C. echinata yields “Brazil-wood.” (See p. 413.)

C. Sappan, Sappan-wood, India.

Cassia auriculata, Turwar or Tanghadi bark, Southern India. Used for tanning so-called “Persian” sheep- and goat-skins, contains about 17 per cent. of a catechol tannin. Leather tanned with it is of a pale yellow colour, but rapidly reddens in sunlight. Cp. p. 235.

C. fistula, India. Husk of pod, 17 per cent. tannin. The pulp of pod is used as an aperient.

C. elongata and lanceolata. Senna leaves. Upper Egypt.

C. Sophora, “Bali-babilan.”

MIMOSEÆ, a Tribe of Leguminosæ.

Acacia arabica, “Babool,” “Babul,” India, Egypt. Fig. 59. Bark contains about 12-20 per cent. of catechol tannin; one of the principal Indian tanning materials, used for kips and heavier leathers. Pods, used in India for bating, contain about same amount of tannin as bark, but of a different kind, that of the bark being a catechol-tannin, with a good deal of red colouring matter, while the pods contain a paler tannin allied to divi, which is not precipitated by lime-water. In Egypt the pods are called bablah, a name which is also applied to pods of A. cineraria and A. vera, and others. They are used for dyeing glove-leathers.

A. nilotica, Egypt. Pods called neb-neb or bablah.

A. catechu, India. The wood yields cutch or “dark catechu.” A lighter coloured variety called kath, containing much crystallised catechin, is also made in India, and principally used for chewing with betel. A. catechu is a tree 30-40 feet high, common in India and Burma, and also in tropical East Africa, where, however, it is not utilised. In Southern India, A. suma is also used for the same purpose.

Trees of about 1 foot diameter are cut down, and the wood (some state the heart-wood only) is reduced to chips, and boiled with water in earthen jars over a mud-fireplace. As the liquor becomes thick and strong, it is decanted into another vessel, and the evaporation continued till the extract will set on cooling, when it is poured into moulds made of leaves or clay, the drying being completed by exposure to the sun and air. “Kath,” or pale cutch, is made in Northern India, by stopping the evaporation at an earlier point, and allowing the liquor to cool, and crystallise over twigs and leaves thrown into pots for the purpose. It contains a large proportion of catechin, apparently identical with that of gambier, but its tannin is much redder. Good cutch contains about 60 per cent. tanning matter, but is principally used for dyeing browns and blacks with chrome and iron mordants. It contains quercetin, a yellow colouring matter (p. 263).

A. leucophlea, India and Java “Pilang.” Pods and bark equal to A. arabica.

Australia abounds in acacias (mimosas), many of which are used in tanning, but vary greatly in strength, not only according to species, but probably also by situation and growth. Probably the best information is to be found in a pamphlet on ‘Wattles and Wattle-Bark,’ by J. H. Maiden, F.L.S., published by the Department of Public Instruction at Sydney, 1890. His analyses were made by the Löwenthal process, and can only be roughly compared with those by the hide-powder method. The analyses given are by the I.A.L.T.C. method, and mostly on samples furnished by Mr. Maiden.

A peculiarity largely developed in the mimosa family is the tendency for the true leaves to be suppressed, and their place taken by the flattened and expanded midrib (phyllode). Thus leaves of two very distinct forms are common in the genus, and some acacias, as A. heterophylla, may have both forms on the same branch. Compare A. pycnantha and A. decurrens.

The Australian mimosas have been naturalised in India, and grow freely in the Nilgiri Hills, but the bark does not appear to be utilised.

The most important species are as follows:—

A. pycnantha. (Fig. 61.) “Broad-leaved” or “Golden Wattle,” South Australia. One of the strongest tanning barks known. A sample marked “special,” analysed in the Yorkshire College, contained 50 per cent. of tannin; another sample marked “ordinary” contained 40 per cent.

A. longifolia, the Golden Wattle of New South Wales, only contains half as much tannin as A. pycnantha.

A. mollissima, with its two varieties A. decurrens (Fig. 62) and A. dealbata, are among the most important of the Wattle family commercially. Two samples of the former marked “Green Wattle” showed 36-39 per cent. of tanning matter; another sample marked “Sydney Green Wattle,” contained 41 per cent. A sample of A. decurrens, the second variety, was much weaker, showing only 12 per cent. on analysis.

