D. R. P. 51073 Supplement to 47349; Process for Dyeing Hair

This patent was an extension of the original patent to include certain oxy and amido-oxy compounds, the method being essentially the same otherwise as in the original patent. An illustration of the process is as follows:

73 grams para-amido phenol hydrochloride are dissolved with 40 grams caustic soda in a liter of water. The solution dyes hair a golden-yellow, which on subsequent treatment with a solution of ferric chloride turns to a red-brown.

In these two patents is to be found the basis of the modern fur dyes and fur dyeing methods. It is interesting to note that furs were not mentioned at all in connection with the process, which was intended mainly for dyeing hair, especially on the human head. It was only several years later that the value of the method for dyeing furs was realized. So about 1894, the Aktien Gesellschaft für Anilinfabrikation put upon the market three fur dyes under the trade name Ursol, Ursol D, giving dark-brown to black shades; Ursol P, giving red-brown colors; and Ursol C, giving a yellowish-brown shade. Pyrogallic acid had been previously used as a hair dye, and also to a slight extent as a fur dye, so it was used in conjunction with the Ursol dyes for shading purposes. The new fur dyes were not dyes in the ordinarily accepted sense of the term. They were really coal-tar intermediates, substances similar in character to aniline, and their dyeing property depended on the fact that they could be oxidized either by atmospheric oxygen, or by means of oxidizing agents, forming colored insoluble products. When the oxidation of the intermediate was caused to take place on the hair the colored product formed on and in the hair fibre, and remained fast. The reactions bringing about the conversion of the intermediate to the colored insoluble compound are quite analogous to those of the Aniline Black process, though possibly not so complicated, with the important difference, however, that, while in the production of Aniline Black acid is essential, in the present instance the oxidation can be carried on in neutral or even alkaline medium. On account of the character of the method used in applying the new fur dyes, the name Oxidation Colors has been given to them. Strictly speaking, Aniline Black is also an Oxidation dye, but it is usually considered in a class by itself. The methods used at first in the application of the Ursol dyes to furs followed closely the process as described in the patents. The furs were first killed, usually by brushing on a lime mixture, drying, and then beating out the dust. This operation was repeated, if necessary. Then a solution of the desired dye, mixed with an equal volume of 3% peroxide of hydrogen was brushed on and the fur allowed to lie exposed to the air. The dyeing could also be done by the dip process, less concentrated solutions being used. By varying the concentration of the solution, and prolonging or shortening the time of action, the shades could be varied from very light to very dark, and by combining two or more of the Oxidation Colors, many different color effects could be produced. Soon other fur dyes were developed and put on the market; for example, Ursol DB, giving blue to blue-black shades, and Ursol 2G, yielding yellowish tones suitable for mixing with the other colors. Ursol C was discarded shortly after its introduction. The dyeings obtained with the Oxidation Colors seemed to be very fast, resisting successfully the action of cold or hot water, or even hot soap solution. Moreover, a dyed hair examined under the microscope appeared to be colored through the epidermis to the medulla, and no individual particles of dye could be discerned.

The new fur dyes had many evident advantages over the coloring matters in general use at the time. The simplicity of the dyeing operations, the short duration of the process, the great tinctorial power of the new products, were facts which strongly recommended themselves to the progressive fur dyer. The cost of the dyes was higher than that of the vegetable dyes, but this consideration was largely overbalanced by the saving in time and labor in using them. And yet, the Ursol dyes found only a comparatively small market. The majority of fur dyers, always conservative and reluctant to turn from the traditional ways of the industry were skeptical of, and even hostile towards the new dyes and the new methods of dyeing. In a sense, this opposition was justifiable. It was not an easy task to relinquish all at once methods which had been successfully applied for generations back, and with which they were thoroughly experienced, in favor of processes which were radically different, and with which they had no experience at all. But some enterprising spirits among the fur dyers undertook to try out the new products and it was not long before the skeptics had good cause for condemning the work and achievements of the chemists as far as fur dyeing was concerned. The new type of dyes did possess some of the advantages claimed for them, but they also possessed many highly objectionable features, which had never been manifest with the vegetable dyes. First of all, the dyeings were not so fast as had at first appeared, for the color came off the hair when the furs were rubbed, brushed or beaten. Then it was observed that after a short time some of the dyeings changed color, and at the same time the hair lost its gloss and became brittle. The condition of the leather after dyeing was anything but satisfactory. Most serious of all, however, was the appearance among the workers in the dyeing establishments, and also among the furriers who worked with the dyed skins, of certain pathological conditions which had hitherto been unknown. Various skin diseases, eczemas, inflammation of the eyes, asthmatic affections and intestinal irritations were some of the afflictions which were directly attributable to the use of fur dyes of the Ursol type. Medical science was at a loss to know how to treat these ailments, because their nature was not understood.

