FUR DRESSING
TANNING METHODS
After the pelts have gone through the preliminary operations of softening,
washing and fleshing, they are ready to receive the treatment which
will convert the easily decomposing skin into leather of more or less
permanency, depending on the method used.
During the past century, considerable study has been made both by
scientific and technical people, of the problem of leather formation.
Numerous theories as to the nature of the process have been evolved, but
even to this day, no satisfactory explanation has been given which would
account for all the facts as they are now known, so the matter is still a
subject of considerable controversy. Procter, who is one of the leading
authorities on leather today discusses the development of the tanning
theories as follows:
“The cause of the horny nature of dried skin is that the gelatinous and
swollen fibres of which it is composed not merely stiffen on drying but
adhere to a homogeneous mass, as is evidenced by its translucence. If
in some way we can prevent the adhesion of the fibres while drying we
shall have made a step in the desired direction, and this will be the
more effective the more perfectly we have split the fibre-bundles into
their constituent fine fibrils, and removed the substance which cements
them. The separation of the fibres can be partially attained by purely
mechanical means.... Knapp, to whom we owe our first intelligible theories
of the tanning process, showed that by physical means the separation and
drying of the fibres could be so far effected as to produce without any
tanning agent a substance with all the outward characteristics of leather,
although on soaking it returned completely to the raw hide state. He
soaked the prepared pelt in absolute alcohol, which penetrated between,
and separated the fibres and at the same time dried them by its strong
affinity for water. More recently, Meunier has obtained a similar result
by the use of a concentrated solution of potassium carbonate which is even
more strongly dehydrating.
“Knapp made a further step by adding to his alcohol a small quantity
of stearic acid which, as the alcohol evaporated, left a thin fatty
covering on the fibres which completely prevented their adhesion, and
reduced their tendency to absorb water; and he so produced a very soft and
white leather. Somewhat similar are the principles of the many primitive
methods which apply fatty and albuminous matters, grease, butter, milk, or
brains to the wet skin, and by mechanical kneading and stretching, aided
by capillarity, work these matters in between the fibres as the water
evaporates. Such methods are still used, and enter into many processes in
which other tanning agents are also employed.
“Building upon these facts, Prof. Knapp advanced the theory that the
effect of all tanning processes was not to cause a change in the fibres
themselves, chemical or otherwise, but merely to isolate and coat them
with water-resisting materials which prevented their subsequent swelling
and adhesion. True as this theory undoubtedly is in many cases, it can
hardly be accepted as the whole truth, and it seems incontestable that
frequently the fibres themselves undergo actual chemical changes which
render them insoluble and nonadhesive.
“Before Knapp’s work, the prevalent theory, at least as regards vegetable
tannage, had been a chemical one, started by Sir Humphrey Davy. If a
solution of gelatine be mixed in proper proportion with one of tannin,
both unite to form a voluminous curdy precipitate; and, according to
Davy’s ideas, this was amorphous leather. Against this, it was urged
that even the supposed ‘tannate of gelatine’ itself could not be a
true chemical compound, since the proportions of its constituents were
considerably varied by changes in the strength of the solutions, or by
washing the precipitate with hot water; and further, that in chemical
compounds, the form was changed, and no trace of the original constituents
appeared in the compound; while in leather apart from some change of color
and properties, the original fibrous structure remained unaltered.
“This reasoning appears much less conclusive now than it did in Knapp’s
day. Against the last objection guncotton may be quoted as an instance
of profound chemical change with no alteration in outside appearance;
and it is recognized that, especially among complex organic substances,
chemical reactions are rarely complete, but that stable positions are
reached, so-called ‘equilibria,’ in which the proportion of changed and
unchanged substance is dependent on concentration or other conditions;
and that therefore such a precipitate might well be a mixture of gelatine
with a true gelatine tannate from which further portions of tannin might
be dissociated by water.
