Grinding and Leaching of Tanning Materials from Leather Entry in the Encyclopaedia Britannica MDCCCLXXXII
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Grinding and Leaching of Tanning Materials. – Bark, valonia, myrobalans, and other tanning bodies are reduced to a small and as far as possible uniform size by means of grinding or comminuting machinery. The main object in such machines is to produce uniformity of size with as little dust as possible, and the apparatus most commonly used is similar in principle to the ordinary coffee-mill, with breaking arms for the bark and segmental cutters for small materials. Various forms of disintegrator are also used, which produce their effect by violent concussion obtained by the revolution in opposite directions of two large and strong disks armed with projecting spikes on the sides of the disks facing each other. These disks are enclosed within a stout iron drum ; and, as they revolve at a speed rising to three thousand revolutions per minute, some conception of the violence with which the tanning materials are struck and smashed may be formed. The tanning materials so prepared are next leached, latched, or infused for preparing the strongest tanning solutions for use in the “layers” or lay-away pits noticed below. In making these leaches or infusions, some tanners use hot (even boiling) water, others use cold water alone ; some employ only pure water, and by some the weak and exhausted oozes or woozes from the pits are strengthened up by renewed leaching. The sole object of the tanner is to obtain the greatest amount of the tanning principles contained in the materials operated on, and to take care that what he gets is not lost or wasted. The method of leaching commonly adopted in the United Kingdom is to pass the bark through a series of leachers or spender pits. New or fresh bark is put into the first of the series, and over it is pumped cold the well-strengthened ooze from the next leacher. In this first pit the ooze or infusion is brought up to the full strength required for the lay-away tan-pits, and after the infusion is pumped off the tan (now somewhat reduced in strength) is passed over into No. 2 leacher, where it is treated with liquor in its turn also somewhat lower in strength. In this manner the bark passes by stages through a series of pits, diminishing in richness in tannin at each stage, and in the same gradual manner being infused in a weaker and weaker liquor, till in the last of the series it is fully exhausted with pure warm water. Thus pure water is put in at one end of the range and fresh tanning material at the other ; the water as it ascends is gradually strengthened till it reach the maximum richness in tanning principle, while the tanning material as it descends is in like proportion deprived of its extractive constituents, till in the end nothing further soluble remains. From the last pit the bark, &c., are turned out as “spent tan,” usually to be burned in a special form of tan-burning furnace for raising steam. The use of leaches or infusions was first insisted on by Seguin about the end of the 18th century, and the adoption of his suggestion led to the shortening of the time occupied in tanning heavy leather by about one half.