382 L E A T H E R
Tasmania (A. leucophylla), and A. cyanophylla. The red colour of mimosa bark produces a dark leather against which there is a prejudice, and the material has therefore to be used sparingly in mixtures. It is also said that mimosa tanning results in a somewhat hard brittle leather.
Hemlock Bark is the most important tanning material in North America. It is the produce of the hemlock spruce, Abies canadensis, which grows in vast forests throughout Canada and the northern and eastern States of the Union, the principal bark-producing States being Pennsylvania, Michigan, and Wisconsin. Hemlock bark is obtained by cutting down the trees; and, as no provision is made for renewing the denuded forests, the strain on the more accessible portions of the American forests is already beginning to make itself apparent. The bark contains 7 or 8 per cent. of tannin, and the leather it makes has a strong reddish-brown colour. A large and increasing amount of hemlock extract, an inspissated decoction of the bark having a specified richness in tannin, is now imported into Europe, principally to the United Kingdom.
Among barks used to a limited extent and for special purposes are larch and Scotch fir barks, used for tanning sheep skins into basils, &c. Willow bark is used in Russia for tanning russia leather. Mangrove barks (Rhizophora Mangle), which are exceedingly abundant in India, and rich in tannin, have been tried in the United Kingdom, but their use did not prove satisfactory.
Wood. – Quebracho wood (Aspidospermum Quebracho), a wood rich in tannin, obtained from the river Plate, has recently grown rapidly in favour as a tanning substance in the United States and France, and is now coming into notice in the United Kingdom.
Fruits. – Under this head are comprised valonia, myrobalans, and divi-divi, three substances which now play an important part in tanning both in Europe and America. Valonia consists of the imbricated acorn cups of a species of oak, Quercus Ægilops, which is indigenous in Asia Minor, Greece, and southern Turkey, and is mainly shipped from Smyrna. It contains as much as from 40 to 45 per cent. of tannin, and the average annual imports into the United Kingdom are now not less than 30,000 tons. Myrobalans are the dried immature fruit of species of Terminalia, principally T. Bellerica and T. Chebula. They vary in size and appearance, but in general they are oval hard wrinkled nuts rather larger than a filbert. The amount of tannin they yield varies from 20 to 36 or sometimes as high as 40 per cent. Myrobalans grow abundantly throughout India and are largely and increasingly exported from the three presidencies to the United Kingdom. DIVI-DIVI (q.v.) contains as much as 50 per cent. of tannin. Though it is in considerable use, tanning with divi-divi is subject to several grave objections. The abundant mucilage of the pods ferments readily, and thereby some times causes a rotting of the hides. Leather tanned by divi-divi also draws moisture readily in presence of damp, while in a dry atmosphere it is hard and horny, and further this agent leaves finished leather very dark in colour.
Leaves. Sumach. – The leaves of various species of Rhus, under the name of sumach, or sumac, form materials of the first import ance for the tanning of light skins in which it is essential to have a fine white colour, as in the case of bright morocco leather, &c. Of the species the most important is the Sicilian sumach, R. Coriaria, a shrub or low tree indigenous to Italy, Spain, France, and the Mediterranean coasts of Africa. It is cultivated with much care in Sicily, the leaves being gathered from shoots not more than a year old, which thereafter are cut down close to the parent stem. The same shrub is largely cultivated in the southern departments of France, in Spain, and in Portugal. The leaves gathered in June are dried and ground under edge rollers to a fine dust, in which con dition the material comes into the market. It has a bright olive green colour with something of the odour of tea, and should contain in the best qualities from 25 to 30 per cent. of gallotannic acid, the same tannin which is present in galls. The leaves of the Venetian sumach, R. Cotinus, are similarly used in the eastern countries bordering on the Mediterranean. In America a large quantity of sumach for tanning is obtained from two species of Rhus, R. copallina and R. glabra, growing principally in Virginia, the Carolinas, Georgia, and Alabama. These are, however, much inferior to European sumach, both on account of the colour they communi cate to leather, and also from the smaller percentage of tannin they yield, although it is believed that with careful cultivation and proper attention the quality might be greatly improved.
Extracts. – Cutch and Gambir, vegetable extracts obtained from the East, are frequently confounded, and in commerce they in differently pass under the name of terra-japonica and catechu (see CATECHU, vol. v. p. 220). Gambir, which comes almost exclusively from Singapore, is the inspissated juice of the leaves of Uncaria Gambir, a tall shrub belonging to the natural order Rubiaceæ. The extract comes into the market in the form of cubes about an inch in size, of a dull brown earthy appearance. In composition and properties it agrees with Cutch. The exports from Singapore amount to about 80,000 tons annually. KINO (q.v.) is exceedingly rich in a variety of tannic acid, but its high price precludes its use in tanning. These extracts are used in tanning only in conjunc
tion with other materials; they not only hasten the operation, but, judiciously used, they tend to render the leather soft and mellow.
Galls. – Although galls are among the richest of all bodies in tannic acid, they do not form an important item among the materials of the tanner, being most valuable for other industrial purposes, and therefore too costly for use as tanning agents. The "knoppern" galls of Hungary, which are formed on the acorn cups of a species of oak, however, are to some extent used in Continental tanneries. For full information regarding galls, see vol. x. p. 43.
It is to be noted that most of the tanning substances above alluded to may be and are used in dyeing as well as for tanning.
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
smaller 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 principle
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.
Testing Tan Liquors. – The methods by which the tanning value of any substance is determined are numerous, but few of them are at once capable of simple application and minutely accurate. One of the commonest plans for ascertaining the strength of the tan liquor technically called ooze, or wooze, is by means of a kind of hydrometer called a