“With the clearing up of old difficulties, however, the conflict between
chemical and physical theories has, as is usually the case, merely
passed into a new phase. Years ago, it was shown by Linder and Picton and
others, that liquids could be obtained which were not really solutions
of ions or molecules, but merely suspensions like that of clay in water,
or butter-fat in milk; but so finely divided as to appear clear and
transparent, and pass through filters like true solutions. Later, by
means of the ultra-microscope their discrete particles have actually
been made visible, each of them consisting of many molecules of the
suspended substance. Nevertheless, these particles have many molecular
properties, possessing plus or minus electrical charges; behaving like
large ions under the influence of an electrical current; and mutually
precipitating and neutralizing each other when positive and negative
are brought together. Such solutions are called ‘colloid,’ and those of
gelatine and tannin are of the class, so that it is now often said that
the precipitation of gelatine by tannin, and the fixation of tannin by
gelatinous fibre are merely ‘colloidal’ and ‘physical,’ and not ‘chemical’
phenomena. Admitting the facts, the question still arises whether the
distinction between chemical and physical is not here one without a
difference; and whether between the purely ionized dilute solution of a
salt and the coarsely granular clay suspension there is any point where a
definite line of demarcation can be drawn. The writer inclines to the view
that there is not; and that ionic and colloidal combinations are extreme
cases of the same laws, both physical, and both chemical.”
There are several methods which are used in tanning furs, each having
its peculiar characteristics and qualities, and possessing individual
advantages and disadvantages. In order to be able to judge the merits of
the various processes, it is necessary to have a criterion which can serve
as a basis of reference. Fahrion, a recognized authority and investigator
in this field, gives a definition of leather which is usually accepted
as a standard for comparison. He says: “Leather is animal skin, which on
soaking in water and subsequent drying does not become hard and tinny, but
remains soft and flexible; which does not decay in the presence of cold
water; and which does not yield any gelatine on boiling with water.” While
the requirements set forth in this statement are essential for leather,
and a compliance with them would also be desirable for tanned furs, a
somewhat less rigorous standard of conditions to fulfil is satisfactory
for the general needs and purposes of furs. The chief qualities which
tanned furs must possess, with particular reference to the leather side of
the pelt, are retention of softness and flexibility after being moistened
by the furrier for manufacturing purposes, and subsequent drying; and
freedom from a tendency to decay during this operation and thereafter. If
the furs are to be dyed, the effect of the dyeing must also be considered,
and the tanning must be such as to enable the dyed furs to possess the
above qualities.
The most important tanning processes employed for furs are the following:
1. Salt-acid tan, or pickle.
2. Mineral tans.
3. Chamois tan.
4. Formaldehyde and similar tans.
5. Combination tans.
6. Vegetable tan.
1. SALT-ACID TAN, OR PICKLE
This is one of the most extensively used methods for tanning furs, and is
also very cheap and easily applied. A typical formula for this tan is the
following: A solution of salt is prepared containing about 10% of common
salt, sodium chloride, and to this is added 1/2–3/4 ounce of sulphuric
acid for each gallon of tanning liquor. The proportions may be varied
within certain limits, but the figures here given are those which have
proven successful in practise. The solution should be made in a wooden
or earthenware container, free from any metal, as it would be attacked
by the acid. The liquor is then applied to the flesh-side of the fleshed
skins by means of a brush, making sure to touch all parts of the pelt.
They are then placed in a pile and allowed to remain thus until tanned, an
operation which occupies a time ranging from a few hours to two or three
days depending on the thickness of the skins. When the corium has lost its
translucence and has become of a milky-white color throughout the entire
thickness of the skin, as can be seen by viewing a cross-section, the skin
may be considered tanned. In some instances, where the hair of the fur can
stand immersion without injury, the skins are entered into the pickling
solution and allowed to remain for 12 to 24 hours, which is generally a
sufficient time to tan them in this manner.
The acid of the pickle causes the skin to swell, the salt then penetrating
between the fibres of the corium, and at the same time reducing the
swelling of the skin. The acid also neutralizes the alkaline products
of decomposition which may form, while the salt acts as a deterrent to
the progress of the putrefactive processes. When the skin is dried after
tanning, and stretched and finished, a soft white leather is obtained
which is permanent as long as it is kept dry. It is the salt which causes
the fibres of the skin to be completely differentiated and thus prevents
their adhesion.
It is interesting to note that other acids besides sulphuric can be
used for the pickle, organic as well as mineral, formic acid in 1/4%
solution being especially effective and giving excellent results, but
is more expensive than the mineral acid. A method, which in principle
is identical with the pickle, but carried out in an entirely different
manner, is the lactic acid fermentation process, or “Schrot-beize” as it
is called in German. The procedure is in general as follows: “The fleshed
skins are placed on tables, flesh-side up, and covered with a layer of
bruised barley grains, or a mixture of 3 parts of wheat bran and 2 parts
of rye flour. Then the head, tail and legs are turned inward, and the
skins rolled up in little cushions, hair-side out, and placed in a vat.
