Date: 1880
In some instances the knives are provided with projecting points, so as to keep the clay in constant motion, as shown in the engraving, this may be ad- vantageous, but it requires more power than plain knives, and a stronger machine than can be made of wood.  This mill, of course, may be driven by horse-power, as shown, or by a water- wheel, or a steam-engine.  When circumstances admit, it is advan- tageous to temper the clay when warm ; this causes the air or gas in the pores of the clay to expand and escape, so that a close con- tact of the particles may be accomplished.