A. penninervis (Hickory bark) is said to be particularly hardy, but its strength seems to vary. A sample from Bateman’s Bay contained 38 per cent. of tanning matter.

A. binervata, another “Black Wattle” contains up to 30 per cent. tanning matter, as does also the “Weeping Willow,” A. saligna. The latter is poisonous, and is said to be used for killing fish.

A. prominens, the bark of which resembles that of the Golden Wattle, A. longifolia, in appearance contains only 14 per cent. tannin.

The cultivation of wattles in Australia has been somewhat neglected, but would render possible the utilisation of many acres of land lying waste, or which have already been exhausted and rendered unfit for the growth of cereals. It requires so little attention as to make it very profitable, and wattle-growing and sheep-grazing can be combined satisfactorily after the first year, when the young trees in the plantation have reached the height of 3-4 feet. In Natal the Australian wattles, especially A. mollissima, have been acclimatised and cultivated with success, and large quantities of excellent bark are now exported to England. African wattle-barks usually contain about 30 per cent. of tannin.

Wattles grow in almost any soil, even the poorest, but their growth is most rapid on loose, sandy patches, or where the surface has been broken for agricultural purposes. When the soil is hard and firm, plough-furrows should be made at a regular distance of 6-8 feet apart, and the seeds dropped into these. The seed should be sown in May, having been previously soaked in hot water, a little below boiling temperature, in which they may be allowed to remain for a few hours. It should be dropped at an average distance of 1 foot apart along the furrow, in which case, about 7200 seeds would suffice for one acre of land. The seed should not be covered with more than about ¹⁄₄ inch of soil.

On loose sandy soil, it might even be unnecessary to break up the ground in any way; the furrows may be dispensed with, and the seed sown broadcast after the land has been harrowed. After the plants have come up, they should be thinned so that they stand 6-8 feet apart. When the young trees have attained the height of 3-4 feet, the lower branches should be pruned off, and every effort afterwards made to keep the stem straight and clear, in order to facilitate the stripping, and induce an increased yield of bark. It is advisable that the black and broad-leaved kinds should be grown separately, as the black wattle, being of much larger and quicker growth, would oppress the slower-growing broad-leaved one. Care should be taken to replace every tree stripped by re-sowing, in order that there should be as little variation in the yield as possible. In Victoria, the months of September-December are those in which the sap rises without intermission, and the bark is charged with tannin. Analysis proves that the bark from trees growing on limestone is greatly inferior in tannin to that obtained from other formations, differing 10-25 per cent.

The following are South American mimosas:—

A. cavenia, Espinillo. Bark, contains 6 per cent., pods, 18-21 per cent., or more of tannin.

A. cebil, Red Cebil. Bark, contains 10-15 per cent.; leaves, 6-7 per cent. tannin. Argentine Republic.

A. Guarensis, Algarobilla of Argentine Republic. Bark, pods and flowers said to be used for tanning.

A. timbo, Buenos Ayres.

A. curupi, Curupy bark.

A. angico, or Piptadenia macrocarpa, Brazil, yields “angica bark,” a sample of which contained 20 per cent. of tanning matter when analysed recently in the Author’s laboratory.

“White Bark,” South America, probably an acacia, bark internally very similar to angica, if not identical.

A. horrida, “Doornbosch,” Cape of Good Hope, contains 8 per cent. of tannin.

Inga feuillei, “Paypay,” Peru. Pods said to contain 12-15 per cent. of tannin (doubtful). Several other species of Inga known to contain tannin.

Elephantorrhiza Burchellii, Elandsboschjes, Tugwar, or Tulwah, South Africa; a papilionaceous plant. The air-dry root contains 12 per cent. of tannin, and a great deal of red colouring matter. The roots are several feet long, and about 2 inches in diameter, growing by the sides of rivers.

The following additions may be made to the above list:—

EUPHORBIACEÆ.

Cleistanthus collinus, “Kodarsi,” Deccan. Bark stated to contain 33 per cent. of tannin.

Phyllanthus emblica, India, yields emblic myrobalans, which in immature condition contain considerable tannin. Leaves (18 per cent.) and bark used for tanning.

Phyllanthus distichus and nepalensis both yield tanning barks.

COMBRETACEÆ.

Anogeissus latifolia, India. Bark and leaves rich in tannin.

