The methods of tanning may be classified as follow: (1) Vegetable tanning processes; (2) methods of chroming; (3) tawing processes; (4) oil tannages; (5) formaldehyde tannages; (6) sundry tannages, chiefly mineral; and (7) combination tannages. The public is chiefly familiar with types of vegetable tannage in boot sole, bag, and portmanteau leathers; of chrome tannage, in box calf and glazed kid used for boot uppers; of the tawing process, in kid glove leather; and of the oil tannage in wash leather or "chamois."

Vegetable Tanning Processes

The methods of tanning with infusions of barks, leaves, and fruits of trees and plants containing tannin are much more numerous than they were a decade ago, and tanners have now to pay special attention to the selection and blending of the materials they use in order to produce the various qualities of leather required. Formerly, most of the sole leather was tanned in oak bark liquors, and, in the later stages of the process, valonia and gambier were added to quicken the process and give solidity to the leather. This tannage, which may be described as an oak bark tannage, is still used by a few tanners, but the wide choice of materials available has brought other tanning agents into prominence, chiefly because they possess more astringent properties, and, therefore, tan more quickly than oak bark. Moreover, the need of other materials than oak bark became a necessity several years ago, as the supply of the latter would be inadequate to produce the large quantity of leather now required, even allowing for the rapid development of mineral tannages. Further, it is amply proved that a good blend of materials (mixed tannage) produces quite as durable and a firmer leather than the old oak bark tannage; unfortunately, there are other blends, occasionally combined with extraneous weighting materials, which account for the poor quality of a great deal of modern sole leather.

While the mixed tannage is now popular for sole, belting, harness, and other heavy leathers, vegetable tanned light skins, such as calf, goat, and sheep, are in most cases treated with a single material, sumach being used for a good proportion of them.

Whichever method be used, the first essential is the most suitable means of leaching the materials, or extracting the tannin. It seems, however, that this process may be eventually eliminated from the tannery, for most tanning materials are now converted into extracts, which only require dissolving in water to prepare the tan liquor. The manufacture of tanning extracts is quite a separate business, which is generally, but not always, conducted in factories situated near the source of the raw materials. There are important extract works in the Argentine, Paraguay, Canada, the United States, Hungary, North Germany, Borneo, Smyrna, France, Italy, and England.

Where natural tanning materials are used, either entirely or in conjunction with extracts, the leaching is done in a series of large square pits, four of which would suffice for a small yard, while a very large tanning would need twelve or sixteen. Oak bark, which is usually delivered to the tannery in strips measuring 3 to 6 ft. in length, must be chopped or ground into small pieces by machine (Fig. 21bark_mill). A measured quantity (a certain number of baskets or skeps full) is placed in the empty pits, which are then filled with water. The liquor is pumped from these pits, as required, to others in which the hides are tanned. The hard fruit of myrobalans, which somewhat resembles nutmegs, is powdered in a disintegrator or special crusher.

The three principal vegetable tannages are those for sole, dressing, and light leathers, and it is obvious that a careful selection of materials is necessary to obtain the desired effects. For this reason, oak bark, although a good tanning material for dressing leather, is insufficient to produce a firm sole leather, and, therefore, even in the so-called pure bark tannages, valonia or other suitable tannin is used towards the end of the process in order to increase the solidity and waterproof quality of the leather. There are two general methods of tanning sole leather, namely, the pit and the drum tannage. In the first case, the hides are passed through three series of pits containing gradually increased strengths of tannin (Fig. 22). These series are technically described as "suspenders" or "colouring pits," "handlers," and "layers" or "layaways." To facilitate handling and economise tanning materials, hides are "rounded" (i.e., cut into sections) either before tanning or after the hides have passed through the suspenders (Fig. 23). The obvious advantage of the former method is that the offal (shoulders and bellies) removed can be chrome tanned if required. The suspenders may consist of any number of pits from six to twenty-four, according to the size of the tannery. As the name implies, the hides are suspended in the liquors from poles, which extend across the top of the pit. The hides are attached to the poles by means of stout cord or copper hooks. Various mechanical appliances are in use to supersede the old method of allowing the hides to rest during this process. These methods save time and tend to produce even coloration, the latter being a very important point in the early stages of tanning. Some of the methods advocated, however, are too vigorous for the hides at this stage, when the chief object should be to preserve as much gelatine as possible. The best mechanical system is that which gently raises and lowers the hides in the liquors. Some of the American tanners use a mechanical contrivance known as the rocker, which consists of a stout wooden beam rocking from a central pivot, and with a see-saw movement. The objection to this method is that the hides do not receive equal treatment, those in the centre receiving hardly any movement, while others at the ends of the beam are raised too far out of the liquor. Under these conditions, the colouring of the leather must be irregular. Another method, invented in England, consists in suspending the hides from a wooden frame which can be mechanically moved on wheels. As the contact of tanning liquors with iron must be avoided to prevent discoloration (iron and tannin form the basis of inks), the wheels should be galvanized. This mechanical method is very satisfactory, for the hides are moved gently and kept in the liquors. As some tannins oxidise rapidly (i.e., darken in colour by exposure to the air), any process which exposes the hides at this stage for lengthy periods should be avoided.

