The method failed theoretically, because a definite molecular weight had to be assumed for tanninswhich are all different.
Oak bark produces the best leather known, proving that a blend of the two classes of tannins gives the best results.
The number of tannins which exists has not been determined, nor has the constitution of those which do exist been satisfactorily settled.
Pyrogallol tannins include some of the lightest coloured and best materials known, and, speaking generally, the leather produced by them is not so harsh or hard as that produced with catechol tannins.
Pyrogallol tannins give a blue-black coloration or precipitate with ferric salts, and catechol tannins a green-black; and whereas bromine water gives a precipitate with catechol tannins, it does not with pyrogallol tannins.
The fact that tannins retard intestinal peristalsis, whereas coffee promotes this digestive action, lends further proof to the non-existence of tannin in coffee.
Tannins precipitate proteins, gelatine, and connective tissue, and thus act as astringents, styptics, and antiseptics.
The different tannins are not equivalent in these respects.
Bearing in mind that concentrated extracts of vegetabletannins in some circumstances effect a "dead" tannage (cf.
As is well known, synthetic tannins frequently possess the practically important property of rendering natural tanninseasily soluble in water.
Such substances, in our present-day terminology, are termed pseudo-tannins (e.
In view of this fact it is doubly pleasing to see that British chemists have found new ways, and are able to produce equally good and more varied synthetic tannins than has hitherto been deemed possible.
Relative Behaviour of an Alkaline Solution of Bakelite and Natural Tannins F.
The production of synthetic tannins employs two quite distinct methods; one is to synthesise the most simple tannin, viz.
First of all, however, it may illustrate the extreme importance and the universal applicability of the synthetic tanninsin the making of leather.
It has been previously pointed out that the tannins differ largely in their penetrating and weight-giving powers.
Pelt (whether delimed or not) is rapidly acidified by the quickly penetrating and strongly adsorbed organic acids of the old tan liquors and becomes positively charged before the tannins are adsorbed.
The valonia tannins have only a moderate affinity for hide, which, like myrabolans, they penetrate very slowly.
Whatever class of leather is being made, and whatever blend of tanning materials is being employed, the tannins must be efficiently extracted by water in order to make the tanning liquors.
The tannins are classified into "pyrogallol tans" and "catechol tans," according to the parent phenol.
A tan liquor usually contains several tannins in addition to other closely similar substances, also in colloidal solution, and is therefore a sol of considerable complexity.
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.
Valuable pyrogalloltannins are obtained from oak wood and chestnut wood, but the woods are not used in tanning as the percentage of tan is so small.
The aqueous infusions of the tannins are in reality colloidal solutions; i.
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 differ from the vegetable tannins in that they contain sulphur and sulphonic acid groups, but they agree in that both are aromatic derivatives.
All the tannins and many associated substances darken rapidly with oxidation when in alkaline solution, so that to place the fully limed hide in a tan liquor would give a dark-coloured leather.
The tannins all give greyish to black shades with iron salts, and it is this fact which renders them important for fur dyeing.
The extra tannage which the skins received from the tanninsused in the dye mixtures for these furs, accounts for this increased resistance to heat.
However, many of these tannins also have dyeing properties, and are used in dyeing the furs.
While the tannins can be used alone with an iron mordant for producing greyish to black shades, the dyewoods alone yield colors which would be too bright to be suitable for dyeing furs.
Yet there are unmistakable indications that the tannins were employed for tanning at a period which reaches back to the dawn of history.
Cutch acts as a tannin, and like other tannins discussed above, can be used for the production of grey or black shades with iron mordants.
For it is only in very recent times that these tannins have been superseded in part by new tanning substances whose use is simpler and more time-saving.
Tannins that rapidly oxidise in contact with light and air have, therefore, to be avoided.
For dressing wool and fur skins, the synthetic tannins are much superior to the alum and salt process.
The latter penetrates the hides in a few hours without contracting the grain, increases the solubility of the vegetable tannins subsequently used, and helps to keep the leather light and uniform in colour.
The pyrogallol tannins give a bluish-black colour, and the catechol tannins a greenish-black, with iron salts.
These tannins can be suitably blended to produce first-class leather.
The use of combinations of vegetable and mineral tannins has lately increased, and it is possible that the blending of the two classes of materials may produce an ideal tannage for certain classes of leather.
Synthetic tannins can also be used alone for light leathers.
Pyrogallol tannins give a light coloured, soft leather, and can be used alone satisfactorily; but heavy leathers, such as sole and belting, need a blend of both kinds of tannin.
Owing, perhaps, to faulty preparation, this tanning extract was not well received at first, but it is now among the principal tannins and increases in importance every year.
It was thought at first that these tannins might play as important a part in the leather trade as the synthetic dye-stuffs have, but it is now generally believed that this will not be the case.
A great many other tannins are known, and their possibilities for technical use in tanning, dyeing, etc.
There seems to be no evidence that tannins perform any such function as this in higher plants, however.
Many conflicting views have been advanced, but a careful review of these leads inevitably to the conclusion that tannins probably do not serve in any significant way as food material.
Tannins are also found in certain special structures, such as gland cells, cells of the pulvini, laticiferous tissues, etc.
The disappearance of tannins during the process of ripening of fruits may be connected, in part at least, with the development of the brilliant red, blue, and yellow pigments which give such rich colors to the thoroughly ripe fruits.
Among the higher plants, tannins occur in a great variety of organs.
Recent investigations show that at least some of the varieties of fruits which are resistant to the attacks of certain parasitic diseases utilize tannins for this purpose.
It may be, therefore, that tannins serve as safety accumulations of excessive condensations of formaldehyde, or other photosynthetic products, under such conditions.
It seems certain that in all such cases tannins are the result of, and not (as some investigators have supposed) the causative agents for, the abnormally rapid metabolism.
The tannins in the Japanese species, although of the same type as in the native tree, are more soluble and reach a level toxic to the fungus.
In the Chinese trees all the tanninsof the bark belong to the toxic pyrogallol groups, and this, combined with their high solubility, results in the high degree of resistance in this species (4).
This, of course, would have a marked bearing on the effectiveness with which the tannins could check the spread of the parasite.
Considering the specificity of the enzyme systems of fungi it is quite possible that different tannins show different degrees of toxicity to a certain fungus.
The information available at present regarding the formation oftannins in plants is not conclusive.
It would be important to know the centers of origin of the tannins in the chestnut, their translocation, and whether they are translocated through or over graft-unions.
In the American and Japanese species they are a mixture of catechol and pyrogallol tannins, while they appear to be pure pyrogallol tannins in the Chinese species.
Furthermore, it has been found that the types of tannins in the three species differ.
There is, however, good evidence that the tannins in the Asiatic species, as a result of the way in which they are bound to other colloids in the cells, are more soluble than in the American species.
It was not realised, however, that these different tanning materials contained tannins of different qualities, capable of producing quite a different leather to that produced by means of oak or sumach.
Oak and sumach tannins (used especially for morocco leather) are those which best preserve the suppleness and capillarity of leather.
Oak wood and chestnut are thetannins least influenced by light.
Cotton and linen possess the remarkable power of attracting tanninsfrom their aqueous solution, and when these substances are prepared with tannins, they are able to retain dyes permanently.
Tannins are used in the dyeing of cotton and linen.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "tannins" in a variety of sentences. We hope that you will now be able to make sentences using this word.
