H. R. Procter

Iron tannages may be very shortly dismissed, as their practical interest is at present either historical or prospective, but iron salts enter in so many ways into the chemistry of leather manufacture, that their properties must be briefly considered. Iron exists in salts in two states, the ferrous, and the ferric, in the first of which it is divalent, and in the second trivalent. Thus ferrous chloride is FeCl₂; ferrous oxide, FeO; ferrous sulphate, FeSO₄; ferrous hydrate, Fe(OH)₂. The compounds of ferrous iron are mostly green, like ferrous sulphate (“green vitriol,” “copperas”): exposed to air and moisture, they easily absorb oxygen, and pass into the ferric form. Ferric chloride is FeCl₃ (or, as it is sometimes written without much reason, Fe₂Cl₆), ferric hydrate Fe(OH)₃, ferric oxide Fe₂O₃, ferric sulphate Fe₂ (SO₄)₃, and so on. The atomic weight of iron is 56. Ferric salts are mostly yellow or orange, ferric hydrate is yellow-brown, and on ignition is converted into deep red ferric oxide, which is very difficultly soluble in acids. Ferric salts in contact with more easily oxidisable matters, readily give up oxygen, and pass into the ferrous state; and especially does this happen in the presence of organic matter, under the influence of sunlight. Thus iron-salts often act as carriers of oxygen, and oxidisers of organic matter, absorbing oxygen from the air, and giving it up again under the influence of light or heat. There are several other oxides of iron which do not form salts, and there is a ferric acid, apparently corresponding to chromic acid, which is so unstable that it has been very imperfectly investigated.

Ferric salts correspond in structure to those of alumina, and like these are powerful tanning agents, and readily form basic salts, while the ferrous salts have no tanning effect until they become oxidised, when they form basic ferric salts. Ferric salts are characterised by giving blue-black or green-black compounds with tannins, and with many other allied bodies, while the corresponding ferrous compounds are mostly colourless, though they rapidly oxidise and darken.

Ferric iron, like alumina, forms an “alum,” a double sulphate of iron and potassium, Fe₂(SO₄)₃K₂SO₄, 24Aq, forming fine pale-violet crystals, but dissolving to a yellow-brown solution. (It must be distinctly understood that iron-alum and chrome-alum contain no alumina, but are simply called alums because of their similarity of constitution, iron or chrome taking the place of the aluminium. Iron-alum, in conjunction with salt, can be used for tanning, giving a pale buff-coloured leather very similar to an ordinary alum leather. Thus the presence of a small quantity of iron in an alum used for tawing is of no consequence, except as affecting the colour of the leather. In impure sulphate of alumina such as “alumino-ferric,” it, however, generally exists in the green ferrous state, and only acquires tanning properties on oxidation. Without common salt iron-salts are still less satisfactory tanning agents than those of alumina under the same conditions, as the acid is yet more loosely held, and though basic ferric salts are taken up in considerable quantities by hide, the leather produced is thin, and usually brittle. Professor Knapp devoted much study to the production of a commercial sole-leather by basic iron-salts; and took several patents, which did not prove practically successful, though the brittleness was to some extent overcome by the incorporation of compounds of iron with organic materials such as blood and urine, of iron-soaps, and of rosin and paraffin in the leather. Like most mineral tannages, the process was far more rapid than that with vegetable materials. Knapp’s basic tanning liquor was made by the oxidation of ferrous sulphate with a small quantity of nitric acid. Patents have also been taken for the oxidation of ferrous sulphate by peroxide of manganese in presence of sulphuric acid, which produces basic ferric sulphate in mixture with manganese sulphate, which has also some tanning properties. Attempts have also been made to tan by treatment of the hide with solutions of ferrous sulphate, and subsequent exposure to the air, in order to oxidise the iron on the fibre and convert it into a basic ferric salt, but have not proved of any commercial value.

The principal use of iron at present in leather manufacture is in dyeing blacks (see p. 413), but in this case, its feeble hold upon acids in the ferric state, and its tendency to act as an oxidising agent, or oxygen carrier, renders the blacks somewhat unstable, and is frequently injurious to the leather. There is also little doubt that the presence of ferric salts in leather blacks has a great tendency to cause the resinification of the oil, known as “spueing,” by promoting its oxidation.

Chrome tannages, from a practical point of view, stand on a very different footing to those which have just been mentioned; having established their position in the manufacture of almost all sorts of light leathers, in competition with all the older methods, and making a serious claim to a share in the production of belting and even of sole leathers.

Chromium is a grey, and very infusible metal, which chemically much resembles iron in its compounds, and has an atomic weight of 52, or a little over. Like iron, it possesses a divalent and a trivalent form, but the divalent has so strong an affinity for oxygen, and passes so readily into the trivalent form, that until easier means are found for its preparation, it is of little practical interest. Its salts are blue. On the other hand, salts of the trivalent form, corresponding to the ferric salts of iron, are very stable, and powerful tanning agents. They are mostly green, but violet modifications are known, corresponding to the violet crystals of iron-alum, but of a much deeper tint. There is also a hexavalent form, probably corresponding to that of iron in the unstable ferrates, but in the case of chromium, of considerable stability. Its oxide is chromic anhydride, CrO₃, commonly called chromic acid, which combines with bases, and especially with the alkalies to form yellow or orange-red salts, and the anhydride itself is almost crimson in the solid form, though dissolving to orange or yellow solutions. Chromic acid though it hardens and preserves animal tissues, has no tanning properties till it becomes reduced to chromic oxide. There is also a higher, but very unstable oxide, perchromic acid, possibly corresponding to persulphuric acid, which is soluble in ether to an intensely blue solution. The name chromium is derived from the intense colour of many of its compounds.

