_Rounding_ is usually the next process. The unhaired and fleshed hide is spread out flat and cut up into b.u.t.t, shoulder and a pair of bellies.

These parts have different commercial values, and may afterwards be tanned by different methods for very different purposes--for dressing leather, and sometimes even for sole leather. _Scudding_ is the last piece of beam work. The fleshed hides (whether rounded or not) are washed, or at least rinsed, with water, and again placed on the beam grain side up. They are then sc.r.a.ped with a rather sharp concave knife, to remove "scud," which consists of hair roots and sheaths, lime soaps, fat, pigment and other dirt. Short hair is shaved off by a very sharp hand knife.

The beam work demands a certain amount of skill from the workmen, especially from the flesher, whose sharp knife may prove very wasteful in incompetent hands. Hand labour was slowly but surely being replaced by machinery before the war, and war-time conditions have greatly accelerated the rate of transition. Beam house machinery is rapidly becoming universal. The machines are c.u.mbrous and expensive in cost and in power, but machine work is quicker, less laborious, and needs much fewer workmen. Many types of machine have been suggested, but the most useful are those in which the hides pa.s.s over rollers and are simultaneously acted upon by a rapidly revolving cylindrical knife with spiral blades, one half being a left-handed and the other a right-handed spiral, so that the hide is sc.r.a.ped outwards as well as in the direction of motion. The part of the hide being acted upon rests on a pneumatic roller. By changing the type of spiral knife cylinder the machine will unhair, flesh or scud.

=Deliming= is a general name covering a number of similar operations whose primary object is the neutralization and removal of the caustic lime and soda in the plumped pelt, or at any rate on the surface of the hide. This is a preparation for the tan liquors. All the tannins and many a.s.sociated 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. A short insertion in a bath of weak acid would secure the elimination of surface lime and the disappearance of this difficulty, but there are other purposes in deliming. The more completely lime is removed the more the plumped pelt "falls" into a soft, pliable, unswollen and relaxed condition, and this change a.s.sists very materially in the production of a soft dressing leather, suitable for boot uppers, bags, etc. For such leathers, therefore, the deliming must be much more complete than for sole leather, in which the object is to obtain a firm and plump leather.

In the case of the softer dressing leathers, experience indicates the advisability of allowing some further bacterial action on the interfibrillar substance in order to produce the requisite pliability and softness. This is secured by "bating" the hides. This process consists in immersing the goods into a cold fermenting infusion of hen or pigeon dung. The infusion is made in a special tub or pit with warm water and allowed to stand for a day or two until the fermentation has commenced, and then run into the bating pit through a coa.r.s.e filter such as sacking. The hides are immersed for some days, but are handled frequently to ensure an even effect. The bate is always slightly alkaline. The caustic alkalinity increases rapidly at first owing to the diffusion of caustic lime, then at a slower rate, afterwards slowly declining. This is explained by the production of organic acids, and their salts with weak bases from the dung infusion by the action of bacteria. The total alkalinity of the bate liquor increases rapidly at first owing to the diffusion of lime and its liberation of organic bases, then very slowly, but towards the end of the operation the total alkalinity increases very rapidly indeed, owing probably to the commencement of a violent anaerobic fermentation which produces ammonia and other organic bases, and which heralds the approach of a putrefactive action, which if allowed to continue for even a short time will ruin the hides. Bating is consequently a risky process, and needs experienced oversight. For goods which need only a mild bating, there is the alternative of giving a longer liming in older limes. This of course involves more bacterial hydrolysis, and perhaps does it in a safer, more economical and certainly in a less offensive manner. Bating is often followed by a further deliming by acids. Boric, lactic, acetic, formic and butyric acids are all used, and with care even hydrochloric and sulphuric acids may be employed. Innumerable "artificial" bates have been put on the market, but most are merely weak acids, acid salts or salts of strong acids with weak bases. An American "bacterial bate"

consists of a lactic fermentation of glucose in the presence of glue.

Closely similar to bating is "puering," investigated by Wood.

Drenching is another fermentive deliming process. In this the goods are inserted into an infusion of bran. This is made by scalding the bran with hot water, and allowing it to stand until it is about 70-90 F.

The infusion is then "inoculated" with a few gallons of old drench liquor, and the goods are immersed. This fermentation has been examined carefully by J. T. Wood. First the enzyme cerealin converts bran starch into glucose, which is then fermented by the drench bacteria with the production of lactic acid, some acetic acid and small amounts of formic and butyric acids. The butyric fermentation is liable to become too violent. These acids, as they are formed, neutralize the lime in the hides and plump the pelt slightly.

