274 CLAYS AND CLAY INDUSTRY. 



The latter are turned around or over during the drying, to 

 facilitate shrinkage and drying in all directions. After thorough 

 air drying the green ware is taken to the spraying room, where 

 the slip which is to- form the surface coating is sprayed on it, 

 'thus forming a thin layer over all the surface, and also 1 being 

 somewhat absorbed by the body. The slip, which is commonly 

 a mixture of kaolin, quartz and feldspar, to which the proper 

 -coloring ingredients are added, forms an impervious layer on the 

 surface of the terra cotta, and also produces the color effect on the 

 ware. It is sometimes made so as to- burn to a dull enamel. 



Terra cotta is commonly burned in circular down-draft kilns, 

 whose diameter ranges from 15 to> 25 feet; the kilns are often of 

 the muffle type. 1 The different objects are set in the kiln sur- 

 rounded by a framework of quarries (slabs of fire brick) so that 

 during the burning no- object has to bear any weight other than 

 its own. The total shrinkage in drying and burning is com- 

 monly about 8 per cent., and the temperature reached is commonly 

 between cones 6 and 8 where No. 2 fire clays, or a mixture of 

 these with No. 1 clay is employed. If the terra cotta is made 

 from lower grades of clay, as is seldom the case now except at a 

 few small factories, much lower temperatures are of course used, 

 such as cone 1 or 2. 



NEW JERSEY TERRA-COTTA INDUSTRY. 



Although a number of large terra-cotta works in surrounding 

 states are dependent on New Jersey for their raw materials, there 

 are comparatively few terra-cotta factories in the State itself. 

 The most important centre is Perth Amboy, where 3 factories 

 are located, and there are 4 others in the State respectively at 

 Beverly, Moorestown and Rocky Hill. 



The terra-cotta industry in New Jersey was established first at 

 Perth Amboy, in 1849, tne ^ rst WOI "ks being known as the Hall 



1 A muffle kiln is one in which the flames do not come in contact with the 

 ware, but pass upwards through double walls, and then sometimes down 

 through a flue in the center of the kiln. 