When this is filled with the skins a solution of common salt is poured
over them, and they are allowed to remain thus in a moderately cool place
for 24 hours. After this time, the skins are carefully unrolled, so as
not to remove any of the adhering solid materials, and turning the skins
hair-side inward, they are laid flat together in pairs and placed in an
empty vat. After another 24 hours they are again unpacked and replaced in
another vat, care being taken each time to keep all the solid particles
adhering to the flesh-side. This operation is continued and repeated until
the skins are properly tanned, which takes from 10 to 14 days, depending
on the weather and the temperature. The skins are then removed, rinsed
free of the tanning substances, pressed, dried and finished.” A somewhat
modified form of this process is the so-called Russian tan, which is
usually done in the following manner: 5 parts of bruised barley grains
are mixed with ten parts of luke-warm water in a vat, which is then
covered up. A small quantity of brewers’ yeast is also added to aid in the
fermentation. As soon as the mixture develops a slight heat, one part of
fresh whey is added, and the fleshed skins entered into the tanning liquor
in which they remain for about 12 hours. They are then tramped in the
mixture so as to effect greater penetration, and left until the tanning
process is complete. Whey is the milk fluid left after the casein and most
of the fat have been removed from the milk by coagulation, and consists
practically of a solution of all the milk-sugar or lactose, and the lactic
acid of the milk, together with a small percentage of mineral salts, and
a slight amount of fat. By fermentation, the milk-sugar is converted into
lactic acid, which helps to effect the tan by swelling the skin.
The effectiveness of the fermentation processes depends to a considerable
degree on the action of certain bacteria and yeasts. Bacteria are
one-celled organisms belonging to the vegetable kingdom, and some are so
small as to be scarcely visible under a microscope, while some indeed
cannot be seen by any means, their existence being inferred from their
effects. As they vary in size, bacteria also vary in shape, some being
spherical, others in the form of long, thin rods, while still others are
of a spiral shape; another common form is the dumb-bell shaped bacterium.
Some types are provided with what are known as flagella, which resemble
fine hairs attached to the body of the organism, and which enable it to
move about actively in liquids. The food of bacteria is always in liquid
form, as only in this condition can it be absorbed. However, some kinds of
bacteria attack solid substances from which they obtain their nourishment,
but this is done in an indirect way, by secreting certain fluids known as
enzymes, which dissolve or digest the material and convert it into a form
that can easily be absorbed by the bacteria. The enzymes are non-living
chemical substances, which possess the peculiar property of bringing
about the chemical change of an almost indefinite amount of material
upon which they act, without themselves being in any way changed. Yeasts
also act in a manner similar to the bacteria in causing various chemical
changes, particularly inducing fementations. In the simple “Schrot-beize,”
the starch contained in the bran or barley grains is first converted to
a soluble sugar by means of enzymes secreted by the bacteria which are
always present. This sugar then undergoes an acid fermentation, with
the formation of lactic and acetic acids, due in this case to organisms
known as the _bacterium furfuris A_ and _B_. The action of the Russian
tan is similar, but quicker. In this case, the sugar is already present
in soluble form, and the yeast cells cause its fermentation with the
production of lactic acid. In both cases, the acids as they form swell
and loosen up the skin fibres slowly, the salt penetrating between them,
and keeping them separated on drying. Both methods give results which
are equally good, but by the Russian tan the skins acquire a disagreeable
odor, which makes this method of dressing objectionable.
The lactic acid fermentation processes have an advantage over the pickle,
in that the slow formation of weak organic acids with their gradual action
produce a softer leather, with a gentler ‘feel,’ the presence of the flour
and the grains of the tan, aside from their tanning action, contributing
to the fullness and softness of the leather. There is also less likelihood
of the leather being subsequently affected by the presence of the acid
in it, as lactic and acetic acids are much less injurious than sulphuric
acid to leather. These disadvantages of the pickle can to a large degree,
be overcome without any great difficulty. On the other hand, the matter
of the length of time of the tanning process, shows the acid pickle at
a great advantage, and so, especially for furs other than those obtained
from sheep and goats, the pickle is in most cases used as the principle
method of tanning. In Austria, Russia, and to a certain extent in Germany
also, the “Schrot-beize” is still considerably employed, chiefly for
dressing sheep and lamb skins. The dressing of the various kinds of
Persian lambs, caraculs, astrachans, etc., in the native center of the
industry in Buchara and surrounding districts, is also a “Schrot-beize,”
barley, rice flour or rye flour, and salt water being used to prepare the
skins, the manipulations being essentially the same as those described
above, although carried out in cruder and more primitive fashion.
