SECTION III.—VEGETABLE TANNAGE
All tannages have for their object the conversion of the readily putrescible hide tissue of the corium (the pelt) into an imputrescible, insoluble and permanent material called "leather" which, possessing considerable strength and pliability, is capable of application to a variety of useful purposes. The conditions necessary for this transformation have been clearly stated by Procter. [1] For the production of leather from pelt "it is not only necessary to dry the fibres in a separate and non-adherent condition, but so to coat them or alter their chemical character that they are no longer capable of being swelled or rendered sticky by water." Whatever substance will secure this permanent dehydration of the hide fibres in a separate condition is called a "tanning material." The change from pelt to leather is known as "tannage," the process is termed "tanning," and those who undertake it are "tanners."
[1] "Principles of Leather Manufacture," p. 184.
In "vegetable tannage" the tanning materials are of vegetable origin, and contain a group of organic compounds called "tannins" which are extracted by the infusion of these materials with water. Pelt, when immersed in these infusions, is converted into leather, rather slowly; but a gelatin solution gives an immediate precipitate of "amorphous leather," even if the tannin infusion be exceedingly dilute. The tannins are aromatic compounds of phenolic character, and contain carbon, hydrogen and oxygen only, but our knowledge of their chemical constitution is exceedingly small owing to their instability and colloid nature, which make impossible their preparation in a pure state. They are all, however, derived from either catechol or pyrogallol, and yield these substances if carefully heated to about 200° C. The tannins are soluble in water, alcohol, acetone, ethyl acetate and acetic acid, but insoluble in benzene, chloroform, carbon disulphide, petroleum ether, dilute sulphuric acid and pure ethyl ether. The aqueous infusions of the tannins are in reality colloidal solutions; i.e. heterogeneous systems of two phases. The systems are lyophile, or, more particularly, hydrophile, i.e. there is an affinity between the two phases. As usual with lyophile systems the two phases may be considered as both liquid, and an aqueous infusion of tannin forms an emulsoid sol, which therefore is subject to the phenomenon of adsorption. The tannins are all precipitated by solutions of basic lead acetate and copper acetate, and many of them with varying completeness by solutions of many other metallic salts and hydroxides, of basic dyestuffs and of alkaloids. They give dark colorations with ferric salts.
The tannins are widely distributed in plant-life, but only in a limited number of cases do the plants contain sufficient tannin to render them of commercial importance. Tannin is found in all parts of plants, but usually in greatest amount in the bark or fruit. The tannins are classified into "pyrogallol tans" and "catechol tans," according to the parent phenol. This classification is confirmed by their chemical, analytical and practical behaviour, and the vegetable tanning materials may be classified into the same two groups, for, although even the same plant contains both pyrogallol and catechol tans, it is usual to find in any one part of the plant that one group is predominant.
Pyrogallol tans, which are oftenest obtained from fruit or leaves, contain usually about 52 per cent. of carbon. Used alone they produce a rather soft and porous leather. Associated with them—in many cases probably as decomposition products—are certain other substances of well-known properties and constitution. These substances are not only typical of the group, but also form the most valuable clue to the chemical constitution of the group and the key to their chemical behaviour. One of these substances is gallic acid (3:4:5 trihydroxy-benzoic acid C6H2(OH)3COOH), which possesses properties very similar to the tannins, but does not precipitate gelatin and will not itself make leather. Another of these substances is ellagic acid C14H6O8, a double lactone of a hexa-hydroxy-diphenyldicarboxylic acid. This is deposited as an insoluble yellow powder from infusions of many pyrogallol tans, by boiling with dilute acids only, allowing them to stand for a few days. In practice the deposit is found as mud at the bottom of the tan pits, and also upon the leather, to which it strongly adheres. It is technically known as "bloom." It is insoluble in acids and cold alcohol, but soluble in alkalies. It is a feeble dye-stuff. The pyrogallol tans yield very different amounts of bloom. Other associated substances are the sugars. In practice these sugars ferment to lactic, acetic, and other acids which cause "sour" liquors. Such liquors plump the hides and tend to give firm, thick leather. These acids also probably cause increase of adsorption of tannin by the hide and therefore assist in giving "good weight." Solutions of pyrogallol tans all give a blue-black colour with a dilute solution of ferric alum. If a solution of sodium arsenate be added to an infusion of pyrogallol tan diluted until no longer distinctly coloured, and the mixture allowed to stand for about two hours, a green colour develops at the surface of the liquid. The reaction is due to gallic acid or a similar grouping, and is, in the author's experience, the most satisfactory qualitative test for the group. Another test is to mix equal volumes of a 0.4 per cent. infusion of tan and a 10 per cent. solution of sodium bisulphite; a few drops of 10 per cent. potassium chromate are added, and either a transient blood-red colour or a more permanent deep purple is obtained. The former colour is due to gallic acid. If a tannin infusion be largely diluted with hard water and a little iodine solution added, the pyrogallol tans yield either a purple-red or a dark blue colour, the former being a reaction of gallic acid. Pyrogallol tans yield no precipitate with bromine water. They yield a yellow or brown colour when one drop of infusion is added to concentrated sulphuric acid.
