Chapter XI: Fur Dyeing

MORDANTS

The hair of furs has the peculiar quality of fixing the oxides or hydroxides of certain metals from dilute solutions of their salts. Advantage is taken of this property to mordant the furs, that is, to cause a certain amount of the metallic oxide or hydroxide to be permanently absorbed by the fibres. The term mordant comes from the French word “mordre,” meaning to bite, it being formerly considered that the purpose of a mordant was to attack the surface of the hair in such a way as to permit the dye to be more easily absorbed. In fact, killing mixtures, which were intended for this same object, used to contain the various chemicals which have a mordanting action, in addition to the alkaline constituents. The mordants were not applied as such, but always as killing materials. It was later realized, however, that the mordant was instrumental in the production of the color itself.

Mordanting may be considered as having a two-fold object: first, to help fix the dye on the fibre in a more permanent fashion, thus rendering the dyeings faster; and secondly, to help obtain certain shades of color, as the various mordants produce different shades with any given dye. Some classes of dyes can be applied to furs without the use of mordants, but other types are taken up only in a very loose manner, being easily washed out from the hair with water, and it is only when such dyes are brought on to the hair in the form of a metallic compound, producing what is known as a “lake,” that really fast dyeings are obtained with them. The substances which are used for mordanting the hair are certain metallic compounds, but not all metallic salts which are used in dyeing are mordants. Sometimes such a compound is employed to develop the color of the dyeing by after-treatment, as in the case of after-chroming, the action of the metallic salt being directed only to the dye, and is not fixed by the fibre as a mordant must be. In order for a metallic compound to act as a true mordant, it must be fixed by the hair, and it must combine with the dye, thus forming a sort of connecting link between the dye and the hair. It is not absolutely essential that the mordant be applied first, although this is the customary and commonest practise. There are three ways by which the mordants can be fixed on the fur hair: First, by the absorption of the metallic oxide or hydroxide from a solution of the mordant prior to the dyeing; second, the mordant may be fixed on the fibre at the same time as the dye; and third, the mordant may be applied after the fur has been treated with the dye. The last two methods will be discussed in connection with the dyes, as they are special cases.

The salts of metals which are comparatively easily dissociated in water, with the formation of insoluble oxides or hydroxides, are most applicable as mordants for furs, and among them are compounds of aluminum, iron, chromium, copper and tin. The constituents of the hair seem to bring about the dissociation of the metallic salt, and the oxide or hydroxide as the case may be, is absorbed and firmly fixed by the hair. Just what the manner and nature of this fixation are, is still uncertain. It is supposed that chemical combination takes place between the hair and the metal. The course of this process may, as far as is known, be described as follows, taking, for example, the case of chromium sulphate: In dilute solution, this compound gradually dissociates first into its basic salts, and finally into the hydroxide, the breaking up of the neutral salt being induced by the presence of the fur-hair.

Cr₂(SO₄)₃ + 2H₂O = Cr₂(SO₄)₂(OH)₂ + H₂SO₄
chromium water first basic sulphuric
sulphate chrome salt acid

Cr₂(SO₄)₂(OH)₂ + 2H₂O = Cr₂(SO₄)(OH)₄ + H₂SO₄
second basic
chrome salt

Cr₂(SO₄)(OH)₄ + 2H₂O = Cr₂(OH)₆ + H₂SO₄
chromium
hydroxide

These reactions take place within the fibre, after the hair has been impregnated with the solution of the neutral salt, and when the compound has been rendered completely basic, in other words has reached the form of the hydroxide, it is supposed to combine with the acid groups contained in the hair substance, forming thus some complex, insoluble organic compound of the metal within the hair. According to some authorities the mordant is supposed to be present in the hair simply as the hydroxide, being tenaciously held by some physical means. The facts seem to indicate, however, that the metal is actually combined in some chemical way with the hair. For, if the mordant were present as hydroxide, then on white hair it would show the color of the hydroxide, which it does not. The same facts obtain with regard to other metals.

In order for the hair to be properly mordanted, it is necessary that the metallic compound which is taken up by the hair be held in such a manner that the mordant cannot be removed by water or even dilute acids or alkalies. Salts which dissociate too readily produce mordants which are only superficially precipitated on the hair and subsequently come off. Usually some substance is added to the solution of the salt to cause slower and more even dissociation of the salt, so that the hair substance can be quite saturated with the metallic compound before any insoluble precipitate is formed. Dilute sulphuric acid, organic acids like acetic and lactic, and cream of tartar are used to facilitate the uniform absorption of the mordant salt by the hair.

When the skins are mordanted before dyeing, they are immersed for 6 to 24 hours in a solution containing 1 to 20 grams of the metallic salts per liter of water, together with the corresponding quantity of the assistant chemical. The skins should be so entered into the mordant solution that the hair is uniformly in contact with the solution, and all the skins so that they are acted upon alike. Machinery such as is used for killing is suitable for mordanting also. The duration of the mordanting, and the concentration of the solutions are varied according to the depth of shade required, and also according to the nature of the dye to be employed. By suitably combining several mordants a considerable range of colors can be obtained with a single dye.