The theory of the vegetable tanning process is not perfectly understood, and the principal trade chemists are not yet agreed as to whether the changes are chemical, physical, or both. Many are inclined to believe that both changes take place, and this view is probably correct, as a pure tannic acid produces a thin, empty leather, while a tanning material which contains a fair proportion of non-tannin matters, yields a full and firm leather. The practical tanner who adapts his work to the theory that tanning is a process of feeding the hides with gradually increasing strengths of tannin, is, in any event, on the right track. The suspender liquors are always weak in tannin, and are pumped from "handler" liquors through which packs of hides have already passed. It is important that these liquors possess a certain degree of acidity, and, if the natural acids of the tan liquor are too weak, or neutralised completely by the lime in a pack of hides that has been treated, a small quantity of acetic, formic, or lactic acid is added. Formic acid has the additional advantage of making the liquor antiseptic. It is absolutely necessary that the liquors have acid properties, otherwise tanning cannot proceed. Acid assists penetration of tannin. Hyposulphite of soda, formaldehyde, or synthetic tannin may be used for the same purpose, while these also help to keep the leather light in colour, owing to their bleaching properties. After passing through three or four suspender pits, in which the tan liquor is gradually strengthened, the hides are often rounded and the butts (Fig. 23) are transferred to the handlers. The first handler liquor should register about 25 degrees by the barkometer (Fig. 24), an instrument which registers the density of tan liquors. The handlers consist of a series of six or eight, and the strength of the liquors should be increased gradually. After a day's immersion in the first handler liquor, the hides are hauled out with a sharp two-pronged hook fixed to the end of a long wooden pole, similar to that used in the liming process. After they have been piled up for a few hours, two workmen, one on each side of the pit, place the hides flat in the next pit. The tan liquor is then run off the first handler to the suspender and fresh liquor is pumped into the empty pit, which then becomes the head or strongest liquor of the series. The succeeding packs will, therefore, follow in rotation: the first going to No. 2 pit, the second to No. 3, and so on. In the last two or three handler pits, the hides are sometimes dusted with freshly ground tanning material, such as oak bark, myrobalans, or divi-divi, the object being to strengthen the tan liquor so that it penetrates the hides before they are transferred to the layers. The handler liquors, which are originally sent back from the layers, are also frequently strengthened by the addition of oakwood, chestnut, or quebracho extracts, although some tanners only use extracts in the layers. Gambier is a useful material at this stage, as it keeps the liquors mellow and plumps the leather. In the layers, the last series of tan pits, the hides are treated with very strong extracts, and are also heavily dusted with such natural tanning materials as give firmness and solidity to the leather. For this purpose, nothing is better than valonia, or valonia extract, which deposits a large quantity of bloom, or ellagic acid. Where a mixed tannage of natural materials is used, the density of the liquors may reach 50 degrees barkometer, but if strong extracts be added, the barkometer may exceed 100 degrees. The relation between the layers and handlers in regard to the disposal of the used liquors is similar to that between handlers and suspenders; the first layer used is sent to the last handler.