Here indeed, were obstacles threatening to destroy all the hopes which the discovery of the new class of dyes had aroused, and to check at the outset the possibility of rational progress in the fur dyeing industry. But the men of science were not content to let the matter drop thus. Difficult problems had been solved before, and surely there must be some way of overcoming the objections and deleterious features of a system of fur dyeing which had so much potential merit. Where hindrances sprang up in the path of progress, it was the duty of the chemist to remove them, and when difficulties arose, it was up to him to resolve them, as far as was humanly possible. So the chemists who had been responsible for the introduction of the Oxidation Colors set themselves to the task of eliminating the undesirable or injurious qualities. It was many years before the results of painstaking effort and persistent study cleared up the causes of all the objectionable aspects of the fur dyes, and suggested means of overcoming them satisfactorily. The work had been directed to the improvement of the dyes and of the methods of dyeing with them. Purer intermediates were produced, and more easily soluble ones, so that there would be no possibility of ultra-microscopic particles of the dye being deposited on the surface of the hair from the dye solution, instead of being taken up within the hair fibre. It was this superficial deposition of minute crystals of the dye or of the only partially oxidized intermediate, on the hair, crystals so fine as to be invisible in the ordinary high-power microscope, which caused the color to come off when the furs were brushed or beaten, giving rise to a dust which was frequently very injurious to the health. Then, mordants were adopted to help fix the dyes, compounds of copper, iron, and chromium being used as formerly with the vegetable dyes, and the range of shades was also increased thereby. Certain of the Oxidation Colors had a tendency to sublime off the hair, so the dyed hair was chemically after-treated in such cases to prevent this. The causes of the pathological aspects of dyeing with the Oxidation fur dyes were not so readily disposed of. But the adoption of devices to prevent the formation and circulation of dust during the handling of the dye, the employment of adequate protection against contact with the dye or its solutions, the use of the most dilute solutions possible in dyeing, the thorough washing of the dyed skins to remove any excess of the coloring matter, the prevention of dust formation in the drying of the skins, and the rigid observance of, and adherence to hygienic laws, were all factors in the elimination of the health-impairing phases of dyeing with the Oxidation Colors.

It was only after all these improvements had been accomplished that the fur dye intermediates began to acquire a degree of popularity among fur dyers, and strange as it may seem, there was a more ready market for these dyes in America, than in Germany where they were manufactured. Other manufacturers of coal-tar intermediates also began to produce fur dyes, and so, in addition to the Ursols, there were the Nako brand, the Furrol brand, the Furrein brand, and one or two others. New dyes were invented, until the whole range of colors suitable for fur dyeing had been produced. The black dye, however, presented some difficulty. A black dye which would rival logwood blacks could not be attained. Ursol DB in conjunction with Ursol D was being used to produce bluish-blacks, but the dyeings were not fast, turning reddish after a time. In 1909, a patent was taken out for a dye mixture, which was made up like the DB brand, but instead of using toluylene diamine with para-phenylene-diamine, the new dye was made up of a methoxy, or ethoxy-diamine with para-phenylene-diamine, and it yielded brilliant bluish-blacks, which were fast, and which very nearly approached the logwood black in luster, intensity, and bloom. For some purposes, however, the production of a black color is still dependent on the use of the logwood dye.

When the Great War cut off to a large degree the importation of skins dyed in Europe, the American fur dyeing industry developed tremendously, and in a comparatively short time was able satisfactorily to accomplish in the way of dyeing furs, what had taken foreign dyers a much longer period to attain. It had been previously considered that furs could be dyed properly only by European fur dyers, but the achievements in this direction by Americans fully dispelled this belief. But the success of the fur dyers in America might not have been so marked or rapid, had it not been for the work of the American chemists. The war had also shut off the supply of German dyes, upon which the dyeing industries of America had formerly been dependent, so enterprising chemists in this country undertook to fill the need, and in a surprisingly short time, American fur dyes, in every respect the equal of the foreign product were offered to the American fur dyers, and at the present time, the requirements of the fur dyeing industry in this country are being adequately met by domestic producers. Among the brands on the market are the Rodol, Furamine, Furol, and several others. The Oxidation Colors are now being offered in a high state of purity, and easily soluble, free from any poisonous constituents, and there is absolutely no reason for the appearance of any pathological conditions among workers on dyed furs, or users of such furs, provided the necessary precautions have been taken in the dyeing process. The occurrence of any affection which can be traced to dyed fur, cannot possibly be due to the dye itself, but to gross carelessness and negligence in dyeing, and in any such event, the dyer responsible should be brought to account.