Our supplies of chromium are derived from chrome-iron-ore, a mineral which contains oxides both of chrome and iron. This is furnaced with a mixture of lime, and soda or potash, when it absorbs oxygen from the air, the chromium becoming converted into chromic acid which combines with the alkali present, while the iron remains undissolved as ferric oxide. Lixiviating the mass, and evaporating the solution, lime and potassium or sodium chromates are obtained, according to the alkali used, and on adding sufficient sulphuric acid to combine with half the base, potassium or sodium dichromate (or as it is commonly called “bichromate”) can be crystallised out. Potassium dichromate is most commonly made, because it crystallises well, and is not deliquescent, but sodium dichromate is somewhat cheaper, though less convenient. Dichromates, at least in the crystallised state, are not hydric salts like bisulphates, but anhydrochromates corresponding to the potassium anhydrosulphate obtained by fusing ordinary bisulphate, and to fuming sulphuric acid. Thus the formula of potassium dichromate is

-		CrO2OK
		O	, or Cr2K2O7
		CrO2OK

and its molecular weight is 294, while that of sodium dichromate, which is similar in constitution, but crystallises with 2Aq, is 298. The molecular weight of CrO₃ is 100. Chromic acid, and acidified potassium dichromate are powerful oxidising agents, and are used as such in many processes, and especially in the manufacture of alizarine. If sulphuric acid be used in molecular proportions, the product of the reaction is chrome-alum: 4H₂SO₄ + Cr₂K₂O₇ = 3O + 4OH₂ + K₂Cr₂(SO₄)₄. This, like ordinary alum, crystallises with 24Aq, and hence has a molecular weight of 998. It forms dark purple, almost black crystals, which are a fine garnet-red by transmitted light. In cold water it dissolves to a violet solution, which becomes green on boiling, but very slowly resumes the violet condition when cold. This change, which is not uncommon in chrome solutions, is probably due to a partial decomposition into free acid and a basic salt, the basic salts of chromium being generally green. It has been noticed that raw pelt swells much more in the green, than in the violet solution. Being derived from waste products, chrome-alum is often a cheap and valuable source of chromium for chrome tanning.

For the analysis of chrome compounds see L.I.L.B., p. 141 et seq. Chrome oxide, and basic chrome salts, when strongly ignited, become insoluble even in concentrated acids, and their analysis is therefore attended with some difficulty. If, however, the ignited residue (for instance a leather-ash) be finely powdered, and intimately mixed with a fusion-mixture consisting of equal parts of pure calcined magnesia and pure dry sodium carbonate, and ignited (preferably over a Teclu burner), in a platinum crucible, in which it is occasionally stirred with a platinum wire, it will be quantitatively converted into chromate, which may be dissolved in acid, and estimated with potassium iodide and thiosulphate in the usual way. If it is desired at the same time to estimate sulphuric acid, it is sometimes preferable to substitute lime or calcium carbonate for the magnesia, which is apt to be contaminated with sulphates.

Chrome is not only of importance in tanning, but in dyeing; on account of its power of forming insoluble colour-lakes with many mordant colouring matters. For this purpose normal or basic chromic salts are sometimes used, sometimes chromic acid or dichromates, the latter acting not only by yielding chrome-oxide on reduction, but as oxidising agents to the colouring matters. Most of the colours produced with chrome mordants are of dark shades, that with logwood being deep violet or black. The mordanting power of chromium is important in the dyeing of chrome leather. Bichromate of potash is often used in dilute solution for darkening the shade of leather dyed with other materials, but is not to be recommended on account of its destructive action on the leather.