GUTTIFERÆ.

Garcinia mangostana, India. The rind of the mangosteen fruit contains much tannin.

CHAPTER XIX.
THE CHEMISTRY OF THE TANNINS.

The essential constituents of tanning materials are members of a large group of organic compounds known as “tannins” or “tannic acids,” which are widely distributed throughout the vegetable kingdom, and said to have one representative among animals, in the body of the corn-weevil. Their use in vegetable physiology is as yet uncertain, and indeed they appear in some cases to be waste products of organic change. The tannins, though varying considerably in their chemical constitution, and in many important characteristics, are all marked by the power of precipitating gelatine and some allied bodies from their solutions, of converting animal skin into the imputrescible material known as leather, and of forming dark-coloured compounds with ferric salts which are often utilised as inks. They are also precipitated by lead and copper acetates, stannous chloride, and many other metallic salts, and form insoluble compounds with many organic bases, such as quinine, and with the basic aniline colours. They are possessed of feeble acid character.

All tannins are soluble in water to a greater or less degree; they are also soluble in alcohol, in mixtures of alcohol and ether, in ethyl acetate, acetone, and a few similar solvents, but are not dissolved by dry ether alone, nor by chloroform, petroleum spirit, carbon disulphide, nor benzine.

As the tannins are uncrystallisable, and incapable of being distilled without decomposition, they are exceedingly difficult to obtain in a state of purity, and, owing to the considerable differences in their character, no one method is equally applicable to the whole group. As their successful separation requires considerable chemical training, and experience, detailed description is outside the scope of the present work, but some particulars of the more important methods employed are given on p. 43, L.I.L.B.

Their chemical constitution is complex and in most cases imperfectly understood, but all the natural tannins which have been investigated prove to be derivatives of the trihydric phenol, pyrogallol, or of the dihydric phenol, catechol, the latter of which is often accompanied by a trihydric phenol, phloroglucol, which is isomeric with pyrogallol. The phenols are, themselves, a class of derivatives of benzene, C₆H₆, in which one or more of the hydrogen atoms are replaced by OH groups. Common phenol or “carbolic acid” is their simplest representative. Many of them, including pyrogallol and catechol, are used as photographic “developers.” The phenols on replacing another hydrogen by carboxyl (CO.OH) form true acids, of which salicylic corresponds to common phenol, protocatechuic to catechol, and gallic to pyrogallol; and the tannins are apparently complicated acids, in which one of the two latter acids is linked to a second molecule of the same or another acid as an anhydride, in some cases possibly with the addition of phenols or other organic groups. For more detailed information, see L.I.L.B., p. 45. Gallotannic acid is apparently digallic acid, in which two molecules of gallic acid are linked together after giving up the elements of a molecule of water. Natural gallotannic acid and many other tannins are glucosides, or at least contain glucose, which in many cases can be removed by purification.

From what has just been said, it is obvious that a classification of the tannins according to constitution, is at present impracticable, not only from our imperfect knowledge, but from the difficulty of separating and determining the products of their decomposition. It is not, however, difficult to distinguish the catechol- from the pyrogallol-tannins by their chemical characteristics, apart from actual separation of the phenols, and the division is important as it is marked by certain broad differences in their properties which affect their use in tanning.

The catechol-tannins, dissolved in water, yield a precipitate when bromine-water is added till the solution smells strongly of it. The precipitate is occasionally crystalline, but generally amorphous, and of yellowish or brownish colour. When the infusion of tannin is very weak, the precipitate is sometimes only slight, or forms slowly. Pyrogallol-tannins give no precipitate with bromine-water. Another reaction, which is generally characteristic of catechol-tannins, is that if concentrated sulphuric acid is added to a single drop of the infusion in a test-tube, a dark red, or crimson ring is formed at the junction of the two liquids, and on dilution with water, the solution is generally pink. Pyrogallol-tannins on the other hand give a yellow, or at most a dark brown ring, which dilutes to a yellowish solution. This reaction is of great delicacy, which may be further increased by the use of an alcoholic instead of an aqueous extract. It is often given also by the non-tannin residue of catechol-tannins which is left after treatment with hide-powder, in which case it is probably due to the presence of catechins allied to the tannins. With ferric salts (preferably a solution of iron-alum), pyrogallol-tannins give blue-blacks, while catechol-tannins generally give greenish blacks, though the reaction is apt to be rendered uncertain by the presence of colouring matters, or perhaps in some cases by the constitution of the tannin. Thus aqueous infusions of common oak-bark (Quercus robur) give a decidedly blue black with iron, though the tannin is a catechol one, and the purified tannin gives a green-black. Most of the barks of American oaks, such as Q. prinus, give green-blacks without purification. The Australian mimosas generally give dull purple-blacks with iron-salts, though they all contain catechol-tannins. The iron test was first proposed by Stenhouse as a means of classification. Trimble has shown that while the purified pyrogallol-tannins only contain about 52 per cent. of carbon, the catechol-tannins have about 60 per cent.[150]