Myrabolans is one of the most important of the pyrogallol tanning materials. It is a name given to the dried fruit of Terminalia chebula and other species of Indian trees. The nuts resemble an elongated walnut. They are dried and exported from many parts of India to all parts of the world, but largely to this country. The varieties of commerce are named according to origin and quality: thus we have "J1's," i.e. Jubbelpore, No. 1 quality, "R1's" (Rajpore, No. 1), "B1's" (Bhimley, No. 1), etc. The little difference in tannin strength (about 32 per cent.) in these varieties is usually compensated by corresponding differences in price. The quality of myrabolans cannot be safely judged by appearance. Much bloom is deposited by myrabolans liquors, especially by "J's." Myrabolans are amongst the most sugary of tanning materials, containing up to 5½ per cent. It is therefore one of the best materials for giving a plump leather. Broadly speaking, those varieties which yield most sugar yield least bloom, and vice versâ. Myrabolans tannin has a small affinity for hide substance and penetrates the hide very slowly. It gives a "mellow" tannage, but a bright, good colour, which characteristics are imparted to the leather when the material is blended with other materials containing dark or astringent tannins. When used alone it yields a rather spongy leather, and it is not considered a good weight-giving material, though its acid-producing powers are very helpful to other more astringent tannins.
Valonia has been the other staple tanning material of the heavy leather trade. It is the acorn cup of oaks common in Asia Minor and Greece, chiefly the Turkish oak (Quercus ægilops). The fruit is gathered when ripe and dried in layers of about one foot deep until the acorn drops out, Smyrna is the great export centre. Greek valonia is obtained from many parts of the Archipelago and mainland. It is gathered in a more immature condition and includes the acorn. It is considered slightly inferior in strength and colour to the Smyrna valonia. The exterior of the acorn cup is covered with rather scaly protuberances known as "beard," which contains usually about 40 per cent. of tannin. The cup alone contains usually about 25 per cent. tannin, and the whole about 30 per cent. The valonia tannin has been thought to contain two chemical individuals, only one of which produces bloom. Parker and Leach [2] found that the tannin of the cup produces more bloom than that of the beard, and that Smyrna valonia yields more bloom than Greek. The more bloom is deposited, the less acid will be produced. Under all conditions the yield of bloom is large, and its deposition in and on the leather assists materially in giving the weight and water-resisting powers associated with sole leather which has been largely tanned with valonia. The valonia tannins have only a moderate affinity for hide, which, like myrabolans, they penetrate very slowly. When used alone the leather is less yellow than that from myrabolans, and is also duller. After most of its bloom has been deposited valonia makes a very suitable tannage for dressing leather, and in conjunction with gambier has been largely thus used. Since the outbreak of war the Turkish product has, of course, not been available for importation.
[2] J.S.S.I., 1903, 1184.
Sumach[3] is the other pyrogallol tan of commercial importance. It consists of the leaves and small twigs of the Sicilian sumach (Rhus coriaria) cultivated in Italy extensively for export. The leaves are hand picked, dried and often ground to powder. It contains 26-28 per cent. of a tannin which yields little or no bloom, but much gallic acid. It is an unstable tannin, and its infusion rapidly ferments. Sumach is a very valuable tanning material, and when used alone gives an exceedingly durable leather of excellent light colour. It gives a soft mellow tannage, and is therefore most suitable for light leather tanning, and is extensively used for this purpose. It is used, nevertheless, in large quantities by the heavy leather tanners for finishing purposes, for it contains some organic reducing agent which exerts a powerful bleaching action on other tannages, and which assists to brighten as well as lighten the rather dull appearance of leathers largely tanned with valonia. It is rather an expensive tannin, but most manufacturers find that its results are worth its cost.