236 CLAYS AND CLAY INDUSTRY. 



4. Increasing density with rising temperature, the maximum 

 being reached at the vitrifying point of the clay. 



Effects due to variation in the clay. — Burned clays may be of 

 many different colors. Although the majority of clays contain suffi- 

 cient iron oxide to burn red, nevertheless it is not safe to predict,, 

 from the color of the clay, the shade that it will burn, since some 

 bright red or yellow clays may yield a buff brick. If consider- 

 able iron oxide is present, 4 to 5 per cent., the brick burns red, 

 unless much lime is also present. If only 2 to 3 per cent, are 

 present, a buff product is obtained, whereas, with 1 per cent., or 

 under, the clay burns white, or nearly so. An excess of lime in 

 the clay will, however, counteract the effect of the iron oxide 

 and yield a buff brick, but a brick owing its buff color to this 

 cause will not stand as much fire as one which owes its buff 

 color simply to a low percentage of iron oxide. 



Where a clay is mottled, as red and white, for instance, the 

 colors of the different spots will retain their individuality most 

 plainly after burning, unless the clay is thoroughly mixed. Some 

 clays of South Jersey contain lumps of whitish clay, much 

 tougher than the rest of the mass. These resist disintegration in 

 the tempering machines, SO 1 that after burning they can be plainly 

 seen, as white spots in the red ground of the brick. 



The normal iron coloration may often be destroyed by the 

 effects of the fire gases. When these are reducing in their action 

 (i. e., taking a part of the oxygen from the ferric compounds and 

 reducing them to ferrous compounds, pp. 57-59), the red color 

 may be converted to gray, or even bluish black, if the reduction 

 is sufficient, so that in some districts the bricks, on account of 

 lack of air in the kilns and carbonaceous matter in the clay, do 

 not burn a very bright red. Moreover, other things being equal,, 

 the higher the temperature at which a clay is burned, the deeper 

 will be its color. 



The surface coloration of a burned brick may often be different 

 from the interior. This is due to several causes. 1) Soluble 

 salts may accumulate on the surface, sometimes causing a white 

 coating, because they have been drawn out by the evaporation 

 of the water during the drying of the brick. 1 2) The deposition 



See "Soluble Salts in Clays," pp. 75, et seq. 