By the English method, the leather is not, as a rule, transferred immediately from one pit to another, as it is found preferable to lay the hides in pile for a few hours, by which means the great weight helps to force the tannin through them. By draining the hides, they are in good condition for the absorption of the next tan liquor. The English method of handling is too slow for most of the American tanneries, where the greater proportion of the hides are cut straight down the centre to form "sides," as they are technically known. In America, the pieces of leather, whether hides, butts, bellies, or shoulders, are often tied together and transferred from one pit to another by means of a skeleton reel, worked either by hand or mechanically. The latest type of reel is a great improvement on the old reel. The cost of manual labour in many English tanneries is very great, in view of the fact that the hides have to pass through many pits, and the work of transferring the hides from one pit to another is done entirely by hand. It seems that there is plenty of scope for mechanical ingenuity in overcoming this difficulty, and it may yet be found possible to construct a perforated brass platform, connected with a crane, by which the whole of the hides could be lifted out in a second, left to drain for half an hour or more, and then bodily transferred to, and lowered in, the next pit. Naturally, it is easier, and quite within the bounds of practicability, to raise the hides from suspenders by mechanical power, as it would not be difficult to fix a stout cross-beam to the pulley from which the hides are suspended. The leather can be safely treated with very astringent tannin in the last series of pits, and the use of strong extracts is, therefore, a common practice at that stage, not only to give solidity and firmness to the leather, but to increase its weight.

It would be quite fatal to the production of good leather if strong tannins were used in the early stages of the process, because the surface on both sides would tan quickly, the grain of the hide would be badly drawn owing to the sensitiveness of gelatine to astringent solutions, and it would be impossible to make the tannin penetrate the hide. The final product would be a half-tanned leather which would be extremely brittle and poor in quality; in fact, quite unsuitable for sole leather. Despite the well-known axiom that the vegetable tannage (but not the mineral) must proceed slowly and gradually in the early stages of the process if the hides are to be properly tanned, the modern tendency is to speed up the tanning, with the result that, in some cases, the so-called sole leather is really not fit for the purpose for which it is intended. The root of the evil is the desperate attempt made by many boot manufacturers to produce lower-priced boots than those of their competitors.

Taking into account the development of leather trades chemistry, it is not surprising that the problem of reducing the time required to complete the old processes of tanning has received much attention. The results have been successful in some instances and have certainly proved that very good sole leather can be made without leaving the hides in the pits for a year or longer.

It has also been shown that the absorption of tannin can be accelerated by treating the hides, before putting them in the suspenders, with a solution of acid (lactic or butyric for preference), or hyposulphite of soda, or synthetic tannin. 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. It is of no use alone for heavy leathers, however, as it lacks the physical properties necessary to make the leather firm and resistant to water. The use of "soluble" oils in the tan liquors is another late innovation which has given good results. The term "soluble" in connection with oils merely means that they are made miscible with water by treating them with acids. Solubilised cod oil might be specially useful, as it has tanning properties and would increase the wearing and waterproof qualities of the leather. The chief objects of using oil in tanning, however, are to increase the weight of the leather and permit the use of strong liquors.

The tanning process outlined is the oak bark tannage, which is now supplanted in many yards by the mixed tannage, in which various tannins, chiefly exotic, are skilfully blended to produce the kind of leather wanted. Each tannin seems to have special characteristics, although the materials are classified into only three groups, namely: pyrogallol, catechol, and mixed (pyrogallol and catechol) tannins. Generally, the best mixed tannages for solid leather consist of a combination of both pyrogallol and catechol tannins. Reviewing the characteristics of some of the principal tanning materials, it may be said that oak bark produces a nice, fawn-coloured leather of strong texture, but tans slowly; valonia makes the leather solid, durable, and waterproof; myrobalans quicken the process and lighten the objectionable colour of other tannins; divi-divi and algarobilla are very rich in tannin, and are, therefore, useful in later stages of the process; gambier mellows the astringency of other liquors and plumps the leather; while most of the tanning extracts on the market penetrate the hides much more quickly, often give a lighter colour than that produced by solutions of the natural materials, and expedite the process considerably.

The most popular tanning materials are: oakwood, quebracho, chestnut, valonia, myrobalan, mangrove, mimosa, hemlock and spruce extracts, oak bark, valonia, gambier and sumach. The following are some of the combinations in use—

For Sole Leather.

1. Oak bark and valonia, or valonia extract.

2. Oak bark, valonia, and gambier.

3. Oak bark, quebracho extract, and myrobalan extract.

4. Chestnut, quebracho and valonia extracts.

5. Chestnut and oakwood extracts.

6. Oakwood and quebracho extracts, and divi-divi or algarobilla. 7. Quebracho extract, myrobalans, and valonia.

8. Quebracho, mangrove, and valonia extracts.

9. Oakwood, mimosa, and valonia extracts.

10. Hemlock and oak extracts (American union tannage).

For Dressing Leathers.

