CLAY 315 



by iindorminin^ it and letting it fall. Sometimes explosives 

 are used for the Insl named pm-pose. 



In nnderground clay niininjz; the ni(>(li()(ls of working; are 

 the same as those usually followed in coal mining, namely, 

 by shaft or tunnel, with drifts, slopes, and chambers; there 

 are, however, comparatively few deep mines in the clay in- 

 dustr}', except where the clay is mined in connection with 

 coal. Kaolin is sometimes mined by sinking through the 

 vein a round shaft, the sides of which are timbered. 



The commoner grades of clay are generally sold as mined, 

 because to prepare them for the market in any way would add 

 so greatl}^ to their cost as to make them unprofitable. Even 

 the common clays, however, are sometimes screened or ground 

 to get rid of limestone pebbles or other small stones; screening 

 removes them mechanically, while grinding overcomes their 

 bad effects through comminution. In this country the com- 

 mon clays are not washed. The kaolins and ball clays are, 

 however, sometimes washed at the mines, but they are again 

 washed or prepared at the pottery before being manufactured 

 into white ware. Another method of separating the clay 

 from its undesirable impurities is b}^ fans. This method 

 can only be employed where the impurities are in the 

 form of coarse particles or grains of sand, and consists in first 

 grinding the clay to a ver^' fine powder, after which it is car- 

 ried to a flue through which a strong current of air is forced; 

 this immediateh' carries the finer particles to the end of the 

 flue where they are dropped into a bin. The coarser particles 

 are dropped into the flue and carried back to the pulverizer 

 to be ground over again. In the manufacture of potterv', and 

 also of brick by the dry press process, the clay is cured, or 

 weathered, often for months, before being used. 



Clay is used most extensively in the manufacture of com- 

 mon building brick ; next in importance, as measured bj^ the 

 quantity of material consumed, is front or pressed brick, with 

 its almost limitless variety in color and shape. Then there 

 are vitrified brick, with a large and growing field of usefuhiess, 

 and fire brick, the shape and utility of which are almost with- 

 out limit, stove lining being an important subdivision. Other 

 important products are drain tile, sewer pipe, ornamental ter- 



