813 NEW YORK STATE MUSEUM 



source. "When complicated forms are placed in tlie sagger the over- 

 hanging or greatly projecting portions are supported by pieces 

 which have the same composition as the ware itself, so that in 

 burning the shrinkage of both will be the same. 



The proper placing of the ware in the kiln as well as in the 

 saggers is a matter of great importance. 



The condition of the fires in the burning of porcelain or earthen- 

 ware has to be taken into consideration. In the burning of spar 

 china from redness up to the point of vitrification, it is desirable to 

 have the fire reducing in its action, while above that point it should 

 be neutral or weakly oxidizing. In using coal which contains 

 pyrite, if the fire is oxidizing, sulfuric acid is set free; and this 

 tends to unite with any lime carbonate or alkalis v/hich the glaze 

 may contain, the lower the temperature of the kiln the m^ore rapid 

 this union, for the lime and alkalis will unite with the sulfuric 

 acid, as long as they have not entered in coiubination with the silica 

 of the glaze. When the glaze has once melted, the danger that this 

 will take place is far less. If the gases are reducing, any sulfate 

 salts formed are broken up and sulfurous acid gas escapes. If the 

 glaze particles have not yet been thoroughly fused the gas just 

 mentioned escapes without causing any trouble; but, if the fusion 

 has already occurred, blistering or scaling of the glazed surface re- 

 sults. 



Both the body and the glaze may sometimes have a small amount 

 ' of gj^psum, which may come from an Alsing cylinder, if such a 

 machine is used for grinding the clay. The reducing action of the 

 fire must, however, not be too strong, otherwise any organic matter 

 which the clay contains will not burn off at the proper time, and 

 will subsequently be liable to cause bursting in the ware during 

 burning. A reducing fire tends to insure a whiter color in the ware 

 by reducing any ferric combination of iron, thus carrying the color 

 from reddish over into whitish gray, or the pale green of complex 

 ferrous silicates. The latter are hardly noticeable so that the whole 

 body -appears white. It sometimes happens that during the slow 



