FUR DYEING
“KILLING” THE FURS
If dressed furs are treated with a paste or solution of a dye properly
prepared, and at the right temperature, the hair will show very little
tendency to absorb the coloring matter. Even after prolonged treatment
with the dye, only a small amount will be taken up by the hair, and in a
very irregular fashion. Soft, woolly hair, like that of lambs and goats
will be colored more easily than that of furs with harder hair, and the
under-hair of a fur will generally have a greater affinity for the dye
than the harder and stiffer top-hair. Moreover, in some parts of the same
fur, the hair will absorb more color than in other parts. In other words,
the hair of furs resists the action of dye materials to a greater or less
degree, depending upon the character of the fur, and also upon the part
of the pelt. In order to overcome this resistance of the hair, and to
render it uniformly receptive to the coloring substances, the furs are
treated with certain chemical agents, the process being known technically
as “killing.”
The origin of the term is obscure, but it is interesting to note that
in the fur dyeing countries other than the United States and England,
the corresponding expression is used: in Germany, “töten,” and in France
“tuer.” The explanation of the process is as follows: The surface of the
hair is covered with a fine coat of fatty material which renders the hair
more or less impervious to dye solutions and solutions of other substances
which may be used for dyeing purposes. This fatty coating of the hair
cannot be removed by mechanical means, otherwise the hair would have
been freed of it during the dressing operations. Chemical solvents must
therefore be resorted to, and naturally alkaline materials are used, these
being usually cheapest and also most effective in their dissolving action
on fatty substances. Alcohol, ether, benzine, and other similar liquids
also serve as killing agents on furs, since they too, are fat solvents.
In all these cases, the fatty substance on the hair is dissolved away,
and the protective coat which previously rendered the hair impervious
to the dye, is now removed. There are certain chemicals however, which
normally do not dissolve substances of a fatty nature, but are strongly
oxidizing, such as peroxide of hydrogen, hypochlorites, permanganates,
perborates, nitric acid, etc., and exert a killing action when they are
applied to the hair, in that the hair is made capable of taking up the
dye from its solutions. In this case the killing can hardly be said to
be due to a degreasing process. The fact that killing can be brought
about with other substances than alkalies or fat solvents, has led to
the belief on the part of some investigators in this field that killing
is more than a degreasing operation, although the removal of the fatty
material of the hair undoubtedly takes place. Some authorities consider
that the killing process changes the pigment of the hair, which thereby
becomes more receptive to the dye. It is quite possible that some such
change in the structure of the hair fibre does take place, the surface
of the hair becoming slightly roughened, and therefore more capable of
fixing the coloring matter. The question is still an open one, and since
no conclusive researches have been made as yet, it will be assumed that
killing is simply a degreasing process, inasmuch as the modern practise
is based on this supposition, and very satisfactory results are obtained.