2. MINERAL TANS
The basis of the tanning of furs by means of solutions of mineral
compounds is the fact that the basic salts of certain metals are capable
of producing leather. It has been found that compounds of aluminum such as
alum or aluminum sulphate, or any other soluble neutral salt of aluminum,
possess tanning powers. Other metals which are capable of forming salts
of the same type are also endowed with the quality of converting skin
to leather under suitable conditions, chromium and iron being the most
important metals in this connection. Chemically these metals all belong
to the same group, and have properties which are very similar in many
respects, the characteristic of most importance for tanning purposes
being the quality of forming soluble basic salts by the addition of
alkalies or alkaline carbonates to solutions of their neutral salts, or
in certain instances simply by the action of water upon these neutral
salts. By neutral salts are meant those in which the metallic content is
combined with the normal proportion of acid; while basic salts are those
in which the acidic portion is present in less than the normal ratio,
being partially replaced by a hydroxide group. When the acid part of
the salt has been entirely replaced in this way, the compound is called
a hydroxide or hydrate of the metal. Between the neutral salt and the
hydroxide several different basic salts are possible, some being soluble,
while others are insoluble. If into a solution containing a basic salt of
either aluminum, iron or chromium a skin be entered, a part of the basic
salt will be precipitated on it in insoluble form. Inasmuch as neutral
salts of these metals when dissolved in water split up to a small degree,
into free acid and soluble basic salt, a skin immersed in such a solution
will also absorb the basic salt in an insoluble form. Upon these facts in
general, depends the action of the mineral tans used in tanning furs.
A. _Alum Tan_
The alum tan is one of the oldest methods of producing leather, being
employed by the Romans about two thousand years ago, and it is believed,
by the Egyptians at a much earlier period. Its extensive use in Europe,
however, dates from the time of the conquest of Spain by the Moors, who
introduced the process.
At the present time, rabbits and moles are tanned by this process, as are
also at times other furs such as muskrats, squirrels, sables, martens,
etc., when a better tan is desired than that produced by the pickle.
Ordinary alum, which is a double sulphate of aluminum and potassium, and
aluminum sulphate are the chief compounds used for this tan. In recent
years, the aluminum sulphate has to a considerable degree replaced the
alum for tanning, inasmuch as it can be cheaply obtained in a sufficiently
pure form, and contains about one and one-half times as much active
aluminum compound as does alum.
While the aluminum salt can be used alone for tanning, it produces a
stiff, imperfect leather, so salt is always added. The ratio of the salt
to the aluminum sulphate or alum can vary within rather wide limits, the
mixtures used in practise ranging from one part of salt to four parts
of the aluminum compound, up to equal parts of both, or even in some
formulas, a greater proportion of salt than of the other constituent.
Ratios which are most common are four of alum to three of salt, or two of
alum to one of salt.
When aluminum sulphate is dissolved in water, a small part of it splits
up into a soluble basic salt and an equivalent amount of free acid. The
reaction may be shown as follows:
Al₂(SO₄)₃ + 2H₂O = Al₂(SO₄)₂(OH)₂ + H₂SO₄
aluminum water basic aluminum sulphuric
sulphate sulphate acid
When a skin is entered into such a solution, the free acid is absorbed,
causing a swelling of the pelt. While this is taking place, a further
quantity of the neutral aluminum salt splits up into more basic salt and
free acid. At the same time the basic aluminum sulphate is also taken
up by the skin, probably attaching itself to some of the acidic groups
contained in the skin substance, in a manner analogous to the combination
of the acid with the basic groups of the skin substance. A point is
reached, however, when the skin is no longer able to take up more of the
basic salt, for the presence of the acid undoubtedly acts as a deterrent.
The skin, if dried after such a treatment contains a small amount of
aluminum, which is insufficient to tan the pelt properly, and as a result
this comes out in an undesirable and quite useless condition. If to the
solution of the aluminum sulphate salt is added, a different result is
obtained. To a certain extent the salt acts here as in the pickle. The
skin on absorbing the free acid of the solution naturally swells, but the
salt reduces this swelling, and at the same time, by penetrating between
the fibres and dehydrating them, produces a leather as in the pickle.