1. Oak bark and sumach.

2. Oakwood and quebracho extracts, and sumach.

3. Synthetic tannin and oakwood extract.

4. Synthetic tannin and myrobalans.

5. Gambier and sumach.

6. Wattle bark and myrobalans.

Sumach is often used alone for some classes of goat, sheep, and calf skins. Synthetic tannins can also be used alone for light leathers.

Many other variations may be tabulated, for nearly every tanner has his own recipe, having found by experience the blend of tannage that best suits his trade.

Apart from these innumerable combinations, the only method whereby the vegetable tanning process may be hastened is the mechanical, for which purpose either the paddle (Fig. 25) or the drum (Fig. 13) is used. The former consists of a wheel constructed of wooden shelves which, when in motion, dip a little way into the tan liquor in a vat, and so keep it in constant circulation. The drum is fitted inside with wooden shelves or pegs, which carry some of the hides or skins to the top of the drum at each revolution. Were it not for these shelves, the tannage would probably be irregular or otherwise unsatisfactory, as the hides would always be heaped together at the bottom of the drum. The paddles and drum are more often used for light than for heavy leather, as they not only have the effect of making the leather loose on the grain, but also make it soft and supple, characteristics which are not required in most of the heavy leathers.

However, the drum is now largely used on the Continent for the tannage of sole leather. A great saving in the cost of production is thereby effected, but the leather, although of satisfactory appearance, lacks the durability and waterproof quality of pit-tanned leather. The process is much simpler than the pit method, and less room is required. There are only two stages of tanning: (1) by placing the hides in colouring pits or suspenders, in which the hides are nearly struck through with tannin; and (2) running them in slowly-revolving drums containing solutions of tannin which are gradually strengthened until the process is finished. There can be no doubt that the tannin is rapidly forced between the fibres of the hide by mechanical action, but it is not so firmly combined as that slowly absorbed by the hide in the pit method. This can be proved by placing two pieces of weighed leather—one tanned in pits and the other in a drum—for twenty-four hours, drying the leather and reweighing it, when it will be found that the drum-tanned leather has lost the greater percentage of weight. To obviate this disadvantage as far as possible, use is made of special oils, which serve to fix the tannin more firmly between the fibres and render the leather more resistant to water. Other frequent drawbacks of drum tanning are looseness of grain and lack of substance. In sole leather tanning, the former is modified to some extent by extra pressure in rolling the leather during the finishing operations; while the latter difficulty is sometimes overcome by swelling the leather with acid and then fixing the swollen condition of the hide by treating it with a weak solution of formaldehyde. This chemical also has tanning properties, so that the process is hastened; but leather prepared in this way cannot be as durable as that produced by a natural tannage.

It seems that the aid of the engineer is necessary to overcome the difficulty of looseness of the grain caused by the severe pounding of the hides in the drums. The constant circulation of the tan liquor is required, but the hides should only be subjected to gentle motion. It is true there are tanning drums on the market which are said to obviate all the difficulty found in making sole leather in ordinary drums fitted with shelves or pegs, but a drum that meets the ideal conditions for the production of a solid yet flexible sole leather has not yet been invented. It seems that a kind of inner framework, to which the hides can be attached and which rotates much more slowly than the main drum, may solve the problem. The drum tannage permits the use of an excessive quantity of tannin, which, of course, adds to the weight of the leather. By the drum method of tanning, heavy hides can be tanned in two days after leaving the colouring pits; lighter hides are, naturally, tanned in less time, proportionate to their substance. The pit method occupies any time from one to fifteen months, although nowadays very few hides are left in the tan pits for a year.

The methods of tanning just described relate chiefly to sole leather, but there is an enormous production of leather known as dressing hides, which are tanned, dried in the rough state, and sent to curriers or leather dressers for finishing. These hides are used for numerous purposes, including bags, portmanteaus, harness, saddlery, straps, belting, and boot uppers. The tannage of dressing hides differs slightly from that of sole leather; the liquors must be mellower and contain less insoluble matter, in order to obtain the necessary pliability, and a good, clear colour on the grain. A satisfactory tannage is obtained by treating the hides in oak bark liquors, which, in the later stages of the process, may be strengthened with oak wood, or myrobalan extract, or pure gambier, and completing the process in a tepid bath of sumach, which clears and lightens the colour. The drum is more suitable for the tannage of dressing hides than it is for sole leather. A quick method of drum tanning would be to treat the hides first in a 5 per cent. solution of neradol, the artificial tannin, and then complete the process with oakwood, chestnut, or quebracho extracts, or even in mixtures of these extracts. In this way, fairly good leather could be made in about two days. Neradol prevents the drawn grain and dark coloration that would result from the use of vegetable extracts alone.

