CLAYS OF NEW YOEK 



567 



The physical condition of tlie iron in tlie clay may also exert a 

 marked influence. If tlie iron be distributed evenly tbrougli tbe 

 clay in a finely divided condition or as a film around tbe clay grains, 

 the coloration produced will be more even than if it were scattered 

 through the clay as isolated grains. The percentage of iron oxid 

 shown by analysis might in either case be the same, but the effect 

 produced in burning would be an even color in the former case, and 

 a speckled appearance in the latter. 



Ferrous oxid may form in burning, under several conditions; it 

 may be due to the presence of organic matter, or to reducing action 

 of the fire, or it may have existed in the unbumed clay. It is not 

 as strong a coloring agent as the ferric oxid. Alone it produces a 

 green color in burning, but variable mixtures of ferrous and ferric 

 oxids are capable of producing a variety of shades. (See " Division 

 on iron ") 



Manganese oxid in general produces darker colors than iron. 



Other coloring substances might be present in clays in small 

 amounts. Cobalt oxid might produce a blue color, and chromium 

 a green color. 



Both cobalt and ohro^mium are sometimes added to white or light 

 burning clays to color them artificially, 5^ of the. former producing a 

 bright blue, and ^^-1^ of the latter giving a green. A black color 

 can be produced by adding a mixture of 6fo iron oxid, and 6^ manga- 

 nese superoxid. 



Seger -^ classifies clays according to the color assumed in burning 

 as follows : 



1 Aluminous clays, poor in iron, which bum white or very 

 slightly yellowish 



2 Aluminous, moderately ferruginous clays, whose color when 

 burned is pale yellow to light brown 



3 Aluminous, ferruginous clays, such as brick clays, whose color 

 when burned is brick red 



4 Nonaluminous clays, rich in iron and lime, whose color when 

 burned is yellow 



1 Seger. Ges. Schrift, p. 85. 



