744: NEW YOEK STATE MUSEUM 



Min. per Max. per Avarae-p 

 cent cent Average 



Moisture .5 3.0 1.5 



Silica 49.0 Y5.0 56.0 



Alumina ■ 11.0 25.0 22.5 



Ferric oxid 2.0 9.0 6.Y 



Lime .2 3.5 1.2 



Magnesia .1 3.0 1.4 



Alkalis 1.0 5.5 3.7 



Water (loss on ign.) 3.0 13.0 7.0 



Total fluxes . . ; 13.0 



In addition to liaving the proper cliemical composition, it should 

 also possess the necessary physical properties. 



Proper plasticity is of vital importance, but its excessive develop- 

 m.ent is equally injurious. The reason plasticity has such import- 

 ance is that clay when molded by the stiff mud process is very apt 

 to ■ tear when issuing from the die, unless of proper plasticity. 

 Excessive plasticity tends to produce a laminated brick when auger 

 machines are used. The effect of these laminations will be seen in 

 the tests given below. 



As paving brick, unless made of fire clay, should be burned to the 

 point of vitrification, it is essential that in clays used for this pur- 

 pose the points of viscosity and incipient fusion sihould lie well 

 apart, not less than 250° F. and preferably 400° F^ 



The color of a paving brick is no indication of its quality. 



The clay should not show any disposition to blister as the point 

 of vitrification is approached, but this is likely to occur if an excess 

 of iron is present. 



In order to demonstrate somewhat definitely what are the char- 

 acters of a good paving brick shale, the tests of a sample utilized 

 in Illinois for the manufacture of paving blocks is given herewith. 



The shale is rather fine-grained, and breaks up quite easily in 

 grinding. It was ground to pass through a 30 mesh sieve. 28 fo of 



. . ■- — f ■ 



^ Olchewsky in Post. chem. tech. analyse. 1890; Wheeler in Vitrified 

 paving hrick. 1895. 