Only two tannins of the pyrogallol group have been definitely distinguished, though it is very possible that more exist. These are ordinary tannic acid of gall-nuts (probably digallic acid), which yields gallic acid when heated with dilute acids, or by the action of certain unorganised ferments or zymases (p. 16) which are generally present in tanning materials; and ellagitannic acid (usually present in greater or less proportion in mixture with the gallotannic acid), which, under the same conditions yields “bloom” (an insoluble deposit of ellagic acid), as one of its products. Hence it happens that most pyrogallol tannins deposit “bloom” on leather, though in very different proportions, gall-nuts and sumach giving very little, and myrobalans, valonia and divi-divi a great deal. English oak-bark deposits a good deal of “bloom” on leather, though it is certain that its principal tannin is a catechol one, but it is possible that the blue-black which it gives with iron salts may be due to the presence of ellagitannic acid, though gallotannic acid is known to be absent. The tannins of oakwood, chestnut and valonia are principally if not entirely pyrogallol derivatives, closely allied to, if not identical with the two just named, but, if so, very difficult to obtain in a pure condition. It is noteworthy that so wide a difference exists between the various products of the oak; galls, bark, fruit and wood yielding tannins of very varied properties. The tannin of other galls, such as those of the sumach and pistacio, generally contain gallotannic acid, even when, as in the last case, the remainder of the plant yields catechol-tannins.

The tannins of the catechol group appear to present much more variety than the pyrogallol-tannins, though it is possible that many apparent differences may be due to the presence of impurities. It is, however, at least certain that the tannins of gambier and cutch contain phloroglucol as one of their constituents, while it is absent from most other tanning materials. Its presence is easily detected by moistening pine-wood (a deal shaving) with an infusion of the tannin in question, and applying a little concentrated hydrochloric acid, when after a few minutes, a bright red or purple stain is produced. The catechol-tannins, on boiling with acids, yield no gallic acid, or bloom, but generally a deposit of “reds,” insoluble in water but soluble in alkaline liquids, and in alcohol, and which are closely allied to resins, and especially to the red resin known as “dragon’s blood.” These reds are anhydrides of the tannins, that is, are produced from them by the abstraction of water; and are consequently formed by any agency which tends to remove water, such as long boiling or high temperature. The lower anhydrides (that is, those from which least water has been abstracted) are not wholly insoluble, but form the “difficultly soluble” tannins which are naturally present in many materials. They are much more readily soluble in hot than in cold water, which is one of the causes why liquors made by the aid of heat generally give darker colour to leather than those extracted cold. They exist in large quantity in hemlock extract and quebracho. Attempts have been made to utilise their alkaline solutions for tanning, but without much success; though alkalies or alkaline sulphites are frequently used to obtain “soluble” quebracho extracts (p. 338).

Many catechol tanning materials, and especially gambier, cutch and quebracho, contain in addition to the tannin, considerable portions of colourless bodies called catechins, which are only slightly soluble in cold water, but readily in hot, and which crystallise out on cooling. These bodies do not tan, but are in a sense the source of the tannins, which appear to be their first anhydrides, the reds being formed by the successive loss of further molecules of water. These bodies very probably ultimately become converted into tannins by changes in the tanyard. The change may be brought about very rapidly by heating to a temperature of 100 to 120° C.[151] The catechin of gambier, by crystallising on and in the leather, is the cause of a trouble known as “whites,” which is common where gambier is largely used.

An unfortunate peculiarity, apparently common to all catechol-tannins, is that, however light-coloured the leather produced by them, it darkens and reddens rapidly by exposure to strong light, and ultimately becomes quite friable and rotten.[152] Cp. pp. 234, 272.

