642 NEW YOKE STATE MUSEUM 



is far more pronounced in the case of buff ware. In red burning 

 clays the effect is often marked by a superficial reduction of the 

 iron which the clay contains, or a slagging of the surface due to the 

 deposition of fusible impurities, specially alkalis, from the fire 

 gases. 



In calcareous clays many of the elements of the material show 

 a strong affinity for the sulfuric acid of the fire gas; the result of 

 this is that sulfate of lime is formed on the surface, and the ferric 

 oxid, not being able to unite with the lime, imparts a red color 

 to the brick. In the interior the color of the brick remains yellow, 

 for the sulfuric acid gas has not been able to penetrate to that point 

 and take the lime away from the iron. This point can be easily 

 proved by determining the amount of sulfur in the yellow and the 

 red portion of the brick, and, if the theory is correct, the latter 

 should show the greater amount of the acid. That this is so is 

 well shown by two clays analyses made by Soger (Ges. 8chrift. p. 

 2 77). The outer or red portion of the brick which he analyzed 

 showed 14.43^ of sulfuric acid, while the inner or yellow portion 

 showed only 1.04^. 



One fact that this emphasizes is that in burning calcareous clays 

 it is important that coal should be used which contains but a very 

 small percentage of sulfur. 



The slower the burning proceeds, the more completely will the 

 iron in all portions of the clay be oxidized, and the greater the ac- 

 cess of air the brighter will be the red color. 



The time required in drying and burning is affected not only by 

 the clay, but also by the process. The more water which has to be 

 driven off in the kiln, the slower must the burning proceed, unless 

 the clay is coarse-grained. 