In addition, the presence of the salt enables a greater amount of basic
aluminum sulphate to be formed, and thus a greater quantity is taken up
by the skin. On drying and stretching after such a treatment, a soft,
flexible and stretchable leather is obtained.
The number of formulas for tanning furs by this process is legion, the
principle being the same in every instance, and mixtures of salt and alum
or aluminum sulphate form the basis of the various tans. Following are a
few typical formulas, which have been found to be of practical value:
A solution is prepared by dissolving 7.5 lbs. of alum and 3 lbs. of
common salt in 20 gallons of water. When cool, the clean, fleshed skins
are entered, being paddled or drummed for a short time and then allowed
to remain until tanned. By this method the hair also takes up some of the
alum, and if the skins are to be dyed, unevenness may result. In order to
avoid this, the tanning may be effected by brushing a stronger solution
on the pelt. A mixture of 4 lbs. of alum and 3 lbs. of salt, dissolved
in 8 gallons of water, and made into a paste by the addition of 4 lbs. of
flour, is applied to the flesh-side of the skins. These are then placed in
pairs, flesh-side together, and allowed to remain in a pile until tanned.
Sometimes a second application is given. The flour may be omitted, but it
serves to cause the tanning mixture to adhere better to the skins.
Still another method is the following: Into the flesh of the moist,
fleshed skins is rubbed a mixture of two parts of dry powdered alum with
one part of salt. After allowing time for it to be absorbed, another
application is given, rubbing in well, and especially treating the thick
parts. The pelts are then folded up, or rolled together, flesh-side in,
and placed in a vat or tub, which is covered up to prevent drying. They
are left so until tanned, as shown by examination and test. They are then
rinsed, hydro-extracted and dried, and after stretching and finishing, a
soft, white, pliable leather is obtained.
B. _Chrome Tan_
By using chrome alum instead of ordinary alum, together with salt, skins
can be tanned, but the leather formed is not altogether satisfactory.
The basic principle here is the same as in the alum tan, depending on the
formation of soluble basic chrome sulphates in the solution of a neutral
sulphate. The method employed at the present time, the so-called one-bath
process as distinct from the two-bath process, which cannot be applied
for tanning furs, involves the production of the basic chrome sulphate by
the addition of an alkali or an alkaline carbonate to the solution of the
neutral salt. It was Prof. Knapp who first published this process as early
as 1858; but it was not until 1893 that it was shown to be of practical
value, and was then patented in this country by Martin Dennis. Since that
time it has been in general use with but slight modifications.
The chrome tan is used only to a limited extent in the tanning of furs,
the method requiring very careful treatment and accurate supervision
during the various stages of the process, and the leather coming out
colored a pale-blue-green tint, which for some purposes is objectionable.
In some plants ponies and rabbits are tanned with chrome; and when the
skins are to be dyed by means of certain coal tar dyes, they have to
receive a chrome tannage. The leather produced by a chrome tan is very
durable, and possesses great resistance to the action of water.
Any salt of chromium, with either mineral or organic acids, can be used,
but chrome alum is the one most commonly employed. If a skin is entered
directly into a solution of a chrome salt made basic with an alkali, the
precipitation of the insoluble basic salt will take place very rapidly,
and the tanning will be only superficial. The procedure is therefore
first to treat the skins with a chrome solution which forms only small
quantities of the basic salt. After the skins are impregnated with the
solution, this is made basic, so that the real tan will take place within
the skin tissues among the fibres of the corium. A common formula is the
following: 5 lbs. of chrome alum are dissolved in 10 gallons of water.
The skins are entered into the solution at about 70° F. and paddled for
about 2 hours, or drummed for one hour. Then a solution of three pounds
of washing soda is added slowly to the liquor which is then stirred up
well, and the skins drummed or paddled again for an hour or two, and then
left in the liquor for 12 to 24 hours till completely tanned. The skins
are rinsed, and washed in 1/2% solution containing 2/3% of the weight
of the skins of borax. The pelts are then well washed in clean water,
hydro-extracted and dried.
C. _Iron Tan_
Tanning by means of iron salts has thus far been merely a matter of
scientific interest and has not found any practical use. The principle
involved is identical with that of the preceding mineral processes.
