634 NEW YORK STATE MUSEUM 



of better grades of ware, sucli as front brick and terra cotta, the 

 preparation of tbe clay is often a matter of the greatest importance, 

 in order to provide a mass of material wbich. will be bomogeneous 

 tbrougbont, and whose physical properties shall not vary. It some- 

 times happens that this operation means simply the breaking np of 

 the clay thoronghly or the loosening of all the clay particles. The 

 greater the care with which these operations are carried on the 

 more homogeneous will be the material and the better the grade 

 of the wares produced. 



Removal of foreign matter 



This can be sometimes rendered harmless either by distributing 

 it in a finely divided condition through the clay, or by the addition 

 of chemicals, or sometimes it may be removed entirely, the method 

 employed depending on the character of the clay. 



Cleansing' clay. This includes the removal of roots, pyrite, lime 

 pebbles, and similar substances. The simplest method is by hand- 

 picking, which is slow and incomplete. The custom followed at 

 the present day is either to dry the clay and pass it through a sieve 

 of the proper mesh, or to treat it to a washing process or even to 

 an air separation. 



Cleaning dried clay. Most clays are naturally moist, but when 

 occurring in the form of shale the percentage of water is usually 

 very low; very sandy clays are also apt to run low in moisture. 

 "With dried clays, the purification can be accomplished by first pul- 

 verizing the material, and then allowing the product to fall through 

 a strong air current, the effect of this being to separate the particles 

 according to their specific gravity, those of clay being carried far- 

 thest, while heavier particles, such as pyrite, are dropped first, a 

 fairly complete separation taking place. 



Wet process of purification. This is done by subjecting the clay 

 to a washing process. (See "Preparation," p. Y99) 



Separation of iron particles. In the manufacture of certain 

 products, and also certain glazes, it is necessary that the material 