In order to get a better understanding of the nature and action of the Oxidation Colors, a typical one will be studied in some detail. The most important one in this class is para-phenylene-diamine, usually designated by the letter D in all commercial brands of this fur dye, while its chemical formula is represented as C6H4(NH2)2. When pure it occurs in colorless, crystalline lumps, which rapidly turn brown when exposed to the air; the technical product of commerce is of a dark-brown color. It dissolves readily in hot water when pure, and also in acids. At one time the hydrochloride was used instead of the free base, on account of its greater solubility, but now a base is made which is sufficiently pure to be very soluble in water. There are several methods of preparing para-phenylene-diamine: first, by the reduction of amido-azobenzol, the product obtained in this way always containing a slight amount of aniline, which reduces the solubility, and also gives rise to poisonous oxidation products during the dyeing process; second, by the reduction of paranitraniline, the quality and solubility of the product in this case depending on the purity of the starting material; and third, by the treatment of para-dichloro-benzol with ammonia under pressure, the best product being obtained by this method. The crude para-phenylene-diamine, made by any of the above processes, is generally distilled in vacuo, the refined base being obtained as lumps with a crystalline fracture.

The first step in the oxidation of the para-phenylene-diamine is the formation of quinone di-imine, NH:C6H4:NH. This is a very unstable compound in the free state, and even in aqueous solution it decomposes within a comparatively short time, or combines with itself to form a more stable substance. Quinone di-imine has a very sharp, penetrating odor, and produces violent local irritations wherever it comes in contact with the mucous membrane. If a small quantity of para-phenylene-diamine is absorbed into the human body, by breathing the dust, or otherwise, the formation of quinone di-imine takes place internally with consequent irritation of the mucous lining throughout the body. The various pathological conditions mentioned before may be ascribed to irritation caused by quinone di-imine. In any dyeing process where there is a possibility of the formation of quinone di-imine, as is the case with most dyes containing para-phenylene-diamine, special precautions must be taken by the workers in handling the dye or coming in contact with its solutions, and no one who is particularly sensitive to irritation should be permitted to work in a place where such dyes are used.

The next step in the oxidation of the para-phenylene-diamine is the formation of what is called Bandrowski’s base. Three parts of the quinone di-imine combine with themselves, forming a substance of a brown-black color, which was formerly regarded as the final oxidation product. The formula of Bandrowski’s base is represented by the following chemical hieroglyphics:

(NH2)2.C6H3.N:C6H4:N.C6H3(NH2)2 . 

Further investigation has shown that the oxidation proceeds beyond this stage with the formation of a compound of what is known as the azine type, which is depicted by the chemist as

NH NH
(NH2).C6H3< >C6H2< >C6H3.NH2.
NH NH

It is by no means certain that this substance is the true coloring matter obtained by the oxidation of para-phenylene-diamine, for the reactions may continue still farther, producing even more complicated oxidation products. Scientific research and study has not as yet gone beyond this stage.

The reactions of the other dyes of the Oxidation type are quite similar to those of para-phenylene-diamine, some being simpler, and others being even more complex. The presence of certain chemical groups in the intermediate, or the relative position of such groups are factors responsible for the variations in shade.

With the various mordants, the Oxidation Colors give different shades, and a great range of colors can be produced either by combining mordants, or combining dyes, or both. The following tables illustrate the shades formed with the customary mordants.

ChromeCopperIronDirect
Ursol Dbrown blackcoal blackcoal blackdark brown to brown black
Ursol Pdull red browndull dark browngrey brownlight brown
Ursol 2Gyellow browndull yellow brownyellow browndull yellow
Ursol A......blue blackblue to blue-black
Ursol 4Glight brownmedium brownyellowpure yellow
Ursol 4Rorange brownlight yellow brownred brownorange red
Ursol Grey Bgreenish greygreenish greymouse grey...
Ursol Grey Rbrownish greybrownish greyreddish grey...

Fur dyes of American make being equal in every way to the German product, show the same color reactions with the various mordants. The following table shows the shades produced with the same mordants as above:

ChromeCopperIronDirect
Rodol Dbrown blackcoal blackcoal blackbrownish black
Rodol Pred browndark browngrey brownlight brown
Rodol 2Gyellow brownyellow brownyellow browndull yellow
Rodol 4Glight brownlight brownreddish brownpure yellow
Rodol A...blue black...blue black
Rodol Grey Bgreenish greygreenish greymouse grey...
Rodol Grey Rgreenish greybrownish greymouse grey...

