Tannin

“ In a formaldehyde-tanned leather, however, no trace of tannin can be detected; and the yield (of leather, based on the pelt employed) , which, from a practical standpoint, is so important, is so very low that it is hardly possible to speak of it as a tannin in the ordinary sense of the word. Formaldehyde must, therefore, be termed a pseudo-tannin.
The tanning effect of formaldehyde is, according to Thuau, [Footnote: Collegium, 1909, 363, 211.] increased by those salts which bring about colloidal polymerisation of the formaldehyde, the resultant compounds being absorbed by the hide fibre. ”

“ In woods in level districts the proportion of tannin is greatest in localities exposed to the light, while darkness seems to have an unfavorable effect. Here, also, we must refer to the observation that leaves exceptionally exposed to the light are relatively rich in tannin. ”

“ Those trees which flourish in southern countries grow very rapidly, and as tannin is necessary to the development of leaf structure, etc., it is absorbed to a greater extent than is the case with the slower-growing tree of the north. The tannin contained in the sap does not increase in the same ratio as does the rapid growth, and it follows that the remainder in the bark is less than in the tree of slower growth. ”

“ Ure says:—“Various menstrua have been proposed for the purpose of expediting and improving the process of tanning; among others, lime-water and a solution of pearlash; but these two substances form compounds with tannin, which are not decomposable by gelatine; it follows that their effects must be prejudicial. There is very little reason to suppose that any bodies will be found, which, at the same time that they increase the solubility of tannin in water, will not likewise diminish its attraction for skin.” ”

“ This method is very tedious, the end reaction is difficult to hit, the standard solution is very unstable, it is inapplicable to gambier and cutch because the « 115 » mixture will not filter clear, and its results are irregular, probably from the power of tannin to combine with various proportions of gelatin. A plan, which has a seductive appearance of simplicity, is that of Hammer; he takes the sp. gr. of the infusion, then absorbs the tannin with slightly moistened hide-raspings, again takes the sp. gr., and from the difference calculates the percentage of tannin, a difference of 5 per cent. ”

“ It mellows exceedingly, and keeps the leather fibre open; it may be said that it only goes in the leather to prepare and make easy the way for other tannins. Block gambier contains from 35 % to 40 % and cube gambier from 50 % to 65 % of tannin.
Hemlock generally reaches the market as extract, prepared from the bark of the American tree. It contains about 22 % of tannin, has a pine-like odour, but yields a rather dark-coloured red leather.
Quebracho is imported mainly as solid extract, containing 63 % to 70 % of tannin; it is a harsh, light-red tannage, but darkens rapidly on exposure to light. ”

“ The value of substances containing tannin in the preparation of leather is well known. In its pure form it is used as an astringent in medicine; internally, in diarrhœa, hæmorrhages, as a tonic in dyspepsia, &c.; externally, made into a gargle, injection, or ointment.—Dose, 1 to 10 gr., in the form of pills or solution. See Gallic acid, &c. ”

“ Tanning Materials.—Tannin or tannic acid is abundantly formed in a very large number of plants, and secreted in such diverse organs and members as the bark, wood, roots, leaves, seed-pods, fruit, &c. The number of tannins which exists has not been determined, nor has the constitution of those which do exist been satisfactorily settled. ”

“ As regards the first, it will be found on looking at the dripping pile of leather that each butt is covered with a fawn-coloured deposit, known technically as “bloom”; this disguises the under colour of the leather, just like a coat of paint. The theory of the formation of this bloom is this. Strong solutions of tannin, such as are formed between the hides from dusting materials, are not able to exist for long without decomposition, and consequently the tannin begins to condense, and forms other acids and insoluble anhydrides ”

“ The optical activity of tannin is expressed in this formula and its probability is corroborated by Nierenstein, who was able to resolve the acetylated tannin by fractional precipitation into pentacetyl tannin (M.P. 203°-208° C.) and pentacetyl leucotannin (M.P. 166° C.) . By oxidation, the former is converted into ellagic acid, and on hydrolysis with dilute sulphuric acid readily yielded gallic acid. Hydrolysis of the pentacetyl leucotannin, however, yielded gallic aldehyde, and oxidation yielded purpurotannin (a naphthalene derivative) in addition to ellagic acid. ”