An account of the historical development of the killing process brings
out many interesting and enlightening facts, so it will be given here
briefly. One of the first substances used for killing, or degreasing the
hair of furs, was decomposing urine. Urine contains about 2% of urea which
gradually changes to salts of ammonia, and in the presence of the air,
largely to ammonium carbonate. This substance has a weak alkaline action,
but sufficiently effective to be used for killing the hair of certain
types of furs. Woolly furs, such as those derived from the various kinds
of sheep and goats, were degreased with stale urine, the skins being
washed in this, and then rinsed in water. The fat was emulsified by the
ammonium carbonate present, and could thus be easily removed. For other
furs, a stronger mixture was necessary. An example of a killing formula
used on wolf, skunk and raccoon, which were to be dyed black, is the
following:
350 grams beechwood ashes
200 grams unslaked lime
150 grams copper vitriol
100 grams litharge
60 grams salammoniac
40 grams crystallized verdigris
3.5 liters rain water
Beechwood ashes were a very important constituent of the old killing
formulas. The reason for that lies in the fact that beechwood contains
a comparatively high percentage of potassium, which occurs in the ashes
of the burned wood as potassium carbonate, or potash. The ashes alone
were frequently used, being applied in the form of a paste, which in
some instances had an advantage over a solution, in that the killing
could be limited to certain parts of the skin where it was more desired
than in other parts. By extracting the wood ashes with hot water, and
evaporating the clear solution to dryness, potash could be obtained, which
was considerably stronger than the original ashes. Next in importance
for the killing was unslaked lime. This substance was also often used by
itself, being first slaked with water, and using the milk of lime thus
formed, after cooling. Salammoniac, although a salt, and consequently
without any killing action, in contact with the beechwood ashes or the
lime in solution or paste, liberated ammonia slowly, and so also acted
as a degreasing agent. The other chemicals in the formula took no part
in the actual killing of the hair, but acted either as mordant materials
or as mineral dyes. The copper salts, in this mixture present in two
forms, as sulphate in copper vitriol, and as acetate in the verdigris,
were important constituents of the dye formula, being essential to the
production of the proper shade. These substances properly had no place
in the killing formula. The litharge, also was not a killing agent, but
in the presence of the alkaline materials of the killing mixture, it
gradually combined with the sulphur contained in the hair, forming lead
sulphide, and thereby darkening the color of the hair. In this case,
the metallic compound acted, not as a mordant, but as a mineral dye. The
mixture was applied to the hair by means of a brush, the skins let lie for
some time, then dried, brushed and beaten. Many applications were usually
necessary to sufficiently degrease the hair. Inasmuch as the killing paste
was prepared by mixing the constituents together, and then was brushed
on at the comparatively low temperatures which the proper protection of
the hair required, it is questionable whether some of the metal compounds
were even enabled to act as described above as mordant or dye. In spite of
the trouble and considerable time required in working with such a killing
formula to obtain the hair in the desired condition for dyeing, the use
of such a mixture nevertheless possessed the advantage that the hair was
only very slowly and gradually acted upon, and so the gloss was preserved.
The action of strong alkaline substances acting quickly is more or less
detrimental to keeping the gloss of the hair, while the slow action of
the weak alkaline paste of the old formulas, and the gradual formation
of a protective metal film on the surface of the hair, rendered the hair
suitably receptive to the dye which was subsequently applied, without in
any measure affecting the lustre of the hair.
It would be needless to describe or discuss any more of the old killing
formulas, for the principle involved was the same in all cases, there
being usually a slight variation in the content of metallic salts,
beechwood ashes and unslaked lime being constituents of the great majority
of the mixtures used. Modern killing processes employ substances quite
similar to those of the old formulas, the operations, however, being much
less laborious and less time-consuming, and the cheap, pure products
which chemical science has been able to develop being used in place of
the crude products crudely obtained from natural sources. The chemicals
used at the present time for killing furs, are chiefly ammonia, soda ash,
caustic soda, and caustic lime. The choice of the killing agent depends
upon the nature of the fur, the hair of some furs being sufficiently
killed by treatment with weak alkalies, while in other furs the hair may
require stronger treatment. The ability of the hair of a particular fur to
withstand the action of the different alkaline substances must be taken
into consideration, there being a great divergence in this regard among
the different classes of furs. Raccoon, for example, is not appreciably
affected by a solution of caustic soda of 5 degrees Beaumé, while some
wolf hair cannot withstand the action of a solution of soda ash of less
than 1 degree Beaumé. Frequently much stronger alkalies are necessary to
kill the top-hair than the under-hair, so this accomplished by treating
the skins in a solution which is suited to kill the under-hair, and
subsequently the top-hair is treated with a stronger solution, this being
applied by the brush method.