All these shades are produced by dyeing in a bath containing a neutral solution of the dye. Sometimes the dye comes in the form of a salt of a mineral acid, like hydrochloric or sulphuric acid, in which case a sufficient amount of an alkali, usually ammonia, is added to liberate the free base. According to the Cassella Co., German manufacturers of the Furrol brand of fur dyes, the dyeing can also be carried on in slightly alkaline or in slightly acid solution, a different series of shades being obtained in each instance. Ammonia is used to render the bath alkaline, and formic acid to make it acid. The most customary practise, however, is to use neutral solutions of the dyes.

For preparing the mordant solutions much smaller quantities of the metallic compounds are used than in the case of the vegetable dyes. With chrome mordants cream of tartar is always employed as an assistant, and occasionally also with copper and with iron mordants. With copper, and also with iron mordants no addition is made at all, or sometimes a small quantity of acetic acid is added. The temperature of the mordant solution is kept about 30° C., and the duration of the mordanting varies from 2–24 hours according to the depth of shade desired. The concentration of the solution may also be varied, it sometimes being just as well to use a strong mordant solution and less duration of mordanting. Chrome may be combined with copper, and iron may be combined with copper, but chrome and iron do not go together as mordants. Some typical average mordanting formulas are as follows:

Chrome mordant.
Bichromate of soda2.5gms.
Cream of tartar1.5gms.
Water1liter
Copper mordant.
Copper sulphate2gms.
(Acetic acid 50%2gms.)
Water1liter
Iron mordant.
Ferrous sulphate2gms.
(Acetic acid 50%2gms.)
Water1liter

or,

Iron pyrolignite 30%10gms.
Water1liter
Chrome-copper mordant.
Bichromate of soda2gms.
Copper sulphate0.25gms.
Cream of tartar1.0gms.
Water1liter
Copper-iron mordant.
Copper sulphate2gms.
Ferrous sulphate2gms.
(Acetic acid 50%2gms.)
Water1liter

The killed skins are immersed in the mordanting solution, and allowed to remain the required length of time. They are then thoroughly rinsed to remove any excess of the mordant, and are hydro-extracted. Under no circumstances should mordanted skins be permitted to dry, for they would be unfit for use again.

The dyebath is next prepared by dissolving the necessary quantity of the dye, varying from 0.1 gm. to 10 gms. per liter. Then if the solution must be neutralized, the ammonia is added and the temperature of the bath is brought to 30–35° C. by the addition of cold water. This temperature is maintained throughout the dyeing operation. To the solution is added the oxidizing agent. Ordinary commercial peroxide of hydrogen containing 3% by weight is the usual oxidizer, although perborates have been suggested. 15–20 parts of peroxide of hydrogen for every part of dye are added, and the dye solution brought to the proper dilution. As soon as the dyebath is ready, the skins are entered, and worked for a short time to effect even penetration. They are then left in the dyebath for 2–12 hours or longer according to the depth of shade. After being satisfactorily dyed, the furs are rinsed thoroughly, hydro-extracted and dried and finished. Where the dye is to be applied by the brush to the tips of the hair, stronger dye solutions are used, the brushed skins being placed hair together and let lie for about 6 hours in order to permit the color to develop, after which the furs are dried and drum-cleaned.

Some shades, particularly black, have a tendency to rub off slightly. In order to overcome this, the dyed furs, after rinsing, are treated with a cold solution of 1⁄2 part of copper sulphate per 1000 parts of water, for 3–4 hours, then without rinsing, hydro-extracted and dried. Furs which have been tipped are brushed with a 1–2% solution of copper sulphate and dried. Care must be taken in this after-treatment, for the use of too strong a solution of copper sulphate, or too prolonged action of such a solution will materially alter the shade of the dyed fur.

A few typical formulas will serve to illustrate the general methods of employing the Oxidation Colors:

Brown Sable Imitation on Unsheared Rabbit

The skins are killed with soda, soured, and washed, then mordanted with

Bichromate of soda2grams
Copper sulphate.25grams
Cream of tartar1gram
Water1liter

for 24 hours. Then washed, and dyed for 24 hours with

Fur Brown 2G[3]3grams
Hydrogen peroxide45grams
Water1liter

Wash and dry the skins, then brush the tips with

Fur Brown D[3]20grams
Hydrogen peroxide400grams
Water1liter

Black on Sheared Muskrat

The skins are killed with soda, soured, and washed, then chrome mordanted for 6 hours. Then they are dyed for 6 hours with

Rodol P1.5grams
Pyrogallic acid.7grams
Ammonia2.0grams
Hydrogen peroxide45grams
Water1liter