Wagner, a German chemist, attempted to classify the tannins into “physiological” tannins, which were produced in the natural growth of the plant, and “pathological” which were caused by the attack of insects such as the gall-wasps, and he further ventured the assertion that only the former class were capable of producing leather. It has since been shown that the tannins produced in galls are identical with some of those found in healthy plants, and galls themselves have been used in tanning from very ancient times. It is only necessary to remind the reader of the use of Turkish gall-nuts, in place of sumach, which was common in the East in the tannage of moroccos, and of the “Knoppern,” or oak-galls formerly so largely used in Austria as a tanning material for sole leather. It is true that the tannin of galls is not very suitable for the latter purpose, consisting as it does mostly of gallotannic acid, which, giving no solid deposit of bloom or reds, is incapable of making a heavy or solid leather. Pure gallotannic acid itself produces a very white and soft leather.

The class to which the tannins of the different tanning materials belong is mostly mentioned in the Botanical List (Chap. XVIII.), but it may be well here to specify a few of the most common. Galls and sumach contain gallotannic acid with a little ellagitannic; myrobalans, valonia, divi-divi, algarobilla, oakwood and chestnut are all pyrogallol-tannins giving ellagic and gallic acid among their decomposition products. All the pine barks, including the American hemlock, and the larch, all the acacias and mimosas, including the Indian Babul (Acacia arabica), the oak barks (though not the oak wood, fruits, or galls), quebracho wood, cassia[153] and mangrove barks, canaigre, cutch and gambier are catechol-tannins, and the two last contain phloroglucol, of which minute traces are also present in many other catechol-tannins (p. 297).

Gallotannic acid, and several artificial tannins with the characteristic reactions of the class have been produced in the laboratory, but there is no present prospect of their manufacture at prices which can in the faintest way compete with those of natural production.

Tanning materials frequently contain mordant colouring matters, often derived from the same phenols as the accompanying tannins. They also usually contain gums, starch and glucose. Oak bark contains lævulose which is not combined with the tannin. Many tannins, however, exist in nature in combination with the sugars as glucosides, which are easily decomposed by the action of acids or by fermentation. These sugary matters are important as furnishing by fermentation the acetic and lactic acids of tanning liquors.

CHAPTER XX.
THE SAMPLING AND ANALYSIS OF TANNING MATERIALS.

Although the analysis of tanning materials falls more properly within the scope of a book for chemists than one intended primarily for tanners, and though it has been treated at considerable length in the ‘Leather Industries Laboratory Book,’ a slight sketch must now be given of the methods in general use, since it is of great importance that at least the principles on which they are based should be understood by all to whom they are of practical interest, and also because an approximate analysis of a tanning material by the hide-powder method is within the scope of any intelligent tanner who will provide himself with the necessary implements. Much attention has been paid to the subject area by the International Association of Leather Trades Chemists, and also by the American Official Association of Agricultural Chemists, and as the methods prescribed by one or other of these are with very little exception employed by all qualified chemists throughout the world, their directions, corrected up to date, are given in Appendices A and C. As, however, these directions are addressed to chemists already familiar with the usual course of analysis, a somewhat fuller explanation must here be given.

It must specially be insisted on, that absolute adherence to the methods given is essential to obtaining concordant results, and little points of manipulation which appear in themselves unimportant, are frequently the result of long experience and careful discussion. The members of the International Association, especially, are bound by their rules to make note in their analytical reports of any deviation, however small, from the prescribed process.

The first step in the analysis of any material is to draw a sample truly representing the bulk, which is often by no means easy, while failure to accomplish it is probably the cause of more errors and disputes than any inaccuracy of the method of analysis itself. In very many cases, chemists are blamed for discrepancies which really exist in the samples supplied to them. The chemists of the International Association only hold themselves responsible for the accuracy of their analyses when the sampling has been done strictly according to the rules prescribed by their Association. On this account, all important samples should be drawn in the presence of a principal, or some other responsible person.

