CLAYS OF KEW YORK 515 



is impervious or nearly so, tlie alkalis are added for fluxing to tlie 

 body in the form of feldspar. Mucli feldspar is mined both, in the 

 United States and Europe for potters' use, but in nearly every case 

 it is the potash feldspar. 



Alkalis exert little or no coloring influence on the burned ware 

 in' most instances, but if an excess of feldspar be added to a white 

 burning clay, it will produce a creamy tint when burned. Potash 

 seems to have a tendency to deepen the color of a ferruginous clay 

 in burning. 



The amount of alkalis contained in clay varies. It may sink to 

 a mere trace or rise to 1^ or 8^. The limits for a number of clays 

 are given below the figures being taken from tables at end of report. 



Rauge Aver. 



Kaolins 10-6 .21 1.01 



Fire clays 048-5.27 1.46 



Pottery clays 52-7 .11 2.06 



Brick clays 17-15 .32 2 . 768 



Iron oxid 



Iron oxid is the great colouring agent of both burned and un- 

 bumed clay, and in addition serves as a flux. Furthermore in the 

 form of hydrated oxid it may increase the absorptive power of 

 clay.^ 



It is not only one of the most "widespread and comzaon of clay 

 ingredients, but is also derived from the greatest number of 

 minerals. The compounds which may serve as sources of iron oxid 

 in clays are 



Oxids — limonite, hematite, magnetite, ilmenite 



Silicates — mica, hornblende, garnet, etc. 



Sulfids — pyrite, marcasite 



Sulfates — melanterite 



Carbonates — siderite 



' E. A, Smith. Ala. geoJ. sur., rep't on agriculf. p. 45. 