The dyed skins are washed and dried, then tipped with

Rodol D20grams
Rodol DB2grams
Hydrogen peroxide450grams
Water1liter

Brown on Thibet Sheep Skin

The killed skins are mordanted for 6 hours with a chrome mordant, then dyed for 6 hours with

Ursol P1gram
Pyrogallic acid1gram
Ammonia2grams
Hydrogen peroxide40grams
Water1liter

It is also possible to combine dyeings with the Oxidation Colors with Vegetable dyeings, or with Aniline Black. For example, if it be desired to produce an imitation skunk on a raccoon, and an exceptionally fast and intense and lustrous black on the tips of the hair, the skins are dyed in the bath with the Oxidation dyes, and the tips of the hair are brushed with a mixture such as described under Vegetable Colors for the production of French seal, as follows:

Imitation Skunk on Raccoon

The skins are killed with caustic soda, soured and washed, then mordanted with an iron-copper mordant as described, and then dyed with

Fur Grey R3grams
Ammonia2grams
Peroxide of hydrogen45grams
Water1liter

After washing and drying, the dyed skins are brushed over with a mixture such as used for dyeing French seal with Vegetable Colors.

In a similar manner, the Oxidation Colors may be used to give a base color to furs dyed by the Aniline Black process.

It is apparent from these few illustrations that a great variety of shades can be produced, and the dyeing of imitations of the better class of furs on cheaper skins is a comparatively simple matter, after an understanding of the nature of the dyes has been obtained, and a certain amount of skill acquired in working with these dyes.

CHAPTER XVI
FUR DYEING
Coal Tar Dyes

In addition to the Aniline Blacks and the Oxidation Colors already discussed there are certain of the synthetic coal tar dyes such as are generally used in the dyeing of textiles, which can also be applied on furs. There are several classes of these dyes, varying somewhat in their nature, and consequently in their manner of application; in the main they produce bright shades, such as are but seldom used on furs, yet which may occasionally serve for the production of novel effects. Basic, acid and chrome colors are the types which can be employed.

Basic colors possess great fullness and tinctorial strength, but have a tendency to rub off, and the tips of the hair take a darker shade with these dyes than the rest of the hair. The addition of acetic acid and Glauber’s salt to the dyebath will result in a more uniform dyeing. On account of the comparatively poor fastness to rubbing and washing, basic dyes are used only for dyeing furs which are intended for cheap carpet rugs, such as sheep and goat. They may also find use in the production of light fancy shades on other white furs. The procedure is usually as follows: The furs are killed in the customary manner with soap and soda or ammonia, or if this is insufficient, with milk of lime. A soap-bath is then prepared containing 2.5–6 grams of olive-oil soap per liter of water. The temperature of the bath is brought to 40° C. To this is added the solution of the dyestuffs, prepared by mixing the required color or colors with a little acetic acid to a paste, and then pouring boiling water on the mixture until dissolved. Undissolved particles or foreign matter are removed by passing this solution through a cotton cloth or sieve, and the clear solution then mixed with the soap-bath. The well-washed skins are then entered into the dyebath and immersed for about half an hour, or until the desired depth of shade is obtained. They are then removed, pressed or hydro-extracted and dried. For the production of light shades, the following dyes may be used:

For cream, light sulphur-yellow, maize, salmon, etc.

For greenish-yellows

For light pink

For purple

For sky-blue

For white

To produce very delicate shades, the moist dyed skins are subjected to a sulphur bleach overnight, to lighten the color, then rinsed, and dried. Full, brilliant shades may be obtained by dyeing in a bath of 40° C., acidulated with 2–3 grams of acetic acid per liter of solution, the following dyestuffs being suitable:

For yellow to orange

For pink

For light red

For bordeaux and red

For violet

For blue

For green

For brown

In dyeing skins with harder hair than that of sheep or goat, mere killing is insufficient to render the hair capable of taking up the dye. The skins are therefore immersed before dyeing, in a cold, weak solution of chloride of lime, the affinity of the hair for the dye being thereby greatly increased.

