' CLAYS OF NEW YOKK 811 



coefficient of expansion may also be diminished if the percentage 

 of boracic acid in the clay is increased at the expense of the silica. 

 The amount of alumina exerts but little influence on the expansion 

 or contraction of the glaze, but a small percentage of alumina pre- 

 vents glazes which are poor in alkalis from becoming opaque. 



The tenacity of adherence of the glaze to the body depends on 

 the composition of both and also on the temperature of the kiln. 



We can say that the power of the body to carry a glaze without 

 causing it to crack is influenced by its rational composition, its 

 degree of plasticity, the fineness of the quartz grains which it con- 

 tains, and the temperature at which it is burned. 



Burning white earihenware and china 



This is done in saggers, which are oval or cylindric receptacles 

 about 20 inches in diameter, 8 inches in hight, with a flat bottom. 

 The saggers are filled with the pieces of the unburned ware and 

 are set one on top of the other, so that the bottom of one forms a 

 cover for the one below it, the joint between them being closed by 

 meai^ of a strip of soft clay. The use of these saggers is to protect 

 the ware from the smoke and gases of the kiln fire, which would 

 tend to discolor it. 



The requisite of a sagger clay is that it stand slightly more heat 

 than the ware placed in it. Saggers are generally made from a 

 plastic, refractory clay, with as great an admixture of grog (ground 

 up fire brick or old pottery) as possible, but an excess of the latter 

 is deleterious. The color burning properties of a sagger clay are 

 of little importance. Saggers are made in various ways, sometimes 

 being turned on a wheel, or again being formed in plaster molds, or 

 around wooden forms. In Germany metal forms are now mostly 

 used, because they permit the working of a stifler mass, and, the 

 clay containing less water, the saggers after molding shrink and 

 tear less, while in addition they dry more quickly. The interior of 

 the sagger is frequently coated with a slip of kaolin and quartz, in 

 order that the ware may not receive any discoloration from this 