3. CHAMOIS TAN
The chamois dressing, as previously noted, is undoubtedly the oldest
method of preparing leather from skin, the various fat-containing
substances derived from animals, fish, birds, etc., being used for the
purpose. The chief object of the fat was to coat the fibres of the skin,
thus preventing their adhesion, and at the same time rendering them
resistant to water. In the true chamois tan, the fat seems to have also
a chemical function in contradistinction to the other which is merely
physical or mechanical. For, if skins tanned by the chamois process
be treated with a weak solution of an alkali, all the fatty materials
should be removed thereby, but this happens only to a small extent, the
pelt retaining its softness and pliability, and the other characteristic
qualities of leather, indicating that the fat is combined intimately with
the skin substance in a permanent fashion.
In tanning furs, various oils and fats are used, but not all are capable
of producing a chamois tan. Among the fatty materials are mineral oils,
and vegetable and animal oils and fats. Mineral oils are the distillation
products of petroleum, partially liquid, and partially solid. Being
inert substances, they have no tanning effect, but serve merely as
water-proofing or fattening materials. Except for their oily nature they
have nothing in common with fats, being quite unaffected by solutions of
alkalies or of acids.
Vegetable and animal fats and oils are, when pure, neutral substances
formed by the combination of fatty acids with glycerine. They possess
the property of saponification, that is, of forming a soap when treated
with an alkali, the soap being the alkaline salt of the fatty acid.
Under certain conditions, the fat can be split up into free fatty acid
and glycerine by the action of acids, or even water alone. Some fats
on long standing, split up in this way spontaneously in the presence of
moist air. As a general rule, those fats which exhibit this property to a
marked degree are affected by contact with the air, due to the absorption
of oxygen which reacts chemically with the fats, forming what are known
as oxy-fatty acids, usually less soluble, and having a higher melting
point than the original fats. Vegetable and animal fatty materials are
classified on the basis of this phenomenon of absorbing oxygen from the
air, those possessing this quality to a great degree being called “drying
oils,” others being “partially drying,” or “non-drying.” Olive oil,
castor oil, cocoanut oil and cottonseed oil are examples of non-drying
or partially-drying vegetable oils, linseed oil being the most important
drying-oil in this class. Tallow, lard, butter-fat, neats-foot oil are
non-drying animal fats, the drying oils being seal oil, whale oil, and
cod-liver oil.
[Illustration: FIG. 8. TRAMPING MACHINE OR “KICKER.”
(_F. Blattner, Brooklyn, New York._)]
For tanning purposes, this property of absorbing oxygen is important,
because only with drying oils can a true chamois tan be obtained,
non-drying oils acting like mineral oils only as water-proofing materials.
The details of the chamois process are not quite clear, there being
considerable difference of opinion on the matter. But all the studies
on the subject tend to prove that there are at least two phases to the
process: first, the mechanical covering of the fibres with the fat, this
property being common to all fats or oils which may be used; and second,
the combination of the fat with the skin in some chemical way, as a result
of the oxidation of the fat, a characteristic found only in the drying
oils. During the oxidation of the fats, the glycerine in them is converted
to acrolein or acryl-aldehyde, which also aids the tanning. It was at one
time supposed that the tanning action was due to this aldehyde alone,
but a chamois tan can be made with fatty substances from which all the
glycerine has been removed. The evidence on this question, however, is
not quite conclusive.
In general, the procedure of the chamois tan is as follows: The
hydro-extracted, fleshed skins are rubbed on the flesh-side with a good
quality of seal-oil. They are then folded up, and put into a ‘kicker,’
where they are tramped for two or three hours to work in the oil. The
kicker is a machine such as shown in Fig. 8 consisting of a receptacle for
the skins, and two wooden hammers which work up and down mechanically,
turning and pounding the skins. (As many as 1000–1500 skins of the size
of musk-rats can be worked at one time in such a machine.) The pelts are
then taken out and hung up in a warm room for several hours, considerable
oxidation taking place. Another coat of oil is then applied, which is
again tramped in, and the skins are hung up once more and exposed to the
air to cause the oil to oxidize. After the skins are sufficiently tanned
they are rinsed in a weak soda solution to remove the excess oil, washed
and dried. When skins with fine hair such as marten, sable, mink, etc.,
receive a chamois tan, they are not tramped in kickers as the delicate
top-hair will be broken, and the value of the skin thereby reduced.
Instead they are placed in small drums, together with metal balls of
varying sizes and weights depending on the particular fur treated, and the
oil is worked in by rotating the drum. Such a ball-drum, as it is called,
is shown in Fig. 9.