In liquid extracts, the thorough mixing of the liquid is of the greatest importance. Most extracts contain a portion of “difficultly soluble” tannins (see p. 297), which slowly settle to the bottom, or adhere to the sides of the cask; from which such expedients as merely rolling a full cask are quite inefficient to dislodge them. In fact nothing but taking the heads out of a sufficient number of casks, and actually stirring them with a suitable plunger, which should be specially applied to the sides and bottom, or emptying the entire contents of the casks into a tank in which the whole can be adequately mixed, is really thoroughly reliable, though at times it is necessary to be content with less satisfactory methods. In any case, when it is probable that samples must be submitted to more than one chemist, the whole should be drawn at once, thoroughly mixed and divided, and sealed in separate bottles, and in dividing a sample the same care must be taken to ensure complete mixture, as in drawing the original sample.

Solid and pasty extracts, such as quebracho, cutch and gambier, are still more difficult to sample fairly, as the outside is almost invariably much drier than the interior. Generally the only way is to select such portions as are thought fairly to represent the bulk, to chop them into moderately small pieces, mix and seal in an air-tight tin, leaving it to the chemist to draw from these the smaller sample required for analysis. Gambier is best sampled with a tubular tool like a cork-borer, designed by Mr. Kathreiner, Fig. 63, which should be passed completely through the bale, or the cylindrical sample of gambier cannot be withdrawn. The same tool may also be used for sampling sumach in bags, if the damage to the bag is not objected to. If such a tool is not available, the only fair way to sample gambier is to cut slices completely through the bale with a clean fleshing knife. In any case it is of the utmost importance that the sample once drawn, should be mixed as rapidly as possible, and at once enclosed in an air-tight box or jar, sealed and labelled.

Dry tanning materials, such as bark and valonia, require judgment in selecting samples which fairly represent bulk. If they are of a nature which do not readily separate into dust and fibre, a good method is to grind a sufficient quantity in an ordinary bark-mill, and after well mixing, to draw the sample from the ground portion. In other cases it is best to empty a sufficient number of bags one upon another in layers on a smooth floor, and to take out a section down to the floor. In such materials as valonia and divi-divi, the dust or beard is usually much stronger than the average of the pods or cups.

The same sort of precautions are required in drawing the still smaller sample required for analysis from the larger original sample, but these are sufficiently detailed in the directions of the I.A.L.T.C. given in the Appendix. As materials usually require finer grinding than can be managed with the mills employed in the tannery, a suitable mill must be provided, and one of the simplest, at a moderate price, is a No. 4 drug-mill made by A. Kenrick and Sons, Limited, West Bromwich, Fig. 64. Coffee mills are seldom strong enough for the purpose, but if nothing better is available, the sample must be thoroughly dried before grinding, and its loss of weight noted, and taken into account in calculating the analysis, care being taken that the sample after grinding is so preserved that it cannot re-absorb moisture. Valonia, myrobalans and even barks, may before grinding be broken with a flat-faced hammer, on a thick cast-iron plate, with raised edges to prevent loss from flying fragments.

Preparation of solution for analysis.—As the method of analysis only gives satisfactory results when the quantity of tanning matter in the solution is within certain limits, the International Association prescribes that it must be such as to contain between 3·5 and 4·5 grms. of tanning matter per liter, or as near as possible, on the average, to 4 grms. If, as will rarely happen, the strength of a material is quite unknown, it may be necessary to make a trial test to ascertain the quantity of substance to be used, but the following table gives the quantity with sufficient accuracy for most ordinary materials.

Table showing the Amount of different Materials
to be weighed out for analysis
to make up one Liter of Solution.

The best method of weighing out exact quantities may be here described for those to whom it is not already known, as much time may be wasted by attempting it unsystematically. The material is of course weighed in a basin, which together with the weight which is desired of the material, is exactly counterpoised by weights in the other pan. Where many weighings of the sort have to be made, it saves time to keep one particular basin for the purpose, which should be properly marked[154]; and to make a counterpoise of lead or brass exactly equal to it in weight, so that it is only necessary to add weights corresponding to the quantity required to be weighed out. Supposing now, that it is a liquid extract which is to be weighed, a sufficient quantity is introduced into the basin with a pipette, to slightly exceed the required weight. The pipette is now emptied, and a small quantity is withdrawn with it from the basin. If the basin is still too heavy, the pipette is emptied, and the process repeated until the basin is too light. The true weight now lies between that in the basin and the small quantity retained in the pipette, from which extract is added till the basin is again over-weighed, and the same process is repeated, each time reducing the margin, till a sufficient approximation is obtained. It is not necessary in weighing out the sample, to be accurate to a single milligram; but with practice, this amount of accuracy is easily attained. If the material is solid, a spatula is substituted for the pipette. The weighing of liquid or pasty extracts should be as rapid as possible, as they lose weight on the balance by evaporation.

