CLAYS OF NEW YOEK 641 



terior of the Lurned brick may be due to tbe deposition of foreign 

 substances brought there by the fire gases, which may exert a 

 colorizing action either by their presence alone or by their forming 

 a glaze on the surface of the brick as a result of their union with 

 the silica in it. This is often to be seen on the surface of arch 

 brick in an up-draft kiln, and on the surface of the brick which 

 line the bag walls of any do^^Ti-draft kiln. 



The coloration of most brick is due to iron; unless the brick is 

 heated beyond vitrification it is probable that much of the iron re- 

 mains in the ferric condition. 



With ferric oxid in a clay it is possible to obtain all shades rang- 

 ing from pink to reddish black, and with an excess of lime all 

 shades of yellow, while manganese, which sometimes accompanies 

 the iron in small amounts, tends to give a brownish coloration. 



Ferrous oxid produces colors ranging from green to black. 



It should also be remembered that with any given amount of 

 iron in a clay, the higher the temperature to which the material is 

 exposed the deeper will be the color obtained. Iron in the ferric 

 condition tends to color the mass red as long as it is at all porous, 

 but with the beginning of fusion it generally passes over to black. 



When the clay also contains carbonate of lime, the latter serves 

 as a flux, and causes fusion to set in at a lower temperature than it 

 otherwise would, the result being the formation of a complex 

 silicate, containing iron, alumina and lime, which with the proper 

 proportion of iron and lime shows a yellow color. Up to the time 

 that fusion sets in the ferric oxid still imparts its red color to the 

 clay, but as the heat rises, this gradually turns to flesh red, white, 

 yellow, and finally yellowish green, and at viscosity passes to green, 

 and sometimes black. 



The coloration which is induced superficially is a matter of great 

 importance, and, as before stated, it may be due either to a coating 

 of soluble salts, or a deposit of impurities from the fire gases. The 

 former are described under " Efflorescence on bricks," p. 679. 



The discoloration caused by the action of fire gases on the clay 