“ Some persons have supposed that by infusing tea for a very short time only two or three minutes—the passing of tannin into the infusion would be avoided. This is a delusion; you can no more have tea without tannin than you can have wine without alcohol. Tannin, in the free state, is one of the most soluble substances known. If you pour hot water on a little heap of tannin it dissolves like so much pounded sugar. Tea infused for two minutes was not found sensibly inferior in its retarding power on salivary digestion to tea infused for thirty minutes. ”

“ It is found in many plants, and is prepared commercially from the Turkish oak-galls and the Chinese sumac-galls. It exhibits all the characteristic properties which have been listed above for tannins in general and responds to all the characteristic reactions of a pyrogallol tannin. It is extensively used for the manufacture of blue-black ink, and in many technical processes.
Catechu tannin and catechin are compounds of the catechol tannin type. The latter is obtained from acacia wood, mahogany wood, mimosa wood, etc. It is not a true tannin, since it does not convert hide into leather ”

“ Other animals reject them, except when they can not get anything else. Leguminous plants containing tannin in weak proportions are eaten by horses and cattle, but snails are not fond of them. But the garden snail, which lets fresh clover alone, will eat it freely after the tannin has been extracted with alcohol. It is also probably tannin that inspires snails with respect for vetches, saxifrage, and stone-crop. Many water-plants, likewise, strong in tannin, are respected by water-snails, while the treatment with alcohol converts them into savory dishes for the same animals. ”

“ After thirty minutes' steeping, the quantity of tannin dissolved varied from 1·09 to 4·50 per cent., the average being 2·49, and was in no case equal to half the amount contained in the tea. A larger quantity of tannin was extracted in other experiments in which the tea had been macerated at ordinary temperature before boiling. The tannin in coffee-berry, by all reports, is not more than one third the quantity of that in tea-leaves, and may be considerably less. Six specimens of coffee were steeped for five minutes without yielding any tannin ”

“ If the leaves be soaked so as to wash out the tannin the snail readily accepts them as food. It is true that the small, whitish field-slug (Limax agrestis) does not object to the presence of the tannin, and eats the fresh leaves of the clover; indeed, there is no such thing as absolute protection. In discussing the herbivorous mammals I have already mentioned that many trees and shrubs, mosses and ferns are effectively protected by the large amount of tannin they contain; this protection is effective also against snails, for all these plants are fairly free from their attacks ”

“ Kino is used as a powerful astringent, and is administered in the form of powder and tincture. Some specimens of Butea kino, analysed by Prof. Solly, after the impurities had been separated, yielded 731⁄4 per cent. of tannin.
VALONIA is the commercial name of the cupula or cup of the acorn, produced by the Quercus ægilops and its varieties, the Balonia or Valonia oak, natives of the Levant, from whence, and the Morea, they form a very considerable article of export; containing abundance of tannin they are largely used by tanners. The tannin differs materially from that of nutgalls. ”

“ Nierenstein [Footnote: Ber., 1910, 43, 628.] also succeeded in converting tannin into carboethoxytannin, the latter on saponification yielding crystalline, inactive digallic acid. On acetylating pentacetyl leucotannin with acetyl chloride a hexacetyl derivative (M.P. 159° C.) is obtained, the strychnine salt of which is resolved into both of the active components. This proves the presence of digallic acid and leucotannin in tannin lev. pur. Schering investigated by Nierenstein. ”

“ Such tannin is not by any means an absolutely unitary substance. Its solution, if allowed to stand in the presence of a ferment which is naturally present in gall-nut extract, or more readily if boiled with sulphuric acid, yields a large proportion of gallic acid, which is easily obtained in pure crystals. According to Strecker, glucose is formed at the same time, whence he viewed tannin as a glucoside (see Sugar) . But this is now recognized as a mistake, since Hugo Schiff showed that pure tannin is only digallic acid, C14H1009 = 2C7H605 (gallic acid) minus 1H20. ”