Uniformity of action of the killing material on all parts of the skin,
and on all the skins of a given lot, is absolutely essential to obtaining
satisfactory results in dyeing. And it is by no means a simple matter
to get such uniformity, considering the numerous factors that must be
taken into account. Any operation involving the immersion of the skins
in solutions or even in water alone, has an effect on the leather side of
the skin, inasmuch as some of the tanning materials may be extracted. The
application of some substance of a fatty nature to a great degree prevents
this, and the skin can be killed, mordanted and dyed, and then come out
soft and flexible. But the great majority of substances of a fatty nature
are affected by alkalies, and so when the skins are being killed, the
action of the alkaline materials would be upon the fat contained in the
leather as well as that upon the hair. As a result the hair may not be
sufficiently killed, and so give uneven dyeings subsequently. Either a
certain excess of the killing chemical must be used, and it would be very
difficult to ascertain what quantity would suffice, or the killing action
must be prolonged; but best of all, in oiling the skins, an inert mineral
oil should be used, since it is wholly unaffected by alkalies.
Skins may be killed by the brush process or the dip process, or by both.
For brush killing, the stronger alkalies like lime and caustic soda are
used, the solution being applied to the top-hair with a suitable brush,
and the skins allowed to remain hair to hair for the necessary length
of time, after which they are treated further as skins killed by the dip
process. By this latter process, the furs are immersed in a solution of
the desired killing agent in a vat, or drum, or other appropriate device
which will permit of uniform action of the alkali on the hair of all the
skins. After remaining in the solution the required length of time, the
skins are drained, and rinsed in fresh water, and then entered into a weak
solution of an acid in order to neutralize any remaining alkali, it being
easier to wash out acid than alkali. The furs are then washed thoroughly
in clear water, preferably running water, to remove the last traces of
acid. The skins are then drained and hydro-extracted, or pressed, and are
then ready for the subsequent operations of mordanting and dyeing.
KILLING WITH SODA
Soda is sodium carbonate, which is produced commercially in a very pure
state in several different forms, the chief being sal soda, which is
crystallized sodium carbonate, containing about 37% of actual soda; and
soda ash, or calcined soda, which is anhydrous sodium carbonate. The
latter is the variety most commonly used.
10 grams soda ash are dissolved in
1 liter of water at 25°–30° C.
The skins are immersed for 2–3 hours, after which they are rinsed and
treated with
10 grams acetic acid dissolved in
1 liter of water.
The skins are again thoroughly washed, and then hydro-extracted.
KILLING WITH LIME
Lime, calcium oxide, forms a white, amorphous, porous substance, which
readily takes up water, giving calcium hydroxide, or slaked lime. Only the
best grades of lime should be used, as it is very frequently contaminated
with calcium carbonate and other inert materials.
10 grams of lime are dissolved in
1 liter of water.
The skins are entered, and allowed to remain for a period of time which
varies according to the nature of the fur. During the killing, the
solution must be agitated, in order to evenly distribute the milk of lime,
which has a tendency to settle out. After rinsing, the skins are “soured,”
by treating with weak acetic acid solution, then thoroughly washed, and
drained.
KILLING WITH CAUSTIC SODA
Caustic soda is used only on furs the hair of which is very hard and
resistant to killing. Usually it is applied by the brush process, but in
some instances, the dip method must be used. In order to reduce as far
as possible, the action of the caustic soda on the leather, the weakest
permissible solutions are used, increasing the time of treatment, if
necessary. Caustic soda is a white, crystalline substance, occurring
in commerce in lumps, but more conveniently in a solution of 40 degrees
Beaumé, containing 35% of caustic soda. Various quantities, ranging from
4 to 25 grams of this solution per liter of water are taken, according to
the character of the fur, and the skins treated for 2–3 hours, although
weaker solutions may be used, and increasing the duration of the killing.
By keeping the solution in motion, by means of a stirrer or any other
method of agitation, the best results are obtained. After the skins are
sufficiently killed, they are soured, and washed as by the other killing
methods.
Where the nature of the hair of the fur is such that the top-hair and
the under-hair require different killing treatments, the skins are first
killed by the dip process, with an alkali suited to kill the under-hair,
then a brush killing with a stronger alkali is applied to the top-hair.
The subsequent treatments are the same as for usual dip-killing methods.