 process, and while it is desirable that the tensile strength 

 should be 75 pounds or more, at the same time many 

 good pavers are made from mixtures whose tensile strength 

 is not over 50 pounds per square inch. 



Shales are used to a large extent for the manufacture 