“ It is this tannin which gives the small boy, with his green apples, excruciating pains. It will be recalled that the tannin from the bark of trees, so toughens the elastic skin of animals that we can wear this skin for shoes. ”

“ Tannin is created in the bark of the oak tree, in the first instance, not for the purpose [540] of helping to make leather for man's shoes, but for the purpose of preventing mildew and fungous growths from settling on the bark of the tree and so decaying it. Scent is produced in flowers and shrubs that grow in watery places, not for man's gratification in the first instance, but in order to deodorise the air and make it fit for these scented flowers and shrubs to breathe and to preserve their vitality and vigour. ”

“ The small tree affording these materials grows very abundantly in the swampy lands near the coast in various portions of this country, and the principle of tannin is so strong in the leaves of the manque manse, that it makes a black dye which is indelible. ”

“ From a physiological standpoint the recognition of tannins as esters of glucose and hydroxybenzoic acids, possessing characteristics similar to those of tannin, is of great importance. Especially interesting appears the fact of plants utilising sugars for the esterification of acids, just as glycerol or monohydric alcohols may be employed for the same purpose. Free acids, as a rule, are only tolerated in certain parts of the organism, the latter usually striving to neutralise acidic groups which may be brought about by salt formation ”

“ The reactions with ferric chloride and gelatine should be especially noted, since they are analogous to those given by natural tannins. On the other hand, the reactions with BaCl_2, bromine water and formaldehyde hydrochloric [Footnote: Stiasny carries out the reaction with formaldehyde-hydrochloric acid as follows:—50 c.c. of the tannin solution, plus 5 c.c. concentrated hydrochloric acid and 10 c.c. formaldehyde (40 per cent.) are heated under reflux condenser for ten minutes; most natural tannins are completely precipitated (Collegium, 1906, 435; 1907, 52 et 188) .] ”

“ Many tannins yield glucose, or starch sugar, as one of their products, or as that of closely associated impurities. Of this more must be said later. In addition it will be found that the catechol tannins invariably yield insoluble reddish-brown bodies which have been called phlobaphenes, and which differ from the original tannins in containing one or more molecules less water, and which, in chemical language, are anhydrides of their respective tannic acids. The pyrogallol tannins, on the other hand, yield gallic acid, or ellagic acid (the deposit forming bloom) either alone or in mixture. ”

“ Whereas the catechol tannins (i.e., fir, gambir, hemlock, cutch, mangrove, and quebracho) are coloured black, those of the pyrogallol class (i.e., algarobilla, dividivi, valonea, gallotannic acid, myrabolams, and sumac) bluish-black, and the "mixed" tannins (i.e., canaigre, oak, and mimosa bark) bluish-purple by iron alum, Neradol D is coloured a pure blue. How sensitive this reaction is, the following comparative analyses illustrate: to each litre of tan liquor containing 4 gm. tanning matter prepared from (a) quebracho extract and (b) Neradol D, 10 c.c. of a 10 per cent. ”

“ The sum of her discourse was that they and their women would do well to come down with her to the schooner, recruit at once, and fly to a land of safety where men-of-war never came, where Tanna people reclined all day under the shade of banyan and banana, picked a little cane for their employers occasionally, lived upon tinned meat and sugared tea, and eventually returned loaded with riches in the shape of rifles, cartridges, cotton, and knives. ”

“ Some time after the men had gone, Ais-kaw-ba-wis, who lived by himself in a little lodge, pretending to be too holy to go into a common house, or to mingle with men in their ordinary pursuits, sent for the wife of Gish-kau-ko, and when she came to him, he said, “The Great Spirit is not pleased that you should abandon and lose your property. Go, therefore, and get the kettle that you have left at the sugar camp.” The woman obeyed, and he, soon after she had left the camp, took his gun, and under the pretence of going to hunt, went out in a different direction. ”