The ball mill is used for the grinding of a wide range of material, including coal, pigments and felspar for pottery, and it copes with feed up to about 50mm size. In its simplest form, the ball mill consists of a rotating hollow cylinder, partially filled with balls, with its axis either horizontal or at small angle to the horizontal. The material to be ground may be fed in through a hollow trunnion at one end and the product leaves through a similar trunnion at the other end. The efficiency of grinding increases with hold up in mill, until the voids between the balls are filled. Further increase in the quantity then lowers the efficiency.