Oak-bark is the substance most commonly used to supply the astringent principle, and it is crushed or ground to powder in a _bark-mill_. In the old method of tanning, which is not yet entirely abandoned, the hides and powdered bark were laid in alternate layers in the _tan pit_, which was then filled with water to the brim. After some months the pit was emptied, and refilled with fresh bark and water; and this process was repeated whenever the strength of the bark was exhausted. In this way, the time required for impregnating the hides varied, according to their thickness and other circumstances, from one to four years. The process has been greatly expedited by the improvement, introduced in consequence of the experiments of M. Seguin, a French chemist, of tanning with concentrated solutions of bark, formed by passing water through a mass of powdered bark, until, by successive filtrations, it is completely deprived of its soluble tanning principle. [Illustration: Bark Box. Barrow.] The variations of practice among different Tanners extend to the substance used as an astringent, as well as to the manner of applying it. Ground oak-bark, which was formerly the only material in common use, and is still the most general, produces good leather of a light fawn colour. Valonia, of which considerable quantities are imported for the use of Tanners, produces leather of great solidity and weight, the colour of which is inclined to grey, and which is more impervious to water than that made with oak-bark. Valonia consists of the acorns of the _Quercus Ægilops_, and is brought from the Levant and the Morea. Catechu, or terra japonica, the extract of the _Acacia Catechu_, produces leather of a dark reddish fawn colour, which is light, spongy, and very pervious to water. [Illustration: Fleshing Knife, and Section. Shaving Knife, and Section. Striking Pin, and Section. Unhairing Knife, and Section.] When the process is complete, the hides are hung up in a shed and allowed to dry slowly; and while they are drying they are compressed by beating, or rubbing, or by passing them between rollers, to give them firmness and density. A yellow deposit is now found upon the surface of the leather, to which the name of “bloom” or “pitching” is technically given.