Liquid extracts are most easily dissolved by placing a large funnel in the neck of a liter flask, and after pouring a little boiling water into the flask, holding the basin inclined in the funnel, and washing out its contents with boiling distilled water from a glass wash-bottle, or a perfectly clean copper kettle, till the flask is filled to the mark. The flask is now covered with a small beaker, which must hang loose on its neck, without resting on its shoulders, and is rapidly cooled by placing it under a cold water tap, to a temperature as little above 15° C. as possible, and is then filled up to the mark on the neck with cold water, and well mixed by shaking very thoroughly.

Solid or pasty extracts are dissolved in a beaker by stirring with successive quantities of boiling water, which are poured off into the flask, leaving the undissolved matter in the beaker. When the flask is nearly full, if any small portions remain undissolved or insoluble, they may be rinsed into it with the last portions of hot water, and the flask is now cooled and mixed as already described.

Extraction of solid materials, such as barks, or valonia, is more difficult, but the following is a convenient method, which has been recognised as official by the International Association. An ordinary beaker, of about 200 c.c. capacity, but which may be varied in size according to the weight of the material which it is necessary to treat, is placed in a water-bath, as shown in Fig. 65. A thistle-headed funnel, the stem of which is bent twice at right angles, and of which the head is covered with a piece of fine silk gauze (such as is used by millers) to act as a strainer, is placed in the beaker and held in position by a clamp as shown in the figure. To the free end of its stem a piece of glass tube, six or eight inches long, is attached by indiarubber tube, which is provided with a pinchcock to regulate the flow of liquid. Fine silver-sand, freed from iron and soluble matters by washing first with hydrochloric acid, and then very thoroughly with water, is now poured into the beaker, so as to surround the head of the funnel to about half an inch in depth; and the weighed quantity of tanning material is next introduced. It is best to cover the material with cold water, and allow it to stand all night, but in case of haste, water of 30° to 50° may be used, and the extraction proceeded with after the material is thoroughly soaked. Percolation is started by sucking the syphon, and allowing the liquid to drop slowly into a liter flask, the temperature of the water-bath being maintained by a Bunsen burner, and the beaker being refilled as it requires it with water at the desired temperature.[155] At least 500 c.c. must be percolated before the temperature is allowed to exceed 50°, after which, except in the case of sumach and canaigre, which should be begun about 30°, and at no time allowed to rise above 50°, the temperature may be raised to boiling point. At least 1¹⁄₂ hour should be employed in percolating 800 to 900 c.c. and if the material is not then practically exhausted, the liter flask must be withdrawn, and an ordinary ungauged flask substituted, into which the percolation is continued till the material is exhausted. The very dilute liquor in the second flask is now boiled down till its volume is sufficiently small to be added to that in the liter flask, a small funnel being placed in its neck during ebullition, to prevent spirting and the access of air. Under no conditions must the stronger liquor of the first part of the percolation be boiled down, as this would involve destruction of tannin. The solution is now cooled, and made up to the mark as has been before described. Most ordinary materials may be practically exhausted by the liter of water if percolation is slow, and the trouble of evaporation may thus be avoided.

Total soluble matter.—The solution of which the preparation has been described, must now be filtered, the size and kind of paper, and exact method of filtration prescribed by the International Association being strictly adhered to. All papers and methods of filtration absorb traces of tanning matters, and but few will give a clear filtrate with such solutions as those of quebracho and hemlock extracts; and to obtain uniform results exact uniformity of method is essential.[156] Deviations from the exact method, in the case of quebracho, easily cause discrepancies of several units per cent. in the result. The object of rejecting the first portions of the filtered solution is to prevent, as far as possible, errors which would arise from the absorption of tannin by the paper, and to insure a clear filtrate. 50 c.c. of the clear filtrate is now measured by an accurate pipette, and evaporated to dryness in a weighed porcelain basin, on a steam-bath, in order to determine the “total soluble.” This and succeeding operations should be done in duplicate, even if this has not been the case in making up the original solution, which is certainly desirable.