In a ball mill, hard spheres mixed with the stock to be comminuted are rotated inside a large cylindrical container The motion causes the balls and stock to be carried up the container wall, and then pulled back down by gravity to accomplish a grinding action by a combination of impact and attrition. These operations are often carried out with water added to the mixture, so that the ceramic is in the form of a slurry. In a roller mill stock is compressed against a flat horizontal grinding table by rollers riding over the table surface. Although not clearly shown in our sketch, the pressure of the grinding rollers against the table is regulated by mechanical springs or hydraulic-pneumatic means. In impactgrinding, which seems to be less frequently used particles of stock are thrown against a hard flat surface, either in a high velocity air stream or in a high-speed slurry. The impact fractures the pieces into smaller particles.