Acid dyes are employed when a greater fastness is required than can be obtained with the basic colors. Sulphuric acid in a quantity equal to half the weight of the dyestuffs used, together with four times that quantity of Glauber’s salt is added to the dyebath. Formic acid may be used in place of the sulphuric acid, very good results being obtained. The skins are immersed in the dyebath, and worked until thoroughly soaked with the dye liquor, and then allowed to remain until the proper depth of shade is attained, or overnight. The temperature of the solution is about 40° C., and only very light shades can be produced in this manner. In 1900 and again in 1914, the Cassella Co., a large German manufacturer of dyestuffs, obtained patents for processes enabling the dyeing of furs in hot solution with the acid dyes. The method required that the skins be chrome-tanned in order to render them resistant to the action of hot solutions, the addition of a small amount of formaldehyde to the chrome solution increasing this effect. The skins are then treated with a solution of chloride of lime in order to increase the affinity of the hair for the dyestuffs. The method as it is now practised is as follows: The skins which have been cleaned and washed are chrome tanned by the method as described in the chapter on Tanning Methods, 60 grams of formaldehyde being added to every 10 liters of the chrome solution. After proper tanning the skins are rinsed, and while still moist they are subjected to a treatment with chloride of lime. They are first immersed for 15 minutes in a cold bath containing 120 grams of hydrochloric acid 32–36° Twaddell per 10 liters of water, then without rinsing, they are entered into a bath made up by adding gradually in four portions the clear solution of 2–4 grams of the chloride of lime per 10 liters of water. After working for an hour, the skins are removed and entered again into the acid solution, in which they are worked for another 15 minutes. In order to neutralize and remove the last traces of the chloride of lime from the furs, they are rinsed in a luke-warm bath containing 1–2 grams of sodium thiosulphate, or hyposulphite of soda, in 10 liters of water. The skins are then rinsed again, and hydro-extracted, or pressed, and are ready for dyeing. The dyebath is prepared with the required quantity of dye, to which is added 10–20% Glauber’s salt and 2–5% acetic acid (both calculated on the weight of the skins). The skins are entered at 20° C., then after three-quarters of an hour to 40° C., and then after another hour slowly to 50–55° C. For blacks, the temperature is raised as high as 65° C. After dyeing the skins are treated with a solution containing per 10 liters

90–120grams of olive-oil soap
12–25grams olive oil
12grams ammonia

for 15 minutes, then hydro-extracted and dried, without further rinsing.

For this method of dyeing, the following dyes may be used:

For yellow and orange

For reds

For violet

For blue

For green

For brown, combinations of

For black

For grey

The chrome colors are dyed on furs when very fast shades are desired, all the fancy colors being produced in this manner, but for black, only the acid dyes are suitable. The preparation of the skin is exactly the same as for the acid colors, except that the treatment with chloride of lime may be omitted, although for very full shades it is desirable. The dyeing is carried out as follows: The dyebath is prepared with the requisite amount of the desired dyestuff, which is previously dissolved, and to this is added a solution of sodium bichromate, the amount of this substance being half the weight of the dye. The solution is heated and the skins entered and dyed for 1–2 hours at 70–80° C. Then the dyebath is exhausted by the addition of 1⁄3% acetic acid, the skins being worked for another half hour, then rinsed, hydro-extracted and dried. Any of the one-bath chrome, or after-chrome colors may be used for this method.

Recently methods have been patented for the dyeing of furs by means of the vat colors. Vat dyes are among the fastest coloring matters ever produced, and their application on furs would be a great advantage, if suitable shades could be obtained. The general process for dyeing with vat colors, consists in reducing the dye, which is usually very insoluble, into a soluble “leuco” compound, by means of hydrosulphites in the presence of alkalies. The leuco compound is not a dye itself, but when the fibre absorbs it, and is then exposed to the air, the leuco compound is reoxidized to its original insoluble form, which remains fast and permanent. The use of strong alkalies in vat dyeing has hitherto been a great obstacle in the use of these dyestuffs, but in 1917, the Farbwerke Hoechst, a large German dye works, patented a process as follows: “A process for dyeing furs with vat colors. The dyeing is done in solutions of the vat dyes (after the addition of gelatine or some other protective colloid), which are rendered neutral or only slightly alkaline with ammonia, by neutralizing the caustic soda of the solution of the leuco compound of the vat dyes by the addition of ammonium salts, or suitable acids. The dyeings thus obtained are uniform and fast, the leather is dyed to only a slight degree, and shows no deleterious effects of the dyebath on the tannage.” As a practical application of this process, another patent was taken out by the same company, also in 1917, as follows: “A process for producing fast blacks on furs, consisting of dyeing a ground color with appropriate vat dyes in a hydrosulphite vat, and after oxidation in air, topping with an Aniline or Diphenyl black. The dyeings obtained by the combination of vat dyes which are fast to oxidizing agents, with an oxidation black, have an appearance matching that of logwood black in beauty; and with a dark-blue to blue-black under-color, and a full, deep black top color, cannot be distinguished from logwood. These dyeings also have the advantage of being faster to light than logwood or other blacks.”

While these processes undoubtedly have many meritorious qualities which make them interesting, they do not seem as yet, to have attained any great practical application. However, it is a field of fur dyeing which is worth while developing, and with certain necessary improvements in these processes, the vat dyes may yet supersede partially some of the other methods of dyeing furs.