In the case of light skins, such as calf, goat, and sheep, the method of vegetable tannage again differs from those just described, although there is a fair quantity of calf skins tanned with oak bark, especially those used in the shoe trade. The tendency, however, is to complete the process rapidly by using extracts, such as oakwood, quebracho, or mimosa. A very good tannage for the production of a mellow and plump leather is that of pure gambier, the colour produced forming a good ground for brown shades. Where light, fancy colours are required on the finished leather, this tannage must be completed by placing the skins in a tepid bath of sumach.

A large proportion of the vegetable-tanned sheep and goat skins is produced by sumach alone, which was adjudged by the Commission appointed a few years ago by the Royal Society of Arts to investigate the cause of decay in bookbinding leather to be the best tanning material and the one least affected by exterior conditions, such as gas, sunlight, air and dust.

Many sheep skins are split into two sections by machine before tanning, the top portion, known as the grain, being tanned in sumach, and described as "skivers." The under section, the side near the carcase, is known in the trade as a "lining," and is usually made into the so-called "chamois" leather by means of the oxidation of fish oils.

Other noteworthy vegetable tannages are those used in the production of Russia leather, and a large proportion of East India leather. Real Russia leather, of which the raw material consists of small native hides and calf skins, has a characteristic and pleasant odour, which is derived from the birch and willow barks used in the tanning process. Birch bark contains an essential oil, which is permanently fixed on the fibres of the leather during the process of tanning. This leather is only produced in Russia, and chiefly in one large tannery, although imitations are made in Great Britain, America, and Germany. These are produced by the use of ordinary tanning materials, and the scent is applied, either during or after the dyeing process by the addition of birch tar oil, which is made by the distillation of birch bark. In some respects, for example, in brilliance of finish, smoothness of grain, and freedom from defects, the imitation is better than the real, but the latter has the great advantage that its perfume is of superior fragrance and permanent, whereas the imitation leather only retains the odour for about a year. It is somewhat remarkable, in view of the good demand that exists for the leather, especially in England and Germany, that no firm outside Russia has thought it worth while to produce the genuine article. The tannage would be particularly serviceable for bookbinding leather, as the oil of the birch has both insecticidal and antiseptic properties.

The principal vegetable tannage used for Indian leather, namely, the bark of the acacia arabica, known to the natives as babul, or babool, has quite a contrary effect, for it contains a large quantity of red colouring matter, which is incorporated with the leather in the tanning process, and although it shows very little in the rough-tanned leather, it is apt to darken if the finished leather is exposed to strong light for a long period. This oxidation is accompanied by a gradual weakening of the fibres of the leather, which is, therefore, quite unsuitable for bookbinding. Fortunately, Indian tanners are making rapid progress in using other tanning materials, a good number of which are found in India.

Although the bark of the acacia arabica is not altogether satisfactory, the pods of the same tree, which are commercially known as "bablah" and contain nearly twice as much tannin as the bark, produce a very light-coloured, almost white, leather, and it is asserted that this material is a valuable substitute for sumach.

Mineral Tannages

By far the most important of the mineral tannages is the chrome process, the merits and demerits of which have not only been freely discussed in the trade, but also in the lay press. Fanciful theories of the poisonous character of this kind of leather have been published from time to time in the daily press. Such absurd statements as that prussic acid and mercury are used in the manufacture of chrome leather hardly need refuting, as, even if they could convert skins into leather, the cost would prohibit their use. The only poisonous acid used in one of the many chrome processes is chromic acid, but this is converted into the oxide of chromium in a second bath and is, therefore, made quite innocuous.

The chrome tannage is effected either by the one-bath method or the two-bath. In the former case, the tanning agent, either a basic chromium sulphate or chloride, is present in the one liquor used; in the latter method, the hides or skins are impregnated with a solution of chromic acid, which is reduced to chromic oxide in a second bath consisting of sulphurous acid and a small quantity of free sulphur. The properties of leather produced by the two-bath process render it especially suitable for vulcanising on rubber; hence its large use for non-skidding bands for motor tyres.