Various gases (carbon dioxide, hydrogen, nitrogen, methane and sulphuretted hydrogen) are involved, and the proportion produced in the pelt itself has a peculiar opening effect on the hide fibres. The activity of the drench can be decreased by dilution and by using a less starchy bran, and can be increased by adding pea meal or rye meal.

Drenching usually follows bating. Scudding sometimes follows deliming.

The theory of the volume and elasticity changes of pelt during preparation will be better understood after considering the behaviour of gelatine gels. The determining factors are the _nett_ charge of hydroxyl ions on the disperse phase, resulting from ionic adsorptions, and the lyotrope influence of dissolved substances on the continuous phase.

In softening dried hides the swelling may be due to either influence, but the latter tends to loss of hide substance and the production of soft leather.

In liming, the nett adsorption of hydroxyl ions is the princ.i.p.al factor, but the lyotrope influence of the alkali cations and of the impurities is important. Plump pelts are those in which the contained water is in a relatively greater average state of compression. Few substances can a.s.sist plumping, but many can hinder it. In plumping all lyotrope influence is objectionable, and "sharp" (pure) alkali solutions are required. Mellow limes reduce elasticity and plumpness by lyotrope influence.

In bating and puering the essential change is that before the process the swelling is due chiefly to adsorption of hydroxyl ions, whereas afterwards it is due chiefly to a composite lyotrope influence.

REFERENCES.

"Principles of Leather Manufacture," Procter, pp. 108-184.

"The Manufacture of Leather," Bennett, pp. 49-113.

"Lyotrope Influence and Adsorption in the Theory of Wetwork,"

Bennett, _J.S.L.T.C._, 1920, pp. 75-86.

"a.n.a.lytical Examination of Bating," Bennett, _Leather Trades Review_, 1911, p. 972, and 1912, p. 28.

"The Bating, Puering and Drenching of Skins," by J. T. Wood.

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."

[Footnote 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 const.i.tution 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 cla.s.sified into "pyrogallol tans" and "catechol tans," according to the parent phenol. This cla.s.sification is confirmed by their chemical, a.n.a.lytical and practical behaviour, and the vegetable tanning materials may be cla.s.sified 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. a.s.sociated with them--in many cases probably as decomposition products--are certain other substances of well-known properties and const.i.tution. These substances are not only typical of the group, but also form the most valuable clue to the chemical const.i.tution of the group and the key to their chemical behaviour. One of these substances is gallic acid (3:4:5 trihydroxy-benzoic acid C{6}H{2}(OH){3}COOH), 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 C{14}H{6}O{8}, 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 a.s.sociated 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 a.s.sist 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 a.r.s.enate 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. pota.s.sium 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 5-1/2 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 versa_. 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 aegilops_). 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 a.s.sists materially in giving the weight and water-resisting powers a.s.sociated 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.

[Footnote 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 quant.i.ties 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 a.s.sists 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.

[Footnote 3: Also spelt Sumac and Shumac, and always p.r.o.nounced like the latter.]

Other pyrogallol tans are also used to a limited extent. Algarobilla and divi-divi are the fruit pods of several species of American _caesalpina_.

They are strong in tan (45 per cent.) and yield a light-coloured and bright leather, but are unstable tans, yielding much bloom. Babla is a small pod yielding a mellow tannage and much gallic acid. Celavinia is another pod containing no colouring matter and giving an almost white leather. The tannin is closely similar to that of oak galls. These last were once extensively used for tanning in Austria. Willow bark is used for tanning in Russia and Denmark. Valuable pyrogallol tannins are obtained from oak wood and chestnut wood, but the woods are not used in tanning as the percentage of tan is so small.

=Catechol tans=, often obtained from barks, contain usually about 60 per cent. of carbon. They are seldom used alone, for they usually have little or no sugar a.s.sociated, and hence their liquors do not either "sour" or "plump." They can be used alone if artificially acidified, but without acidifying or blending would give a rather flat leather, though possibly firm. They yield no bloom or gallic acid, but have a.s.sociated with this other characteristic substances. Of these the catechins are the most typical, and have been considered as the parent substances of the catechol tans.