Numerous patents have been taken for processes of chrome tannage. The first practical method was described by Professor Knapp in 1858 (see p. 210), though he did not recognise its value. Some of the patents have a historical interest, though of no importance. Among these may be mentioned that of Cavallin, a Swedish apothecary, whose object was dyeing rather than tanning, but who treated raw hide with a solution of bichromate, which was afterwards reduced on the fibre by one of ferrous sulphate. The leather produced is dark reddish brown, and tender from the amount of basic ferric salt formed at the same time. Mr. J. W. Swan, well known in connection with photographic processes, and electric lighting, also patented a process of chrome tannage (as an addendum to a patent on carbon printing), in which the chromic acid first fixed in the pelt was reduced by “oxalic, or other suitable acid.” Although it is possible to produce leather within the lines of the patent, the strongly acid reaction of the reducing agent renders it unsuitable for practical use. The first chrome tanning process which made any show of practical success, was that patented in 1879 by Heinzerling, which was acquired in this country by the Eglinton Tanning Company, and also worked under their license for a short time by the Yorkshire Tanning Company at Leeds. Though the process was not commercially successful on any considerable scale, it possesses points of interest which make a brief description desirable. The hides or skins, after preparation in the usual way, were treated in a mixed solution of salt, alum (or aluminium sulphate), and potassium bichromate, but no systematic attempt was made to reduce the chromic acid to a tanning form, the product being, at first at least, merely an alum tannage, coloured, and perhaps somewhat hardened with chromic acid, though on keeping for a length of time, reduction gradually took place at the expense of the hide-fibre, and of the fats employed in currying, so that the leather internally became greyish-green, and really chrome-tanned. Specimens of the early products of the process, preserved in the museum of the Leather Industries Department at Leeds, have now all undergone this change, but are still tough and flexible, showing that the rapid tendering of the Heinzerling leather, which was one of the causes of its failure, must have been due to some error in manufacture, and was not inherent in the process. Interesting, historically, is the fact, that at an early stage in the life of the patent, a specimen of the leather was submitted to the late Professor Hummel, in order that he should suggest some means of overcoming the disagreeable yellow colour of the product. He reduced it with a bisulphite, and coloured it with an aniline dye, and a piece is still in the possession of the Yorkshire College, and in perfectly sound condition. If legal publication of this experiment could have been proved, it would have invalidated the important Schultz patents under which most of the chrome-kid of the United States has been manufactured. As bearing on modern chrome-tanning, the most important reaction in the process is that of the alum with the bichromate. It has been shown by Heal and Procter[115] that pelt absorbs practically no chromic acid from bichromate, unless it has been previously set free by acidification. When however alum, or sulphate of alumina is added, its sulphuric acid liberates the chromic acid, leaving a basic alumina salt in solution, and this fact has been utilised in some modern tanning processes.

[115] Journ. Soc. Chem. Ind., p. 251, 1895.

The first really important advance in practical chrome tanning was made by Augustus Schultz, in 1884. Schultz was not a tanner, but a chemist, employed by a New York firm of aniline colour merchants, and his attention was accidentally drawn to leather by a friend who asked him if it were possible to produce a leather for covering corset steels, which would not rust them as ordinary alumed leathers do. The process which he adopted was probably suggested by a method then recently patented for the mordanting of wool by chrome oxide, and depended on the power of the pelt to absorb free chromic acid (as it does all other free acids), and the subsequent reduction of the latter on the fibre to a basic chrome salt, which produced the tannage. The reducing substance employed was the free sulphurous or thiosulphuric acid of an acidified solution of sodium thiosulphate (hyposulphite), and as it was not certain which of the two acids was the really active agent, Schultz duplicated his patent, so as to cover both. Though he made no claim in his patent to having discovered the best proportions of his ingredients, those which he specified have proved practically useful after allowing for the modifications required by different skins, and slightly different methods of working. His first bath consisted of a solution of 5 per cent. of bichromate of potash, and 2¹/₂ per cent. of concentrated hydrochloric acid (or 1·25 per cent. of concentrated sulphuric acid), reckoned on the wet weight of the prepared pelt, and dissolved in sufficient water for convenient use in the paddle or drum which was to be used in the process. In this bath the skins were worked till they took a uniform yellow colour throughout, but without any tanning effect being produced. They were now freed from superfluous chrome liquor by draining or “putting out,” and transferred to the second bath, which consisted of 10 per cent. of “hypo” and 5 per cent. of hydrochloric acid similarly dissolved. In this, they rapidly took a duck-egg green colour from the reduction of the chromic acid; and when this was uniform throughout the skin, the tannage was complete. The exact quantity of water is not of great importance, and good results can be obtained with anything varying from 20 to 50 gallons per 100 lb. of pelt (200 to 500 per cent.) if time be allowed for the weaker solution to act. The quantities of “hypo” and hydrochloric acid given for the second bath are often somewhat insufficient, and have to be slightly increased to complete the reduction. The reactions which take place are represented by the following formulÆ, in which the weights of the materials taking part in the reaction are also given below the symbols. In the first bath—

As ordinary concentrated hydrochloric acid does not contain more than about 30 per cent. of actual HCl,[116] about 2·5 parts would be required to completely decompose 2·94 parts of dichromate, while in Schultz’s formula 2·5 parts of hydrochloric acid are used to 5 parts of dichromate. This excess has been found useful in the production of a good leather, both to prevent accidents from an overdose of hydrochloric acid, and because of the modifying effect of an excess of neutral salt on the action of the chromic acid (cp. p. 82).

The reactions which take place in the second bath are somewhat complicated. Eitner, in a valuable series of articles on chrome tannage, which have been appearing in the ‘Gerber’ since January 1900, states that even better results are obtained by using the hydrochloric acid in slight excess, as the action of chromic acid (in the presence of the potassium chloride of the chrome-bath) is not swelling but hardening to the skin, and the slight swelling action of the hydrochloric acid tends to counteract this, and also to facilitate the subsequent reduction. The two views are not contradictory, as the excess of bichromate behaves to the hide as an alkaline salt, which also produces a slight swelling effect, and it is quite probable that better results are attained when the solution is either alkaline or acid, than when the potassium chromate is exactly decomposed. Eitner recommends the use of four parts by weight of bichromate, and four parts of the strongest hydrochloric acid, dissolved in 400 parts of water, for each 100 parts of wet pelt, which should yield about 40 parts of dry leather. He states that if such a bath be used, it may be safely and economically exhausted by a second pack of skins, which is impossible in a bath containing excess of unacidified bichromate. He gives[117] the following explanation of the successive changes which take place when acid is gradually added during the reduction, but points out that in practice the reactions always to some extent go on simultaneously.

