234 NEW TOEK STATE MUSEUM 



ably in shape and size. At small works they are often small 

 square structures. " The fireholes are left above ground and 

 communicate through the various arches and masonry which 

 separate the floor of the chamber above. This floor is full of 

 holes, through which the fire passes and the gases escape through 

 vents in the roof which is arched." Such a chamber is usually 

 about eight feet in diameter and seven feet high. Firing is com- 

 menced at night and proceeds slowly at first. By morning the 

 ware is fairly dry and the heat is then continued from 20 to 30 

 hours, according to conditions. The slip is usually applied only 

 on .the inside of stonewares, the outside being glazed with 

 salt. 



" The salt is thrown in the fire when heat is highest, the vapors 

 are seized by the clay and the soda silicate forms over the surface 

 of the ware. The heat required to do this is the culminating 

 of the point of burning." Either coal or wood fuel is used. 

 Many of the larger works use down-draft rectangular kilns for 

 burning their wares. At Akron, Ohio, these are 32 by 16 feet 

 and 12 feet high. The fireplaces are at each end and are six in 

 number. The ware is set through lateral doors. Burning 

 requires about 70 hours. It is considered that the cooling of a 

 kiln should occupy as much time as the burning. 



A common trouble, in addition to blistering of the slip, is the 

 pitting of the ware in*spots, due to iron. " It is a peculiarity of 

 stoneware clay that while it needs iron to give it color by far the 

 largest part of the iron present is collected in grains and has no 

 favorable effect on the color. These grains, if the heat is high 

 enough to vitrify them, pimple on the surface or break out in 

 rough spots, which are black and ill-looking from the silicate of 

 iron formed. If the heat is too low to vitrify the clay, the slow 

 change in its volume by heat causes a scale to separate from the 

 ware, causing a light colored cavity with the piece of the oxide 

 of iron at the bottom." 



" Failure of the ware to glaze is also considered due to irregu- 

 lar burning, but another cause is thought to act at times." 



In many of the clays there is sulphate of lime present in the 

 form of fine crystals of a whitish color. When exposed to the 



