CLAYS OF NEW YOKK 565 



tile; drain tile; chimney" flues; chimney pots; door knobs; puddling; 

 Portland cement; fireproofing; terra cotta lumber; paving brick 

 copings 



3 Agricultural. Drain tile, bam flooring 



4 Hydraulic structures. Water conduits; sewer pipe; sewer 

 brick; turbine wheels 



5 Sanitary engineering. Granite ware; urinals and closet bowls; 

 wash tubs; bath tubs; sewer pipe; ventilating flues; foundation 

 blocks; vitrified brick 



6 Industrial uses. Crucibles and other assaying apparatus, acid 

 vats and jars; acid bricks, gas retorts; fire bricks; glass pots; sag- 

 gers; stove and furnace linings; wall and writing paper fillings; 

 porcelain chemical apparatus; grinding mills; insulators; pumps; 

 filters; mineral paint; packing horses hoofs; fulling cloth; ultra- 

 marine manufacture 



7 Ornamental and esthetic uses. All forms of ornamental pot- 

 tery; terra coitta and various forms of tiles either glazed or unglazed 



8 Imitative uses. Food adulterants and paint adulterants 



Coloring agents 



This includes those substances which impart a definite color to 

 the clay in burning. Pure clay would bum to a snow white color, 

 but in nature it is frequently tinged with more or less impurity. 

 The most common coloring agent is oxid of iron or iron compounds 

 which, in burning, change to the oxid. The depth of color pro- 

 duced in burning depends on the amount of iron jfresent. It may 

 vary from the lightest yellow to red and dark brown or bluish 

 black. The presence of other compounds may however have a 

 marked influence on the iron coloration. Some of the purest clays 

 known, though containing a mere fraction of a per cent of iron 

 oxid, will, nevertheless, when burned at a very high temperature, 

 develop a very slightly yellow tint. If such clays have a con- 

 siderable amount of feldspar added to them, they keep this yellow 

 color; on the other hand the addition of quartz tends to minimize 

 it. Magnesia and lime may exert a much stronger effect on the 



