FUR DYEING
OXIDATION COLORS
The year 1888 may be considered the beginning of a new era in the
history of fur dyeing; the commencement of a period which was to see
the time-honored, traditional methods of the masters of the art give way
to newer methods of an entirely different character; and moreover, the
initiation of an age when science with its basis of fact and logic, was
to undertake the rationalization of an industry which had hitherto worked
upon a more or less irrational, empirical and uncertain comprehension
of the fundamental principles involved. It was not the work of a single
day, or even of a year which brought about the virtual revolution in the
dyeing of furs, but the result of long, patient, systematic effort. About
this time, the German coal tar industry was attaining its real stride
along the path of progress and achievement, and had already succeeded in
reaching, to an appreciable degree at any rate, most users of coloring
matters, with the consequence that the natural dyes, with their time and
labor-consuming processes of application were gradually being superseded
by the new synthetic dyestuffs which could be simply and quickly applied.
It was now the turn of the fur dyeing industry to receive the attention of
the scientists and technologists responsible for the growth of the coal
tar dye industry, and so there appeared in the above-mentioned year, the
following patents, taken out by a German chemist named Erdmann:
D. R. P. 47349
A Process for Dyeing Hair and Feathers
If white hair or feathers are soaked in an aqueous or alcoholic solution
of para-phenylene-diamine, and then exposed to the slow oxidation of
the air, or are treated in a second solution with some oxidizing agent,
then the hair or feathers will be dyed. According to the oxidizing agent
chosen, and the concentration of the solution used, the color obtained
will be light or dark, varying from the palest blond to the deepest
blue-black. Particularly suitable as oxidizing substances are ferric
chloride, permanganates, chlorates, hypochlorites, bichromates, and
hydrogen peroxide. The dyeings are fast, that is, they do not come off,
and the color cannot be removed by washing. Following examples may serve
to make the process clear:
20 grams pure para-phenylene-diamine and 14 grams caustic soda are
dissolved in a liter of water. The hair, previously degreased, is soaked
thoroughly in this solution, and while moist is entered into a three per
cent solution of peroxide of hydrogen. The action is not instantaneous,
but after a day, the hair is dyed a dark shade; by repetition of these
operations a blue-black is obtained.
The para-phenylene-diamine can be replaced in this process by other
similar bases, such as dimethyl-para-phenylene-diamine, as well as the
naphthylene-diamines. Since the substances which can be applied by this
process are uninjurious, the method described can be used to dye human
hair on the head or beard, and so seems suited to replace for the dyeing
of hair, the metallic salts and various pyrogallic solutions which are on
the market, and which are harmful to the health.
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.
=========================================================================
| CHROME | COPPER | IRON | DIRECT
---------+---------------+----------------+--------------+---------------
Ursol D | brown black | coal black | coal black | dark brown to
| | | | brown black
Ursol P | dull red brown| dull dark brown| grey brown | light brown
Ursol 2G | yellow brown | dull yellow | yellow brown | dull yellow
| | brown | |
Ursol A | ... | ... | blue black | blue to blue-
| | | | black
Ursol 4G | light brown | medium brown | yellow | pure yellow
Ursol 4R | orange brown | light yellow | red brown | orange red
| | brown | |
Ursol | | | |
Grey B | greenish grey | greenish grey | mouse grey | ...
Ursol | | | |
Grey R | brownish grey | brownish grey | reddish 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:
=========================================================================
| CHROME | COPPER | IRON | DIRECT
---------+---------------+---------------+---------------+---------------
Rodol D | brown black | coal black | coal black | brownish black
Rodol P | red brown | dark brown | grey brown | light brown
Rodol 2G | yellow brown | yellow brown | yellow brown | dull yellow
Rodol 4G | light brown | light brown | reddish brown | pure yellow
Rodol A | ... | blue black | ... | blue black
Rodol | | | |
Grey B | greenish grey | greenish grey | mouse grey | ...
Rodol | | | |
Grey R | greenish grey | brownish grey | mouse 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 soda 2.5 gms.
Cream of tartar 1.5 gms.
Water 1 liter
Copper mordant.
Copper sulphate 2 gms.
(Acetic acid 50% 2 gms.)
Water 1 liter
Iron mordant.
Ferrous sulphate 2 gms.
(Acetic acid 50% 2 gms.)
Water 1 liter
or,
Iron pyrolignite 30% 10 gms.
Water 1 liter
Chrome-copper mordant.
Bichromate of soda 2 gms.
Copper sulphate 0.25 gms.
Cream of tartar 1.0 gms.
Water 1 liter
Copper-iron mordant.
Copper sulphate 2 gms.
Ferrous sulphate 2 gms.
(Acetic acid 50% 2 gms.)
Water 1 liter
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 soda 2 grams
Copper sulphate .25 grams
Cream of tartar 1 gram
Water 1 liter
for 24 hours. Then washed, and dyed for 24 hours with
Fur Brown 2G[3] 3 grams
Hydrogen peroxide 45 grams
Water 1 liter
Wash and dry the skins, then brush the tips with
Fur Brown D[3] 20 grams
Hydrogen peroxide 400 grams
Water 1 liter
[3] Inasmuch as most manufacturers use the same letters to designate
the various dyes, any equivalent brand of fur dye may be used in
place of those here mentioned.
_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 P 1.5 grams
Pyrogallic acid .7 grams
Ammonia 2.0 grams
Hydrogen peroxide 45 grams
Water 1 liter
The dyed skins are washed and dried, then tipped with
Rodol D 20 grams
Rodol DB 2 grams
Hydrogen peroxide 450 grams
Water 1 liter
_Brown on Thibet Sheep Skin_
The killed skins are mordanted for 6 hours with a chrome mordant, then
dyed for 6 hours with
Ursol P 1 gram
Pyrogallic acid 1 gram
Ammonia 2 grams
Hydrogen peroxide 40 grams
Water 1 liter
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 R 3 grams
Ammonia 2 grams
Peroxide of hydrogen 45 grams
Water 1 liter
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.