Before tanning materials can be exhausted, it is almost invariably necessary to crush or grind them, so as to enable the water to get freely at the tannin, which, in most cases, is enclosed in the cellular tissue of the plant. It may be thought that for this purpose it would scarcely be possible to crush too finely, but in practice, a very fine powder is extremely difficult to spend, as it cakes into compact and clay-like masses, through which liquor will not percolate. The object, therefore, is to grind finely enough to allow the liquor ready access to the interior, but not so finely as to prevent liquids running through the mass. The mill most usually employed for this purpose consists of a toothed cone, working inside another cone, also toothed on its interior, precisely like those of a coffee-mill. As bark is frequently delivered "unhatched," or in long pieces, it is necessary to crush it preparatory to grinding, and this is usually accomplished by rollers composed of toothed discs, called breakers. In Fig. 31 is illustrated such a mill, as made by Newall and Barker, of Warrington, combining both utensils. Fig. 32 shows a section of the well-known American "keystone" mill, in which the preliminary breaking is accomplished by the arms A; the bark is then finely ground by the toothed cones N, and discharged at the spout R by the revolving shover M. Fig. 33 shows a somewhat similar mill, made by Gläser of Vienna, in which the axis is horizontal, and driven directly by a belt. It is better to drive bark-mills by a belt than by toothed gearing, as in event of iron getting into them there is less danger of breakage. In America, a cheap cast-iron coupling is frequently used, weak enough to give way before serious damage is done. Safety "friction" clutches are generally ineffective. American bark-mills are run faster than English, up to about 80 rev. per minute, and where the bark is to be used immediately it is frequently damped by a small jet of steam below the mill, which lays dust, and prevents danger of fire. Bark which is damp before grinding can scarcely be ground in these toothed mills, but must be dried, or a disintegrator used. [Illustration: Fig. 31.] [Illustration: Fig. 32.] [Illustration: Fig. 33.]