THE MANUFACTURE OF BUILDING BRICK. 237 



of foreign substances by the fire gases may cause a colored glaze. 

 This is especially seen on the ends of arch brick, and on the bag 

 walls of a down-draft kiln, where the particles of ash carried up 

 from the fires stick to the surface of the hot brick and cause a 

 fluxing action. 3) If the clay contains much lime carbonate, and 

 there is much sulphur in the coal, the latter may unite with the 

 lime, forming sulphate of lime, and thereby prevent the com- 

 bination of the lime and iron. In this case the centre of the 

 brick, not being thus affected by the gases, may show a buff 

 color, whereas the outside has another tint. 



Flashing. 1 — Many bricks used for fronts are often darkened 

 on the edges by special treatment in firing, caused chiefly by 

 setting them so that the surfaces to be flashed are exposed, to 

 reducing conditions, either at the end of the firing or during the 

 entire period of burning. This color is superficial and may 

 range from a light gold to a rich, reddish brown. The principle 

 of the operation depends on the formation of ferrous silicate 

 and ferrous oxide, and their subsequent partial oxidation to- the 

 red or ferric form. This oxidation probably takes place during 

 cooling, for if the kiln be closed so as to> shut off the supply of 

 oxygen, the brick are found to* be a light grayish tint. 



The degree of flashing is affected, 1) by the composition and 

 physical condition of the clay, 2) the temperature of burning, 

 3) the degree of reduction, and 4) the rate of cooling and the 

 amount of air then admitted to the kiln. 



1. The percentage of iron oxide should not be large enough 

 to make the brick burn red, but to produce buff coloration, and 

 the clay should have sufficient fluxes to> reduce the point of 

 vitrification to within reasonable limits, thus facilitating the 

 flashing. Clays high in silica are apparently better adapted to 

 flashing than those low in silica and high in alumina. The con- 

 dition in which the iron is present in the clay probably exerts 

 some influence, that is, whether it is there as ferric oxide, fer- 

 rous silicate, concretionary iron, ferrous sulphide or perhaps 

 ferrous carbonate. Bleininger's experiments showed that of three 

 clays, which were used for flashing, all contained considerable 



A. V. Bleininger. Notes on Flashing. Trans. Amer. Ceramic Soc. II 74. 