The various chemicals used as mordants are essentially the same no matter for which class of dyes they are used, there being only slight differences in the concentrations of the solutions, the manner of application of the mordants being practically the same. It is interesting to note that with the exception of chromium compounds, which are of comparatively recent adoption as mordants, all the chemicals now used for mordants were employed by the earliest masters of the art of fur dyeing. While some of the formulas used by those dyers display a lack of appreciation of the true action and function of the mordanting chemicals, yet it is quite remarkable that they chose, in spite of their limited knowledge of chemical processes and phenomena, just those materials which do act as mordants if properly applied. The most important metallic compounds for mordanting furs at the present time are salts of aluminum, iron (ferrous), copper, tin and chromium (as well as chromates and bichromates). The compounds of the metals with organic acids such as acetic acid are preferable, being more easily dissociated, and also leaving in solution an acid which is less injurious to the fur than a mineral acid. However, sulphates and other salts of the metals are also used extensively, inasmuch as they are cheaper than the organic salts.

ALUMINUM MORDANTS

Chief among the aluminum mordants are the various kinds of alum, which is a double sulphate of aluminum and an alkali such as sodium, potassium or ammonium. All these salts except that of sodium, form large, colorless, octahedral crystals, and are soluble in about 10 parts of cold water, and 1/4 part of hot water. Sodium alum is even more easily soluble, but on account of the difficulty of obtaining it in crystalline form, it is little used. The common commercial alum is the potassium aluminum sulphate.

Recently, aluminum sulphate has to a large extent replaced alum for mordanting purposes, because it can be obtained very cheaply in pure form, and it contains a greater amount of active aluminum compound than does alum. Only the iron-free salt, however, may be used for the needs of fur dyeing.

Aluminum acetate also finds extensive application as a mordant in fur dyeing, and while somewhat more expensive than the alum or aluminum sulphate, it has the advantage over these compounds of being combined with an organic acid, which is preferable when the action on the hair and leather is considered. Aluminum acetate can be obtained in the market in the form of a solution of 10 degrees Beaumé, but can also be prepared very easily as follows:

665 grams pure aluminum sulphate, or
948 grams potassium alum, are dissolved in
1 liter of hot water.
1137 grams of lead acetate (sugar of lead) are also dissolved in
1 liter of hot water.

The two solutions are mixed, and thoroughly stirred. A heavy white precipitate forms, which is filtered off, and discarded after the solution has cooled. The aluminum acetate is contained in the filtrate, and the solution is brought to a density of 10 degrees Beaumé by the addition of water, if necessary, and is preserved for use in this form.

IRON MORDANTS

Ferrous sulphate, iron vitriol, or copperas, as it is commonly known, forms pale green crystals, which on exposure to air lose water, and crumble down to a white powder. It is very soluble in both cold and hot water, but the solutions oxidize very rapidly, turning yellowish, and should therefore be used immediately. Care must be taken that a good quality of iron vitriol be used for the mordant, otherwise very unsatisfactory results will be obtained.

Ferrous acetate is prepared in a manner similar to the aluminum acetate, and is occasionally employed instead of the ferrous sulphate. Inasmuch, however, as the solution of ferrous acetate is very easily oxidizable when exposed to the air, a more stable form is used, and this comes on the market as iron pyrolignite or iron liquor. This can be prepared by dissolving iron in crude acetic or pyroligneous acid, or by treating a solution of iron sulphate with calcium pyrolignite. Iron liquor is really a solution of ferrous acetate that contains certain organic impurities which prevent, or rather, considerably retard the oxidation of the iron salt, but which in no way interfere with its mordanting properties. The commercial product can be had in various concentrations, but 10 degrees Beaumé is the most usual and most convenient.

COPPER MORDANTS

The most important copper salts used in fur dyeing processes are copper sulphate, or blue vitriol, occurring in large blue crystals, very soluble in cold and in hot water; and copper acetate, which is formed by treating a solution of copper sulphate with a solution of the requisite quantity of lead acetate. Copper acetate can also be obtained in the form of blue-green crystals, very soluble in water, the solution becoming turbid on prolonged heating, due to the formation of a greenish basic copper acetate. This insoluble compound is known commonly as verdigris, although it is not usually produced in the manner mentioned. Numerous fur dyeing formulas contain verdigris, but inasmuch as the basic copper acetate is insoluble and thus incapable of reacting with any of the substances used in dyeing, it is assumed that the soluble normal copper acetate was meant, for this compound is also sometimes called verdigris.

In addition, there must be mentioned here a compound which formerly found extensive use in fur dyeing. This is a double salt of copper and iron, analogous to alum, ferrous copper sulphate, known as blue salt. It is very seldom used at the present time, being more effectively replaced by other substances.

