112 



CLAYS AND CLAY INDUSTRY. 



color, had respectively 3.12 per cent, and 12.46 per cent, of ferric 

 oxide. 



The color of a green or raw clay is not always an indication of 

 the. color it will be when burned. Red clays usually burn red ; deep 

 yellow clays may burn buff; chocolate ones commonly burn red. 

 or reddish brown ; white clays burn white or yellowish white, and 



Fig. 33. 



Grains of fine and medium sand from a sample of Clay Marl I, enlarged 115 diameters. 



gray or black ones may burn red, buff or white Calcareous 

 clays are often either red, yellow or gray and may burn red at 

 first, but turn yellow or buff as vitrification is approached. 



SLAKING. 



When a lump of raw clay is thrown into' water it fall to pieces 

 or slakes, but the rapidity with which this takes place varies 