CHAPTER XVII
BLEACHING OF FURS

Bleaching is for the purpose of lightening the color of furs, and is most generally applied to white-haired skins such as white fox, ermine, and occasionally white lambs of all kinds, and white bears. Among such furs, pelts of a naturally pure white tone are relatively scarce, while in the majority of cases the color ranges from a pale creamy white to a decidedly yellowish shade. Colors which vary from the pure white detract considerably from the attractiveness and consequent value of the fur, and indeed, some pelts are so far off shade that they can only be used when dyed a darker color. Most white skins which are but slightly inferior in color can be brought to a pure white by bleaching, and they can then be used natural. Some pelts, on the other hand, are particularly resistant to the action of bleaching agents and cannot be sufficiently decolorized to render them suitable for use natural, so these are also dyed. For the production of certain delicate or fancy dyed shades on white furs, it is often necessary to bleach the skins in order to be able to obtain pure tones. Such instances are not very common, however. Occasionally dark furs, such as beaver, are bleached on the tips of the hair, a golden shade being obtained thereby, which at one time was quite popular, but recently such effects have not been in vogue.

In the bleaching of furs, two steps may be distinguished, first degreasing, and second, bleaching proper. In the preliminary operations of fur dressing, the furs are treated with soap or weak alkalies to cleanse them and to remove excess oil from the hair. During the various processes and manipulations, the hair, especially on white skins, may become soiled or somewhat greasy again, so it is advisable to repeat the cleaning process. This should in every case be as light as possible, using a weak solution of soap for the softer and cleaner pelts, or dilute solutions of ammonium carbonate or soda ash for the more greasy-haired skins. The skins are then thoroughly rinsed to remove all traces of the degreasing material. This step is very essential in order to obtain uniform bleaching.

Broadly speaking, there are two general methods which can be used in bleaching furs, one involving the use of what are known as reducing agents, and the other employing oxidizing substances.

Among reducing agents which can be used for bleaching furs are sulphurous acid, and its salts such as sodium bisulphite and sodium sulphite; hydrosulphites, and derivatives.

1. Sulphurous acid.—When sulphur is burned, sulphur dioxide gas is formed. In the presence of moisture, or when dissolved in water, this gas forms sulphurous acid, which is one of the most commonly used bleaching chemicals for all sorts of materials, and is very effective for decolorizing furs. The procedure usually followed is to hang up the moistened skins on wooden rods in a more or less cubical chamber made of stone or brick, and lined with wood or lead. No other metals may be used, because they are quickly corroded by the sulphurous acid. The requisite quantity of sulphur is placed in a pot in the bleaching chamber, and then ignited, after which the doors are shut tight. The fumes of the burning sulphur in contact with the moist hair readily exert their bleaching action on the furs, and the operation is allowed to proceed for six or eight hours, or overnight. Then by means of fans or other devices, the air filled with sulphur dioxide gas is withdrawn from the chamber, and replaced by fresh air. The door is opened, the skins removed, exposed to the air for a time, then rinsed, and finally dried and finished. Sometimes one operation is not enough to sufficiently bleach the hair, so the process is repeated. Sulphur dioxide gas can now be obtained compressed in cylinders, which are more convenient to handle than burning sulphur. The flow of gas which is introduced into the bleaching chamber by means of a nozzle attached to the cylinder, can be regulated, and the bleaching thus retarded or accelerated.

2. Sodium bisulphite and sodium sulphite.—These salts of sulphurous acid are effective in their bleaching action only when in solution in the presence of acids. The acids liberate sulphurous acid from the salts, so this method is virtually the same as 1. Instead of using the salts of sulphurous acid, sulphur dioxide may be dissolved in water, and the solution used for bleaching by immersing the furs in it. This procedure, while consuming somewhat less time than the chamber process, is more likely to affect the leather, which would have to be retanned. The principle is the same as that involved in method 1.

3. Hydrosulphites and derivatives.—The bleaching agent can be prepared by adding zinc dust to commercial bisulphite of soda dissolved in about four times its weight of water until no more reaction is evident. Milk of lime is then added to precipitate the zinc, and the clear supernatant liquid of 1.5°–5° Tw. is used for bleaching. The skins are immersed for 12–24 hours, taken out, washed and finished. Instead of preparing the hydrosulphite, the commercial products may be used with greater convenience, a solution containing 1–4% of the hydrosulphite powder being used, and the skins treated in this until satisfactorily bleached.

