314 JEFFERSON MIDDLETON 



for pressed and ornamental brick. Red earthenware, such 

 as flowerpots, is also made of brick clay. Clays suitable for 

 the manufacture of common brick are located in every state 

 and almost every county in the union. Of all clays these are 

 usually the most impure, often containing a rather high per- 

 centage of lime. Lime in a finely divided state is not harm- 

 ful; but in the form of limestone pebbles it is very injurious 

 to the finished product, as the process of burning converts 

 these pebbles into quickhme, which is apt to slack, thus break- 

 ing the brick, upon being exposed to the atmosphere. This 

 difficulty is sometimes overcome by passing the clay through 

 a pulverizer, which crushes the pebbles, or by screening it to 

 remove them. 



Pressed brick and ornamental brick require a higher grade 

 of clay, the buff varieties being made sometimes from a re- 

 fractory or semi-refractory clay. The buff color in refractory 

 clay is due to the low percentage of iron; in other clays, notably 

 the Milwaukee clay, from which is made, probably, the best 

 known buff or cream brick, it is due to a large percentage of 

 lime, which enters into chemical combination with the iron, 

 thus neutralizing its coloring effect. 



Slip clay is used in glazing low grade products such as 

 earthenware and stoneware. It must fuse at a low tempera- 

 ture and produce a glaze of even color. The clay generally 

 used for this purpose is mined near Albany, N. Y., and hence 

 is known as Albany slip. It produces a dark brown glaze, and 

 is shipped to almost every state in the union. 



Paper clay is used, as its name indicates, in the manufac- 

 ture of paper. As clay used for this purpose is not molded 

 or fired, plasticity, fusibility, and vitrifying quahties need not 

 be considered; hence any pure white, smooth, gritless clay 

 can be used. Where the clay is suitable in color, but gritty, 

 the grit is sometimes washed out. Paper clays are mined in 

 Delaware, Georgia, Maryland, Pennsylvania, and Wisconsin. 



There are several methods of mining clay, but only two 

 are generally followed, viz., open pit and underground min- 

 ing. Open pit mining is conducted in several ways — with 

 pick and shovel, with a scraper, or by falling the clay, that is, 



