CLAYS OF NEW YORK 569 



In the third groaip, which includes the brick clays, the alumina 

 percentage is small as compared with the ferric oxid. They are 

 all easily fused, and the percentage of lime, magnesia and alkalis 

 is low in those which burn to a bright color. The usual color of 

 such clays when burned is red, which becomes deeper with an in- 

 crease in the temperature and greater density, changes to violet 

 red and finally becomes black. 



The percentage of ferric oxid is generally one third to one half 

 the alumina percentage, as indicated by the following figures from 

 five examples given by Soger. , 



Color when burned Dark red ^y^io^errtd" chwTy^red ^^^^ ^^'^ ^^""^ ""^^ 



Eatioof AlA: FeA 1:2.8 1:1.19 1:1.9 1:1.29 1:1.29 



A comparison of the second and third groups shows that those 

 in which the alumina is not more than three times as great as 

 the ferric oxid show a decided red color ; those where it is five and 

 one half times as great show a brown to yellow color. 



Probably other physical properties exert an influence, but these 

 are not clearly understood. 



The fourth group includes calcareous clays, and in this the 

 succession of colors produced in burning is reversed. The ferric 

 oxid exerts its coloring action at low temperatures, but at higher 

 ones the influence of the lime is seen on the silicates of the clay, 

 and the red passes into yellow or yellowish white, which at higher 

 temperatures grades into green, and at viscosity becomes dark 

 green or black. 



The relations between iron and alumina, and iron and lime, 

 and the color when burned are shown below. 



Q 1 With light burning red to flesh red; hard burning yellow white to 



sulphur yellow; at vitrification yellow green to green 



FeA: ALA-.-. 



1:2.3 1:1.16 1:3.1 1:2.5 1:2.5 1:2.4 



FeA:CaO 



1:2.9 1:3.2 1:2.2 1:3.0 1:3.5 1:2.2 



The iron in this group runs about as high ^s in group 3, the 

 lighter color being due to lime, the percentage of which ranges 

 from 11^-19^. 



