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.