The catechins are white crystalline substances, apparently isomers with the general formula C{15}H{14}O{6}. They have different melting-points, and varying amounts of water of crystallization, but are otherwise exceedingly similar in properties. They are sparingly soluble in cold water, but freely in hot, and in alcohol and ether. They are precipitated by lead acetate, mercuric chloride and alb.u.min, but not by gelatin, tartar emetic or alkaloids. In gambier liquors they are especially strong, and sometimes crystallize on the side of the pits, being thus known as "whites." The phlobaphenes or "reds" are also typical of catechol tans from which grow catechins; they can be formed by boiling with dilute mineral acids. They are considered to be anhydrides of the catechol tans. They are difficultly soluble in cold water, but freely in hot, and in cold alcohol and dilute alkalies. They are true tannins and alone are capable of making a red leather, but in practice are often found as mud in the tan liquors owing to their limited solubility. They naturally influence the colour of leather made with catechol tans, which is usually distinctly redder than the leather made from pyrogallol tans. Infusions of catechol (_cp._ catechin) give a green-black colour with iron alum. The sodium a.r.s.enate test gives a red colour due to catechin. The chromate and iodine tests mentioned for pyrogallol tans give negative results with the catechol tans, but bromine water gives a precipitate, and sulphuric acid a crimson colour.

Mimosa bark is one of the most important catechol tans. It is usually obtained in this country from Natal ("Natal bark"); but the tree (Sydney green wattle, _Acacia mollissima_) is a native of Australia. It is being cultivated now extensively in South Africa, and forms a most valuable portion of the Empire's stock of tanning material. Its more extensive use has been long recommended by the author,[4] but its gradually increasing employment in British tanneries has been greatly accelerated by the war, which has prevented its delivery in Germany and has cut off Turkish valonia from Britain. It yields about 30 per cent. of a stable and excellent tannin, and will produce a firm, durable leather, with a colour much less red than that obtained from many other catechol tans.

It is an astringent tan, and if carelessly used yields a harsh or even "drawn" grain. Most of the tannin is easily extracted, yielding a clear infusion which penetrates fairly quickly and gives good weight. It contains less than 1 per cent. of sugar, which unfortunately rapidly ferments to carbonic acid, so that it is not a good plumping material.

It makes in all respects an excellent blend with myrabolans. Like all catechol tans, the resulting leather darkens on exposure to sunlight.

[Footnote 4: J.S.C.I., 1908, 1193.]

Oak bark, from _Quercus robur_, is the ancient tanning material of Britain, and is still used to a limited extent. It contains about 13 per cent. of tannin and is mainly a catechol tan, but also contains a pyrogallol derivative. It yields catechin, and gives a red colour with the sodium a.r.s.enate test, but also will yield some bloom and gallic acid, and gives a blue-black with ferric salts. The tannin itself is exceedingly similar to that of mimosa bark, but the material contains about 2-1/2 per cent. of sugar, which makes it possible to employ oak bark alone for making sole leather. It is noted for yielding a sound, durable leather of good typical tan colour. Its tannin combines well with hide and penetrates quickly. The fatal disadvantage of oak bark is its weakness in tannin strength compared with other materials. This results in heavy freight and heavy cost per unit tannin, bulky storage, expensive handling in the factory, comparatively large bulk of spent tan, after relatively greater trouble in extracting, and the impossibility of making the strong liquors so necessary in these days to produce good weight in a short time. No satisfactory extract has yet been made from it.

Pine bark, from _Pinas abies_, is one of the staple materials of the Continent. It contains up to 14 per cent. of a catechol tan, and, unlike most of this group, contains a high proportion of sugar and will give good results alone. Hemlock bark has been the staple tanning material of North America. It is obtained from the hemlock, or _Pinus canadensis_.

It contains up to 11 per cent. of tan and much phlobaphene, and yields a characteristic red leather of good quality, but which rapidly darkens with sunlight. It contains some sugar, but is usually employed in conjunction with sulphuric acid or with sugary materials. Mallet bark yields another catechol tan similar to that of mimosa, but somewhat less astringent and more yellow in colour. Quebracho wood and mangrove bark have been used, but are now made into extracts.