In the first stage, very slight acidification is required, and if the skins have been chromed with excess of hydrochloric acid, may be altogether dispensed with. The skins become brownish from the conversion of the chromic acid into so-called “chromium dioxide” (probably really a basic chromic chromate, Cr₂CrO₄(OH)₄, which on ignition leaves Cr₃O₆); no sulphurous acid is liberated, or sulphur deposited, but sodium tetrathionate is formed in the bath, and the reaction may be represented as follows:

Further addition of hydrochloric acid brightens the colour of the skins, while the liquid still remains clear, and chromium chloride is formed instead of chromic chromate, the main reaction being:

On still further addition of hydrochloric acid, sulphur is separated according to the following equation, and is deposited partly in the skins, and partly in the bath:

After complete reduction, and consumption of the free hydrochloric acid, further reactions take place at the expense of the excess of thiosulphate which should be present, resulting in the production of basic chromic salts, and the further deposition of sulphur, mostly within the skin, as shown in the following equations:

The thiosulphate bath therefore not only reduces, but precipitates sulphur in the skin, and reduces the chromic salt to a basic state. In boiling solution, thiosulphate precipitates the whole of the chromium as chromic oxide, but in the cold, and in presence of free sulphurous acid, it only reduces to a basic salt. Eitner does not consider the possibility, which certainly requires investigation, that instead of basic salts, sulphite-sulphates are formed at least in the first instance. Such salts of one base and two acids are quite possible, and it is very probable that in the use of chroming baths containing organic acids, they have considerable influence on the tannage.

The free sulphur which is liberated is partially deposited on and among the fibres of the leather, and adds to its softness, and also acts chemically on the oils used in “fatliquoring,” so that it is probably one of the main causes of difference between the products of the Schultz or “two-bath” method, and the “one-bath” processes subsequently to be described.

It does not fall within the scope of this book to describe in detail the working methods for the production of the different kinds of chrome leather, but a few precautions common to all forms of the process may be named. It is not absolutely important in all cases that goods should be completely freed from lime before chrome-tannage, but in this case a sufficiency of acid must be allowed in the first bath to neutralise the lime introduced. Pretty thorough liming is generally advisable, to plump and separate the fibres, but as a rule the bating or puering of goods for chroming should not be excessive,[118] but should be planned not to remove more than is absolutely necessary of the hide-substance, as the chrome tannage is in its nature soft and light, and does not lend itself to artificial fillings, such as the flour and egg-yolk of the calf-kid process. Skins are sometimes freed from lime by “pickling” (p. 89), and pickled skins may be chromed without depickling, which will be done by the dichromate, but in this case the acid contained in the skins must be considered in the composition of the chroming bath. Skins, indeed, which are pickled with a sufficiency of acid may be chromed in a neutral dichromate bath, and this is sometimes a convenient mode of procedure. To prevent drawing of the grain during tanning, skins not unfrequently receive a preliminary tannage with alum, or sulphate of alumina, and these materials, together with salt, may be introduced into the chroming bath, in which case they will liberate a portion of the chromic acid, as has been mentioned in connection with the Heinzerling process. Alum, chrome-alum, and acid salts, such as sodium bisulphate, may be substituted for the acid in the chrome bath, but organic acids must not be used, as they would reduce the chromic acid. The quantity of free chromic acid in the chrome bath is of the most vital importance to success, as it, and not the dichromate (which may be present in considerable excess), regulates the amount of chrome taken up by the skin, and the subsequent degree of tannage. It is very possible to injure leather by overchroming, rendering it rough, harsh and even tender. If a bath containing excess of bichromate is to be re-strengthened, it may be assumed as a rule that all the free chromic acid has been absorbed by the skins, and while it is merely necessary to restore the strength of the dichromate to its original amount, the full quantity of acid must be used which would be required in preparing a new bath. Where, as in Eitner’s acid chrome bath, the whole of the chromic acid is liberated, the bath may be exhausted by a second pack of skins. Many tanners, in order to avoid the complications of remaking a bath, run away their chrome liquors after once using, containing all the excess of dichromate which has been used. With proper chemical control, this should not be necessary, and is objectionable, not only from its wastefulness, but on account of the very poisonous character of the unreduced bichromate. Even weak dichromate solutions, especially if warm, are liable to cause painful and obstinate eruptions on the hands, but this rarely occurs to tanners, as the poisonous action of the solution is removed on reduction. It is well, however, to arrange that men who handle skins in the chrome bath, should subsequently also work in the reducing bath. Methods of analysis of used chrome liquors are given, L.I.L.B., pp. 142 et seq. Those for the determination of acidity are not however easily applicable in the presence of alum and salts of chromic oxide.

