n (pebble mill) A cylindrical or conical shell rotating horizontally about its axis, partly filled with a grinding medium such as natural flint pebbles, ceramic pellets, or hard metal balls. The material to be ground is added to just fill, or slightly more than fill, the voids between the balls. Water may or may not be added. The shell is rotated at a speed that causes the balls to cascade, thus reducing the particle sizes by repeated impacts. The operation may be batchwise or continuous. In the plastics industry, the term ball mill is reserved by some persons for mills containing metallic grinding media, and the term pebble mill for nonmetallic media. For paint, steel balls are frequently used. (Perry RH, Green DW (1997) Perry’s chemical engineer’s handbook, 7th edn. McGraw-Hill, New York; Weismantal GF (1981) Paint handbook. McGraw-Hill Corporation, Inc., New York).