=Leaching.=--Whatever cla.s.s 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. This process is called "leaching." The tanning materials, after being ground, crushed or shredded, are placed in large pits arranged in "rounds," "sets," or "batteries" of 6, 8 or 10 units, through which water is percolated systematically, so as to secure a continuous extraction. Water itself is added to only one of the pits of material. The liquor produced is pa.s.sed on to the next pit, and then to the next, and is continually gathering strength. After pa.s.sing thus through the series, the liquor becomes the source of the strong extracted tan liquors which are used in the tannery proper. With this system the stronger leach liquors are being acted upon by fresh material, and the nearly "spent" material is being acted on by the weakest liquors, and finally by water, thus ensuring a complete extraction. In the press leach system, which is now practically universal, the bottom of one pit communicates with the top of the next, and the liquor presses round by gravity flow caused by a few inches "fall." Liquor is thus constantly percolating downward through the material in each pit. The "head leach" and "tail leach" are always adjacent in a double row of pits, and when the material in the latter is quite spent, it is "cast," and the pit is filled with fresh material.

The liquor is then pressed round into this pit by adding water to the tail leach. Hot water is used to secure better diffusion. At least two such sets of leaches ("taps" and "spenders") are necessary to spend the material of the average tannery and to obtain liquors of the necessary strength.

=The Manufacture of Extracts.=--In addition to the use of the natural tanning materials described above, modern leather manufacturers employ also a variety of "tanning extracts," _i.e._ vegetable tanning materials in which the tannin has been already extracted, and which are supplied in form of a solid or concentrated liquid. Such extracts only need to be dissolved in warm water in order to make a tan liquor, and the cost and trouble of leaching is avoided. They are a great convenience as making strong liquors of definite strength. Many vegetable tanning materials are too weak in tan for the tanner to leach, and indeed to justify the cost of importation have been made available by manufacturing an extract at the source of the material. With such weak materials the extract manufacturer has had to secure a much more complete extraction than in ordinary leaching, and to concentrate his infusions by means of steam-heated vacuum pans. With such experience he has naturally begun to make extracts also from the stronger materials, such as myrabolans and mimosa bark, and it is now possible to have a tannery without any leaches at all. Tanners also have begun to realize the advantages not only of more rapid and complete extraction, but also of doing the work for themselves, and extract factories are beginning to appear as an adjunct to the larger tanneries. The more complete extraction of tan also involves a greater extraction of unwanted colouring matters, hence decolorization is a feature of extract manufacture.

=Chestnut Extract= is from the wood of the Spanish chestnut (_Castanea vesca_), which contains 3-6 per cent. of a valuable pyrogallol tan very similar to that of valonia. Its weight-giving and water-resisting powers are as good as valonia, and its penetrating power is even better, so that it forms an exceedingly suitable material for the modern short tannage, and also for drum tannages. The extract is manufactured extensively in France. The wood is stripped of bark and usually piled for some months to dry and to allow the resins to become insoluble. Some factories, however, use the green wood direct. There are two methods of extraction, viz. in open vats and in closed vats under pressure. The two methods yield extracts which differ in composition and properties. In either case the vats have a capacity of up to 3,000 gallons, and hold up to 6-1/2 tons of wood. They are arranged in series, as in leaching, and the liquor pa.s.ses in succession through all the vats over wood less and less spent. The temperature is highest in the vat containing the fresh water and nearly spent wood. In open vats of wood or copper the temperature is near boiling-point, whilst in the closed autoclaves (copper or bronze) the pressure reaches about two atmospheres and the temperature about 130 C. (266 F.). The series may contain 5, 7, 9 or even 12 vats, and the liquor obtained has a strength of 3 to 4-1/2 Beaume (22 to 33 Bkr.).

After extraction the liquor is allowed to stand, and much insoluble matter settles out--resins, wood, fibre, etc. The clarified and settled liquor is then pa.s.sed through a cooler up to about 55 C., and then run into the decolorizing plant, a deep vat fitted with a copper steam coil and mechanical stirrer attached to power. The best decolorizer is bullock's blood, which is run into the vat and well mixed. The temperature is next raised to about 70 C., causing the blood alb.u.min to coagulate. It carries down with it a little tannin, but much colouring matter. After standing a few hours the settled liquor is run off direct to the evaporator. A multiple-effect evaporator is usually employed, and the concentrated liquor, which has a strength of about 25 Beaume, is run into suitable oak casks. The extracts contain 27-32 per cent.

tannin. An extract made with open vats has about 7 per cent. soluble non-tanning matters, whilst a "pressure extract" may contain up to 12 per cent. of these "non-tans." Pressure extracts obtain also a better yield of tannin, which more than compensates for the slightly lower price. Open extraction yields, however, the purer product and an extract with better penetrating powers, and is consequently the more suitable for drum tannages. Chestnut extract is extensively used by the heavy leather tanners.