The skins, on coming from the chroming bath may be allowed to lie for some time without serious injury, but should be carefully protected from the action of light, which reduces the chrome at the expense of the skin, and renders the subsequent tannage irregular. It is found that skins, if brought into a weak or neutral reducing bath, are apt to “bleed” or lose chromic acid, which is reduced wastefully in the bath. On the other hand a strong “hypo” bath is apt to draw the grain and contract the skins, owing to the tannage taking place too suddenly. A somewhat strong “hypo” bath is therefore often employed as a preparatory “dip,” the skins being simply drawn through it to fix the chrome on the surface, piled on a “horse” and subsequently reduced in a bath of ordinary strength. The tendency to bleed is lessened, but at the expense of the pelt, by the reduction which takes place if the skins are allowed to lie overnight in the chromed state. Eitner states that skins chromed in an acid bath (i.e. where the whole of the chromic acid is in a free state) show little tendency to bleed. After reduction, the skins are well washed with warm water, and their subsequent treatment is the same as that of skins tanned by the one-bath process, which is subsequently described (see p. 211).

Naturally in practical work, the reduction cannot be made to proceed rigidly in the definite steps described by Eitner on p. 206, but all go on in different proportions together, though by supplying the acid in proper quantities, and at proper intervals, they may be made in the main to follow in the given order. Both on this account, and because neither the exact amount of chromic acid in the skins, nor the sulphurous acid lost by escape into the air can be exactly determined, the reduction cannot be conducted on theoretical principles, but the best conditions must be empirically determined. Eitner states that 12 parts of thiosulphate dissolved in 400 parts of water, and 6 parts of (40 per cent.) hydrochloric acid are sufficient for 4 parts of bichromate per 100 of wet pelt employed in the chrome bath, of which not more than one-half to two-thirds is absorbed; and that if equal parts of bichromate and acid are employed in chroming, the acid used in reducing may be lessened to 5 parts. In this case it must not be forgotten, that if the partially exhausted chrome-bath is used for a second pack of skins, which are afterwards finished in a bath of full strength, nearly the whole quantity of bichromate used in making up one bath will be absorbed by the skins. The amount of acid consumed in reduction will be greater, the more rapidly it is added, owing to increased escape of sulphurous acid. It is better to add the acid, previously diluted with water, in 8 or 10 successive portions, more rapidly at first, and more slowly during the latter half of the operation, each portion of acid being added as soon as no further change of colour appears to be caused by that already given. These changes are the more rapid the lighter the goods. The colour darkens at first to olive-brown, then gradually becomes green, and finally blue, and when this colour is uniform throughout the thickness of the goods, no further acid need be added. For goods which have been chromed in an acid bath, Eitner states that no acid will be needed for the first twenty to thirty minutes. It is important to have a sufficient excess of thiosulphate in the bath when reduction is complete, in which case the goods may be left for some hours or overnight in the bath, to complete “neutralisation,” but Eitner prefers to use a fresh bath of 1¹/₂ parts of thiosulphate in 400 parts of water for this purpose, the bath being used, after settling, for making up the reduction bath for the next lot of goods, for which 1¹/₂ parts less thiosulphate is used. The goods must be kept in motion during reduction, either in a drum or a covered paddle.

In a paper on “Die Natur und Wesen der Gerberei” published by Professor Knapp, in 1858, he describes clearly a chrome tanning process with basic chromic chloride, formed by the addition of sodium carbonate to a solution of the normal salt, but he expressly states that the product was not more resistant to water than the ordinary alum tannages. How he fell into this error is hard to explain, for leathers produced according to his directions, resist not merely washing in cold but boiling water. As soon as the Schultz process proved successful, many attempts were made to evade the patent by the use of other reducing agents than the “hypo,” and other salts of sulphurous acid which it covered. Among these, the use of hydrogen sulphide, and acidified solutions of alkaline sulphides, and especially of polysulphides,[119] proved capable of practical use, though less convenient than thiosulphate, but were soon acquired by a combination, the Patent Tanning Company, together with Schultz’s original patents.