The bleaching action of sulphurous acid and hydrosulphite is supposed to be due to the reduction of the coloring matter of the hair to a colorless compound; or possibly to the formation of a colorless compound of the bleaching material with the pigment. The former seems the more probable explanation, because the change is not a permanent one, the original natural color returning after a long exposure of the bleached fur to air and light. However, the results are sufficiently enduring to satisfy the requirements of the trade in the class of furs on which these methods of bleaching are used.

Bleaching chemicals with an oxidizing action generally used for decolorizing furs are hydrogen peroxide and peroxides; occasionally hypochlorites and permanganates are also used.

1. Hydrogen peroxide.—Hydrogen peroxide is usually employed for bleaching in the form of its 3% solution, to which is added about 20 cubic centimeters of ammonia per liter. The ammonia serves partially to neutralize the acid which commercial peroxide generally contains, and also to facilitate the bleaching action. The thoroughly degreased skins are immersed in the solution until the hair is completely wetted by it, are then removed, and evenly pressed or hydro-extracted, after which the pelts are hung up to dry in the air. As the hair becomes drier, the concentration of the peroxide becomes greater, and consequently the bleaching action is stronger. Where there is a likelihood of the leather being affected by the bleaching solution, the ammoniacal peroxide may be applied to the hair with a fine sponge or brush until sufficiently wetted, and then hanging the skins up to dry. Repetition of the process is sometimes necessary to obtain pure white, but the results are always excellent.

2. Peroxides.—The most important of these is sodium peroxide, which comes on the market as a yellowish-white powder, which must be kept dry, and away from any inflammable material, as fires have been caused by the contact of the peroxide with such substances. When dissolved in water, it is equivalent to a strongly alkaline solution of peroxide of hydrogen.

Na2O2+2H2O=H2O2+2NaOH
sodium
peroxide		 water peroxide
of
hydrogen		 caustic
soda

When dissolved in acid, the alkali is neutralized, and a neutral solution of peroxide of hydrogen and a salt is obtained, and this method is used to obtain peroxide of

Na2O2+H2SO4=H2O2+Na2SO4
sulfuric
acid		 sodium sulphate

hydrogen cheaply. 3 parts of sodium peroxide are slowly dissolved in a cold 1% solution of 4 parts of sulphuric acid, stirring during the addition, and making the resulting solution neutral to litmus paper, acid or more sodium peroxide being added as needed. There is then added 3–6 parts of a solution of silicate of soda of 90° Tw. The skins are immersed until properly bleached, taken out, passed through a weak acid solution, then washed and finished. This method generally requires the leather to be retanned after bleaching. Another process, which involves the use of peroxides, but which is not commonly practised, consists in rubbing the hair with a pasty mixture of equal parts of water, barium dioxide, and silicate of soda, hanging up the skins to dry, and then beating and brushing the hair.

3. Permanganates.—The only member of this group that finds practical application for bleaching purposes is potassium permanganate. The skins are immersed in a 0.1% solution of the crystals of potassium permanganate, until the hair acquires a deep brown color. They are then removed, rinsed, and entered into a second bath containing sulphurous acid in solution, prepared by acidifying a solution of sodium bisulphite. The skins are then worked in this until fully bleached. It is the permanganate which does the bleaching, the sulphurous acid being for the purpose of dissolving the brown compound of manganese formed on the hair.

4. Hypochlorites.—Chloride of lime and sodium hypochlorite, which is prepared from the former, are the chief chemicals of this type used for bleaching. The skins are entered into a weak solution of the hypochlorite, and left until the hair is decolorized; then after removing, they are passed through a dilute acid, and subsequently through a weak solution of sodium thiosulphate in order to remove all traces of the hypochlorite. This method causes the hair to acquire a harsh feel, and the yellow color is never entirely eliminated. The hair, however, possesses a great affinity for certain types of dyestuffs, and it is only when these particular classes of dyes are to be applied to the furs, that the hypochlorite bleach is used. (See dyeing with Acid colors).

The various oxidation methods of bleaching are supposed to change the coloring matter of the hair into an entirely different and colorless compound which cannot return to its original form. The bleach is therefore permanent.

In common practise, the sulphurous acid, and the peroxide of hydrogen methods are the two chiefly employed in bleaching processes. Sulphurous acid is used to bleach the cheaper kinds of furs, while peroxide of hydrogen is applied to the finer furs.

Whichever process is used, it is customary to give the bleached skins a subsequent “blueing,” by passing them through a very weak solution of a blue or violet dye, such as indigo-carmine, crystal violet, alkali blue or ultramarine. The furs are then dried and finished off as usual. In drum cleaning white furs, gypsum or white sand, or sometimes even talc are used with the sawdust, or occasionally alone without the sawdust.