CLAYS OF NEW YORK 635 



used shall be thorouglily free from iron, as in burning, this element 

 makes itself very noticeable ; slight specks of it might mar the ware 

 sufficiently to make it unsalable. The removal of iron grains is 

 accomplished by means of a strong magnet, which in case the clay 

 is used in the form of a slip, is suspended in it, or, if powdered 

 clay is used the powder is allowed to pass over the poles of the 

 magnet; in either case the iron particles are extracted, the magnet 

 being cleaned from time to time. 



Purification of fluxes and grogs 



Many of the materials belonging to either of these two classes 

 are often more or less dirty and can be cleaned by washing. If the 

 grogs used, however, contain appreciable particles of iron, it is beet 

 to remove these by hand picking as far as possible, before the 

 material is powdered for use. Feldspar having a red or yellow 

 color frequently contains iron oxid, and such should not be used if 

 the feldspar is to be used in the manufacture of light colored or 

 colorless glazes, while if it is to be used for dark glazes the iron 

 oxid contents are of less importance. 



Many red feldspars, however, when calcined became pure white, 

 showing that the coloration is not due to iron. 