Under these circumstances, Martin Dennis, either by fresh discovery, or otherwise, revived the original process of Knapp, which he patented[120] almost word for word, and offered a basic chrome tanning liquor for sale, without further restrictions on its use. This liquor was made by dissolving precipitated and washed chromic hydrate (easily prepared by precipitating chrome-alum solution with excess of alkali) in hydrochloric acid to saturation, and adding washing soda until the solution was rendered sufficiently basic. Such a solution may be used on skins prepared in the ordinary way, by diluting with water, and strengthening as the tannage proceeds, like a vegetable tan-liquor. It is doubtful if the patent is a valid one, as it was known that the use of such a solution was not new, and it was only granted in America on the representation, which has since been found to be mistaken, that chlorides alone were applicable for tanning, while Knapp had not restricted his statement to these salts. In reality chlorides and sulphates seem equally suitable, but to produce similar results the former must be made more basic than the latter. In any case the patent cannot cover the general principle of basic tanning, but only the particular liquor and mode of preparation specified. It was soon afterwards shown by the writer,[121] that a good chrome tanning liquor might be prepared by direct reduction of dichromate with sugar in presence of such a limited quantity of hydrochloric acid as to produce a basic salt. Suitable proportions are 5 mol. HCl to 1 mol. potassium dichromate, which produces a salt approximately Cr₂Cl₃(OH)₃. The solution is easily made by dissolving three parts of dichromate in a convenient quantity of water, adding six parts by weight of concentrated hydrochloric acid, and then cane-sugar gradually till a green solution is obtained, when the whole may be made up to one hundred parts, and will be approximately of the same strength as a 10 per cent. solution of chrome-alum. A little heat may be needed to start the reaction, but too much should be avoided, as considerable heat is evolved by the oxidation; and as much carbonic anhydride is produced, which causes the solution to effervesce briskly, the vessel used should be of ample size. In place of cane-sugar, a good quality of glucose may be used, but some samples contain some impurity which produces a violet solution which will not tan satisfactorily. This liquor is in regular use in many tanneries, producing a good quality of chrome calf, but is somewhat variable in its effects according to the temperature employed in its preparation, and it appears to have no real advantage over a simple solution of chrome-alum, rendered basic by soda and with some addition of salt. A somewhat similar preparation is Eberle’s “chromalin,”[122] in which some organic substance, probably crude glycerine, is used to reduce the bichromate. The organic matters, and especially the organic acids which result from the oxidation of the sugar or glycerine, are not without influence on the tanning properties of the liquor. Of course these solutions may be rendered still more basic by the addition of sodium carbonate. A good stock-liquor, of approximately the same strength as that above described, is made by dissolving 10 parts of chrome-alum in 80 parts of tepid, but not hot water,[123] and adding with constant stirring a solution of 2¹/₂ to 3¹/₂ parts of washing soda in 10 parts of water. The chrome alum dissolves somewhat slowly without the aid of heat, and the solution is best made either in a small drum driven by power, or by suspending the crystals in a basket near the surface of the liquor, so that the saturated solution can descend.

Eitner[124] has pointed out the important effect that differences of basicity have on the tanning properties of chrome solutions. Normal chrome sulphate or chrome-alum colours the leather quickly and equally throughout, and swells the pelt on account of its practically acid character, but gives a thin and lightly tanned leather, from which much of the chrome washes out, unless it is at once “neutralised” in alkaline solutions. As the chrome solution is made more basic, the tannage penetrates more slowly, but is heavier and more thorough, the colour is darker and bluer, and much less of the chromic salt is removed by washing with water. When the basicity becomes excessive, the solution becomes unstable, and decomposes on dilution with water into a very basic salt which is precipitated, and a more acid solution than that given by a moderately basic salt. The effect of such solutions on the leather is very unsatisfactory, producing the bad effects both of too acid and too basic salts. The pelt is apt to be swollen and lightly coloured by the more acid salt, but at the same time the actual tannage proceeds very slowly, and in extreme cases it is difficult to tan through, while the surface becomes over-tanned, and the grain often tender and even brittle from the incrustation of precipitated basic salt. Eitner likens the effect of the more acid liquors to the quickly penetrating and lightly tanning vegetable tans, such as gambier, and that of the more basic to the heavier tannages, such as valonia; and within limits, advantage may be taken of these facts in adjusting the liquors to the character of the leather it is desired to produce. In sulphate liquors, he considers the salt CrOH.SO₄ as most suited to general use, and in the case of chrome-alum, this is produced by the use of 286 parts of soda-crystals, or 106 parts of dry sodium carbonate (1 molecule) to 998 (or practically 1000) parts by weight (1 mol.) of chrome-alum. (In using washing soda, care must be taken to employ clear crystals of the salt, and not those which have become white by loss of water.) In place of soda, Eitner makes a similar basic liquor by boiling 1000 parts of chrome-alum with 248 parts (1 mol.) of sodium hyposulphite until the whole of the liberated sulphurous acid is driven off, and the sulphur deposited. In comparative experiments by the Author, no difference could be detected between the tanning effects of the two solutions, and that with soda is both cheaper and more easily made. If the solution with hyposulphite is not boiled, a more acid liquor results, in which part of the chromium is combined with sulphurous acid, forming an unstable compound which may prove useful in certain cases.

Eitner states that he has made chrome-solutions of various types, containing organic compounds in combination with the chrome-salt, which combine with the leather, producing a fuller and softer tannage, but he gives no details as to their preparation, as they are made commercially by the “Erste Oesterreichische Soda-Fabrik” at Hruschau. The writer has found that in some cases by the addition of say 3 parts of sugar, or still better of glucose, to 10 parts of the chrome-alum in making up the basic liquor, a much fuller and plumper leather is produced, which dries perfectly soft, even without staking or fat-liquoring; and it is probable that many other organic compounds may be found which produce similar effects. The addition of very small quantities of even neutral tartrates or lactates, and probably of many other organic salts or acids, have a remarkable effect in lowering the apparent basicity of the solution, and it is possible that these may also be usefully employed in combination with very basic liquors. It is highly probable that the unsatisfactory tanning liquors produced by direct reduction with some samples of glucose are due to the presence of small quantities of some organic acid produced during the oxidation. It has been found that these solutions may be made to tan by the liberal addition of soda. It is probable that more satisfactory results in chrome-tanning will be attained by the direct addition of known organic substances to basic liquors of definite constitution, than by the somewhat uncertain products of organic oxidations.