There are several recipes for making one-bath liquors. A favourite mixture consists of chrome alum and sodium carbonate (common soda). Another method consists of reducing a solution of chromic acid with glucose or grape-sugar. This liquor has a greater plumping effect on the leather than the chrome alum liquor has. A third process of making a one-bath liquor combines the use of bichromate of potash and chrome alum, which, when dissolved, is converted into a basic chrome salt by means of a reducing agent. The one-bath liquor can be easily and safely applied to hides and skins, and is used in much the same manner as a vegetable tan liquor, beginning with a weak solution and gradually increasing the strength until the process is completed. The two-bath method needs great care, as a slight difference in the proportions of the ingredients used may alter the character of the leather produced.

The formula now largely used is practically the same as that of the original patentee, Augustus Schultz, an American chemist. The first bath, the chromic acid solution, is made by treating bichromate of potash or soda with hydrochloric, sulphuric, or formic acids. Bichromate of potash and hydrochloric acid (commonly known as muriatic acid, or spirits of salts) are commonly used, and in the proportion of 5 per cent. and 2-1/2 per cent. of the weight of the drained pelts (5 lb. of bichromate and 2-1/2 lb. of acid for 100 lb. of pelts). The chemical reaction is represented by the following equation—

K2Cr2O7 + 2HCl = 2KCl + 2CrO3 + H2O

Bichromate of potash + Hydrochloric Acid = Potassium chloride + chromic acid + water.

This process is most conveniently carried out in the paddle-vat (Fig. 25), which, in this case, should be fitted with a wooden cover to exclude light, since the colour of the chromic acid liquor is affected by strong light. Some chrome tanners prefer to use the drum tumbler (Fig. 13), but the pounding of the skins by this method is apt to make the grain loose. Whichever method be adopted, the hides or skins should be horsed up to drain for several hours before transferring them to the second bath. It is important that they be placed grain to grain and smoothed out, as creases and air bubbles between the skins become fixed in the second liquor, and depreciate the value of the finished leather. To avoid this danger, many tanners pass the hides or skins through a striking-out machine under light pressure. Another detail of importance is to cover the chromed skins with canvas or matting to keep them from the light. Although the skins are preserved by chromic acid, they are not made into leather, for in this condition they would dry quite horny. Hence, it is undesirable that the hides should be dried at this stage and sold as leather, although such a proceeding has been attempted. Further, chromic acid is an irritant poison which may cause an eruption on the hands and arms of workers handling the hides in this solution, unless they are protected by rubber gloves, or by coating the hands with a mixture of vaseline and lanoline. The second bath consists of chemicals which reduce the chromic acid to the oxide, which is quite inert, so that there is no danger whatever of contracting a poisoned foot as the result of wearing chrome leather. The chemicals largely used for the second bath are hyposulphite of soda and hydrochloric acid. Sodium sulphite may also be used without the addition of acid. The skins change from an orange colour to a pale, bluish-grey tint, but the process is not completed until a cross section shows that the colour has changed right through the skin. A suitable proportion is 10 per cent. "hypo" and 2-1/2 per cent. acid (28° Twaddell) calculated on the weight of the drained pelts. Calf, goat, and sheep skins are usually treated in the drum, but hides, especially if a certain degree of firmness is required, are preferably run in the paddle-vat. The reaction of the chemicals in the second bath are somewhat complicated, but the principal point is the reduction of the chromic acid (CrO3) to chromic oxide (Cr23).

The sulphurous acid produced acts as the reducing agent, but is not freely liberated until towards the end of the process, the skins first changing into a dirty brown colour which gradually gives place to a beautiful pale-bluish tint. The last stages of the process are also marked by the formation of free sulphur, which aids materially in softening the leather, and giving the two-bath chrome tanned leather its characteristic rubbery texture. The vats in which this process is carried out should always be fitted with lids to confine the strong sulphurous fumes. By adding an excess of "hypo," the skins can be bleached until they are nearly white. This has no harmful effect on the leather, but makes it softer, though somewhat looser on the grain. When completely tanned, the skins are horsed up again, left to drain for at least twenty-four hours, and are then ready for dressing and finishing.

The recipes for the one-bath process are numerous, but it is becoming a common practice for tanners to buy the liquors or extracts already prepared in chemical works, which are, naturally, better fitted up for the production of a more uniform material than it would be possible to make in most tanneries.

Combination Tannages

Combination tannages have steadily grown in favour during the last few years, and will probably have an important bearing on future methods of tanning. Among those found practicable are: (1) Vegetable and chrome; (2) vegetable and alum; (3) alum and chrome; (4) synthetic and natural tannins; (5) synthetic and chrome; (6) alum and synthetic; (7) formaldehyde and chrome; (8) chrome and iron.