THE MANUFACTURE OF BUILDING BRICK. 221 



done. In the southern half of the State the. extensive mantle of 

 loamv clay found on the surface at so many localities forms a de- 

 sirable material tO' add to the more plastic clay beneath. 



Pressed brick. — Pressed brick call for a much higher grade of 

 clay than is necessary for common brick. The kinds now in use 

 fall in three groups, viz, 1) red-burning clays, 2) white-burning 

 clays, 3) buff-burning clays, usually semirefractory. The com- 

 position of a sample of each of these three types is given below : 



Analyses of clays used for making pressed brick. 



Front brick 



Shale— Kan- White clay — clay — Sayre- 



sas City, Mo. Grover, N. C. ville, N. J. 



Silica (Si0 2 ), 55-75 68.28 56.10 



Alumina (AI2O3), 21.16 18.83 27.42 



Ferric oxide (FeiOs) 5.69 2.60 2.68 



Lime (CaO) 3.25 0.70 .... 



Magnesia (MgO), 2.84 0.13 0.18 



Alkalies (Na^-O, K»0), 3.02 2.29 2.71 



Water (H.O), 8.45 6.47 6.00 



Moisture, 0.76 2.90 



Titanium oxide (TiC^) , 0.27 1.00 



The physical requirements of a clay for pressed brick are 1 ) 

 uniformity of color in burning, 2) freedom from warping or 

 splitting, 3) absence of soluble salts, and 4) sufficient hardness 

 and low absorption when burned at a moderate temperature. The 

 air shrinkage and fire shrinkage, as well as tensile strength, vary 

 within the same limits as common bricks. 



Red-burning clays were formerly much used, and, indeed, are 

 still employed to some extent around Trenton and Philadelphia, 

 but in recent years other colors have found greater favor, and the 

 demand for the former has greatly fallen off. Buff-burning, semi- 

 refractory or refractory clays are, therefore, much employed now, 

 partly on account of their color and partly because coloring mate- 

 rials can be effectively added to them, for since the range of nat- 

 ural colors that can be produced in burning is limited, artificial 

 coloring agents are sometimes used. Manganese is the one most 

 employed. 



The clays must necessarily burn hard at a moderate temperature, 

 and in the case of red-burning clays the temperature reached is 