The quantity of salt to be added depends on the qualities desired in the leather, and upon whether chloride or sulphate liquors are employed; salt in chloride-liquors increasing the softness of the leather, but in excess tending to flatness, while in sulphate-liquors it practically diminishes their basicity by converting the chromium sulphate into the equivalent chloride, which, as Eitner points out, behaves as a less basic salt, and hence but little advantage is to be gained from its use. It is best to begin with a very weak liquor, to avoid drawn grain, and for the same purpose a preparatory tannage with alumina salts, or an addition of alum or sulphate of alumina and salt may be made to the first liquor, as the attraction of the chrome salt for the fibre is sufficient to produce a chrome tannage, even in presence of excess of alumina salts. 10 lb. of chrome alum will tan about 100 lb. of wet pelt, but more must be used for the first parcel; as to avoid loss of time, the skins may be tanned out in a pretty strong liquor. The bath has a tendency to become acid by use, and before strengthening, it may be necessary to add some more soda solution. Very little additional salt is required, as it is only absorbed by the skins to a small extent, probably as chromic chloride. As the liquors gradually become charged with sulphates, it is best to work them out like bark liquors, and not to go on strengthening the same liquor indefinitely. If old liquors are used for green goods, it is not necessary to neutralise them with soda before use, as Eitner has shown that less basic liquors colour more evenly and with less tendency to produce drawn grain.

Basic chrome liquors, such as have been described, may also be used in chrome combination tannage. It is generally best to let the light vegetable tannage precede the chrome, and lightly tanned skins, such as “Persians” and East India kips, acquire many of the qualities of chrome-tanned leather by the treatment. The effect is still further increased by a previous detannisation of the leather with alkaline solutions (see p. 241). Several firms beside Dennis now supply basic chrome liquors ready prepared for use.

The time of tannage will of course vary with the thickness of the goods, and for calf-skins will usually extend over some days, though it can be much quickened by drumming. The tannage is generally best accomplished in the paddle, but can be carried out by frequent handling in pits or tubs, or, where very smooth grain is important, by suspension. When the goods come out of the final liquor, they may be allowed to lie in pile for twenty-four hours, or even for some days, with advantage, as the surplus chrome liquor is pressed out, and the tannage becomes more complete. They are then washed with plenty of warm water, till it ceases to be coloured with chrome. They may be kept for an almost unlimited time in a wet condition, as they do not bleed, and have little tendency to heat even in pile. They have now reached the stage at which we left the “two-bath” leather, and the subsequent treatment may be the same in both cases.

Although by both processes, the chrome-salt fixed in the fibre is of a decidedly basic character, it still contains enough acid to act injuriously on the leather in course of time, and to lead to serious inconveniences in its subsequent treatment. Before proceeding further, this access of acid must be removed or neutralised, and it is not too much to say that most of the troubles experienced in the fat-liquoring arise from neglect or mistake in the washing and neutralisation. The difficulty in the process arises from the fact that while the acid should be reduced to a mere trace, it must not be entirely removed,[125] as chromic oxide itself does not seem capable of tanning, and at any rate the effect of excess of strong alkalies is at once to render the leather hard and pelty. Borax is one of the safest neutralising materials, about 3 per cent. on the wet weight of the pelt being required, in not more than ¹/₂ per cent. solution. Eitner recommends the use of silicate of soda, which, sold as a solution of S.G. 1·5, is somewhat stronger and much cheaper than borax. Hyposulphite of soda and whitening together neutralise more rapidly and completely than either alone. Other salts of weak acids may also be used, the acids exercising a regulating influence which prevents neutralisation going too far. Sodium carbonate or bicarbonate, or ammonia may also be used, but with these it is difficult to get even “neutralisation,” or to avoid the risk of carrying the process too far. Even a thorough drumming with a milk of “whitening” (calcium carbonate) is effective. With the latter there is no danger of overdoing the process, but in some cases the adhering whitening and precipitated calcium sulphate are troublesome in later operations. In any case the neutralising should only be carried so far that the skins show no acid reaction to litmus paper.

It is probable that one of the great causes of difference between “one-bath” and “two-bath” leathers is the presence of free sulphur in the latter. This may also be introduced into “one-bath” leather, by treating it in the wet chromed state, without washing out the chrome liquor, with excess of a solution of hyposulphite, or of an alkaline polysulphide, which at the same time will neutralise the skin. The more acid the chrome liquor, the greater the quantity of sulphur which will be introduced. The simplest means of distinguishing “two-bath” from “one-bath” tannages is to test for the presence of sulphur, by wrapping up a silver coin, with a piece of the leather in paper, and leaving the parcel for an hour in the water-oven, or some other warm place, when the presence of sulphur will be shown by the blackening of the coin. Of course a sulphurised “one-bath” leather will give the same reaction.

