THE POTTERY INDUSTRY. 301 



"wad" clay. The use of these saggers is to- protect the ware from 

 the smoke and gases of the kiln fire. The chief requisite of a 

 sagger clay is that it shall stand slightly more heat than the ware 

 placed in it, repeated firing and cooling, as well as handling with- 

 out breaking. Saggers are generally made from a plastic, refrac- 

 tory clay, with the maximum admixture of grog, *. e. } a pow- 

 der made of old saggers, broken fire brick, etc. The kilns are 

 generally circular down-draft, having a diameter of from 15 to 25 

 feet. The temperature reached in burning varies. White earth- 

 enware is commonly burned at from cone 8 to 9, while porcelain 

 may be fired as high as cone 12 or 13. Since the color of ferrous 

 iron is less noticeable than ferric iron, the fires should be reducing 

 during at least the last part of the firing, and the kiln is then 

 cooled down as rapidly as possible to prevent the oxidation of 

 whatever iron may be in the clay. 



Glazing pottery. — Common red earthenware is rarely glazed, 

 but if this is done, the glaze consists of an easily fusible mixture 

 of metallic oxides, such as lead, together with quartz, and some- 

 times boracic acid. 



Stoneware is usually slip glazed or salt glazed. A slip glaze 

 consists of an easily fusible clay, which will melt to a colored 

 glaze at the temperature reached in burning the stoneware. This 

 is stirred up in water to a slip and the green ware dipped into it, 

 or if the articles are very large, the slip may be applied with a 

 brush. When placed in the kiln and burned, the slip melts to an 

 enamel over the surface of the ware. One of the slip clays most 

 used is that from Albany, N. Y., which has been found suited to a 

 wide range of clays. It melts to> an enamel at about cone 6. 

 Another slip clay is dug near Rowley, Mich., and a third near 

 Seneca Falls, N. Y. 



When the wares are to> be salt glazed, they are placed in the 

 kiln, unprotected from the flames. As soon as the kiln has 

 reached its highest temperature, the salt is put in the fire places, 

 one or two shovels full at a time, at regular intervals, so that the 

 addition of the salt may extend over several hours. When the 

 salt is placed in the fires the heat volatilizes it, and the vapors in 

 passing up through the kiln unite with the clay, forming a glaze 