CHROMIUM MORDANTS

The typical chromium mordant is chrome alum, which is a potassium or ammonium chromium sulphate, constituted just like the aluminum alums, and forming crystals like these. More frequently used, nevertheless, than the chrome alum, is chromium acetate, which is prepared from it, either by treating a solution of the chrome alum with a solution of lead acetate, or in the following manner:

50 grams of chrome alum are dissolved in
500 cubic centimeters of boiling water. To this is added
15 grams of 20% ammonia, diluted with 15 grams of water.

The precipitate which forms is filtered off, and preserved, the filtrate being discarded. After thoroughly washing the residue on the filter it is dissolved in dilute acetic acid, heating if necessary, to effect solution.

Other chromium compounds of an entirely different type are also used in fur dyeing, these being chromates and bichromates, the latter finding greater application than the former. Sodium bichromate is the salt most usually employed. This forms orange-red crystals which are very soluble in water, and in addition to its use as a mordant it also serves as an oxidizing agent for developing or fixing certain dyes on furs.

TIN MORDANTS

Compounds of tin find only limited application in fur-dyeing, the only one of importance being tin salts, stannous chloride, which occurs in the form of white, hygroscopic crystals, which must be preserved in closed vessels. It is very soluble, but in dilute solutions it readily forms a basic salt, so stannous chloride is usually used in very concentrated solutions.

ALKALINE MORDANTS

After the furs have been treated with the solution of some alkali for the purpose of killing the hair, they are always passed through a slightly acidulated bath to remove any alkali which may still be adhering. This operation must always be gone through before the skins can be mordanted or dyed, for if it were neglected, very uneven and uncertain results would be obtained. This process, however, entails the expenditure of no small amount of time, labor and chemicals when large lots of skins are being handled. In order to eliminate this extra step of “souring” between killing and mordanting or dyeing, it has been proposed to use alkaline mordants which combine the killing and mordanting functions, and accomplish these two processes at the same time. The advantages of employing such mordants are easily apparent. Cumbersome manipulation and handling of the skins, with the attendant consumption of much time and labor are reduced to a minimum, and besides there is no needless waste of chemicals as is the case in the ordinary methods of killing the furs.

The principle of alkaline mordants is not a strictly new one. If it be remembered that the old killing formulas used by the fur dyers of an earlier age, contained metallic salts with mordanting properties in addition to the alkaline substances, which alone were effective as killing agents, it would seem that the suggested alkaline mordants were merely a revival in modified form of the old processes. This is undoubtedly true in a large measure, for the killing mixtures which the old masters used certainly embodied the fundamental principle of simultaneous killing and mordanting, although it was not recognized at that time.

Modern alkaline mordants have therefore been devised which can be employed for killing and mordanting furs at the same time. They are prepared as follows:

ALKALINE ALUMINUM MORDANT

250 grams of potassium alum are dissolved in
1 liter of boiling water. To this solution is added
300 grams of soda ash, previously dissolved in
750 c.c. of water, and the resulting precipitate is filtered off,
washed and pressed, and then dissolved in a solution of 65 grams
of caustic soda in 1 liter of water.

ALKALINE CHROMIUM MORDANT

250 c.c. of chrome acetate mordant of 20 degrees Beaumé
320 c.c. of caustic soda solution of 38 degrees Beaumé (32.5%)
10 c.c. of glycerine 30 degrees Beaumé (95%)

The solution of these substances is brought up to a volume of 1 liter by the addition of 420 c.c. of water.

ALKALINE IRON MORDANT

138 grams ferrous sulphate are dissolved in
362 c.c. of warm water. Cool and add
25 c.c. of glycerine. Then slowly and carefully add
25.5 c.c. of concentrated ammonia, taking care that no precipitate
forms.

While these alkaline mordants seem to have much in their favor, there are certain possible objectionable features which must be considered. The solutions of the mordants are generally very alkaline, and not every fur can withstand more than a limited quantity of alkaline substance for longer than a comparatively short time. Suitable mordanting usually requires a longer time than killing does, so with the use of the alkaline mordant, if the skins remain in the solution until sufficiently killed, they may be insufficiently mordanted, while if the furs are treated long enough to be properly mordanted, the hair may have been over-killed. However, the idea of the alkaline mordant is a good one, and it is only a matter of time and patient, scientific experimentation when the difficulties of the method will be eliminated, and a much-desired process will become a practical realization.

The general methods for applying the various mordants of all sorts follow closely the procedure adopted for the killing formulas, and similar precautions must be observed, in order to obtain consistently uniform results. With the exercise of care, there is little reason for the mordanting operations to go wrong.

After proper treatment of the skins in the mordants, they are removed and drained off, then rinsed lightly in running water to remove the excess of mordant liquor, after which they can be directly entered into the dye bath. If it is not feasible to dye the mordanted skins at once, as is often the case, the skins are kept moist, and under no circumstances allowed to dry.