The leather must now be dyed and fat-liquored. Which of these two operations should be first undertaken will depend on circumstances. Most leathers dye more easily before fat-liquoring, but as many dyes are soluble in the alkaline fat-liquor, a good deal of colour is often lost. This may be compensated by dissolving a suitable aniline (acid) colour in the fat-liquor. “Bluebacking” is generally done before fat-liquoring by drumming with methyl-violet, or some other aniline colour (with or without logwood, which gives alone a very dark violet). Any shaving or splitting required must of course be done before bluebacking.

The fat-liquor is an emulsion of soap and oil, which for chrome leather should be as neutral as possible, if the neutralising has been thorough; but if any acid be left on the skins, a neutral fat-liquor will be precipitated as a greasy mass. This can sometimes be remedied by the addition of a little ammonia or borax, or by re-fat-liquoring with soap solution only, but if the washing of the skins has been incomplete, and soluble chrome-salts remain, the mischief is almost irretrievable, as sticky chrome-soaps are formed, often coloured with the aniline violet, which adhere to the skins, and which can scarcely be removed by any solvent which does not injure the leather. As regards the soaps and oils used, there is considerable latitude: 1¹/₂ per cent. of castor-oil soap, and ³/₄ per cent. of castor or olive oil on the wet weight of the pelt has done good service in my hands, but many manufacturers employ soft soaps, curd soaps, etc., with castor, olive, cod or neatsfoot oil, and sometimes sod-oil or degras. Eitner considers olive-oil and olive-oil potash soap the most suitable, and particularly warns against the use either of drying oils or of oils containing tallow (such as neatsfoot), which are not only apt to cause a white efflorescence, but to give the leather a disagreeable rancid smell. Fish-oils are unsuitable, but mineral oils are often useful constituents of fat-liquors. Wool-fat also makes a good fat-liquor, but is unsuitable for goods which are to be glazed. “Turkey-red oil” (which is sulphated castor) may be used as a fat-liquor, simply mixed with warm water, without soap, and has been recommended where delicate colours are to be dyed after fat-liquoring; but it is said to have an unsatisfactory after-effect, hardening and tendering the leather. Some soaps made from the saponifiable part of wool grease, such as “Lanosoap,” also act well in conjunction with olive, castor, or other oils. Where leather is to be glazed, the amount of fat-liquoring must be kept very moderate. Fat-liquors should be thoroughly emulsified, and are generally used warm. They penetrate better if the leather is partially dried by sleeking out, or pressing, or cautious “samming,” but the leather must not be completely dried out before fat-liquoring and dyeing, unless it has been previously treated with glycerine, glucose, treacle or some deliquescent salt, which will enable it to be wet back. Chrome leathers are not “waterproof,” as has often been stated, unless rendered so by treatment with soaps and greases, and are apparently easily wetted, but the fibre will no longer absorb water after thorough drying, and consequently will neither dye nor stuff satisfactorily. In order to enable chrome leather to be kept in an undyed condition, glycerine or syrup is sometimes mixed with the fat-liquor, but as the watery portion of this is not generally completely absorbed, the process is somewhat wasteful. Mr. M. C. Lamb avoids this difficulty by applying a solution of glycerine to the grain-side with a sponge after fat-liquoring. In this case the leather may be dried sufficiently for staking or shaving without risk.

Chrome leather can be dyed by many of the acid aniline colours without a mordant. Basic colours are only fixed when the leather has been first prepared with a vegetable tannin, gambier, or a mixture of gambier and sumach being the most suitable. Considerable care must be employed in the application of tannins to chrome leather, as they have a tendency to harden it and diminish its stretch, or even to render it tender, but traces of tannin in the dye probably facilitate glazing. Before dyeing, it is advantageous to fix the tannin with tartar emetic, or for browns and yellows, with titanium potassium oxalate solution, which itself gives a good yellow-brown with tannin. In place of employing the tannin and titanium salt in two separate baths, they may be combined, using a weight of the gambier or tanning extract (oakwood, chestnut, etc.) about equal to that of the titanium salt, or titanium tanno-oxalate solution may be used. Chrome leather may be dyed with the various dye-woods, which are mordanted by the chromium present, but the colours are mostly dull, that of logwood being nearly black. A good black of a very permanent character is obtained by dyeing with logwood, and saddening with a hot solution of titanium oxalate in the drum. A little iron-alum added to the chrome liquor in tanning will facilitate dyeing the skins black with logwood and help it to penetrate through the leather, which is sometimes desired. Several aniline blacks, and notably the “corvolines” of the Badische Anilin und Soda Fabrik, Casella’s “leather black C,” and Claus & RÉe’s chrome-black give very satisfactory blacks by brushing or dyeing.

Chrome skins may be glazed in the ordinary way with blood or albumen mixtures under glass or agate, but require good pressure and repeated seasonings and glazings, and much care is required in fat-liquoring. The glazing is often assisted by the previous application of barberry juice (Épine vinette) or of lactic or tartaric acid solution with a trace of sugar. Much of the difficulty which has been experienced in glazing chrome leathers is due either to the natural fat of the skin, or to oils used in fat-liquoring in excessive quantity or of unsuitable character.