640 NEW YORK STATE MUSEUM 



sociated witli it, the composition of the fire gases, the degree of 

 sintering and the temperature of the kiln. 



The red coloration usually caused by iron is well known, and the 

 other effects of iron are mentioned on page 517. An excess of 

 lime, magnesia or alumina tends to exert a bleaching action on the 

 iron, and produce a buff tint. 



It is asserted by Soger {Ges. Sclirift. p. 282) that it requires 

 5^ of ferric oxid to give a pronounced red color, which increases 

 with the amount of iron, up to about 20/^. 



Knowing the effect of the different ingredients on the color of 

 the burned clay, it is possible when certain results are desired to 

 add the ingredients to the clay in case they are lacking. Thus a red 

 burning clay might be changed to a buff burning one by adding 

 to it a white or whitish burning clay containing a high amount of 

 alumina, and, depending on the amount added, we should get shades 

 passing from red through brown, yellowish brown, to yellow. Marl 

 produces a similar result. 



The fire gases may be either reducing or oxidizing, and during 

 the burning of a kiln these conditions are apt to alternate at times, 

 but while cooling down the action of the fire is Avith few excep- 

 tions oxidizing. 



One effect of the sintering is to cause the clay to shrink more 

 and become denser, and this of itself is sufficient to deepen the 

 color. 



The color to which a clay naturally burns as a result of its con- 

 stituents, is best shown on the fractured surface of a brick, as the 

 fire gases have not been able to exert any effect on the interior of 

 the product. 



The surface coloration of a burned brick may often be the same 

 as that of the interior portion, but at other times it may differ from 

 it. This is due to the accumulation of soluble salts, which have 

 been drawn from the interior of the brick to the surface, either in 

 burning, water-smoking or drying. 



Another cause of difference in color between the surface and in- 



