Flint may be ground to a fine powder by first raising it to a red heat and quenching it in cold water, then grinding it either under edge runners or in a ball mill. The edge runners are two large granite rollers mounted on a horizontal shaft and revolving within a circular iron pan; for extremely line grinding, water may In-mixed with the flint. The ball mill is a cylindrical vessel built up of iron or steel plates, and having a number of rests or shelves of the same metal. Around the mill there are holes, below which are fitted fine sieves, and steel balls are placed inside the mill. Surrounding the mill is a sheet-iron cover, terminating below in a hopper. As the mill revolves the steel balls are carried up on the shelves and plunged down on the material below, while the pulverised material goes through the sieves into the outer casing of the mill, and is withdrawn from the hopper.