[Illustration: FIG. 9. BALL DRUM.
(_F. Blattner, Brooklyn, New York._)]
In conjunction with the chamois tan may be discussed the process of
oiling, inasmuch as the method of application and the effect are both
similar to the chamois tan up to a certain point. It is customary to
treat skins tanned by any other method but the chamois process, with
some oil in order to render them more impervious to water. The greatest
variety of oils and fats can be used, the action in most cases being
simply the mechanical isolation of the skin fibres by such a substance,
thus corresponding to the first or physical phase of the chamois tan.
The chemical phase, if it takes place at all, is usually slight, and
is merely incidental. Oiling is generally applied either before drying
after tanning, or after drying, the oiled skins being placed in a kicker
and tramped to cause the oil to penetrate. In some instances the oiling
material is put in the same mixture as the tanning chemicals, and the
tanning and oiling are effected simultaneously.
Among fatty substances used for oiling are mineral oils, such as paraffine
oil, and vaseline; animal fats, like train oils, butter, egg-yolk,
glycerine, neats-foot oil; vegetable oils, like olive oil, castor oil,
cottonseed oil; also sulphonated castor oil and sulphonated neats-foot
oil. These may be used singly or in various mixtures, an emulsion of an
oil and a soft soap also being frequently employed.
4. FORMALDEHYDE TANS
Formaldehyde has proven to be of great value in the tanning of furs,
usually in conjunction with other processes. Formaldehyde is a gas with
a strong, irritating odor, and its 40% solution, which is the customary
commercial form, also possesses this quality. When skins are treated for
several hours with a very dilute solution of the commercial product, a
leather is obtained which combines the properties of the alum tan and
the chamois tan. Moreover, in the majority of observed cases, where furs
have been tanned with formaldehyde, the skins seem to acquire a certain
immunity to the attacks of vermin and moths. Although the skins do
not in any way retain the odor of the formaldehyde, nevertheless these
destructive agents seem to be repelled.
Numerous processes have been devised which use formaldehyde in connection
with other substances for tanning. Thus in a German patent is described
a method involving the alternate or simultaneous treatment of pelts
with solutions of formaldehyde and alpha or beta naphthol. Both the
formaldehyde and the naphthol exercise tanning actions, but the process
is not used in practise.
In 1911, Stiasny, a well-known leather chemist produced a synthetic
substance by the condensation of formaldehyde with a sulphonated phenol,
forming an artificial tannin. This chemical, called “Neradol D,” exhibits
many of the properties characteristic of true tannins, although in no way
related by structure and composition. By the use of “Neradol D” a soft,
white and flexible leather is obtained, and it is therefore a suitable
tanning material for furs.
5. COMBINATION TANS
In many instances more than one method is employed in tanning the furs,
and in this way what is known as a combination tan is produced. While the
various individual processes described give more or less satisfactory
results by themselves, they generally possess some features, which
for certain purposes may be undesirable, and which can be eliminated
or considerably reduced by using other processes at the same time or
subsequently. Some of the combination methods are, pickle with chrome
tan, alum tan with chrome tan, and formaldehyde tan with pickle, mineral
tan or chamois tan. By means of such combinations various qualities of
tanned furs can be obtained, and if it is desired to produce a pelt having
certain special characteristics, this can be brought about by combining
two or more standard methods.
Some illustrations of combined tannages are the following: Alum-chrome
tan. The skins are tanned by the regular alum process, then the
constituents of the chrome tan are dissolved directly in the same bath,
and the chrome tan is effected as usual. Chrome-formaldehyde tan. To the
regular chrome tan solution is added 1/2 lb. of formaldehyde for every 10
gallons of chrome liquor. The rest of the process is as ordinarily.
6. VEGETABLE TANS
In practise, the vegetable tanning matters are not used for furs, although
in some special instances gambier cutch may be employed occasionally
with some other tan. However, many of these tannins also have dyeing
properties, and are used in dyeing the furs. In this connection it must
be mentioned that furs dyed with these materials also receive a vegetable
tan, which improves the quality of the leather to a considerable extent.
Comparison of The Various Tanning Methods
In choosing a method for tanning any particular kind of fur, several
factors must be considered. The nature of the pelt, insofar as it is weak
or strong; the time, labor and cost of materials required by the tanning
process; the effect on the leather of the different dyes and chemicals
used in dyeing, if the skin is to be dyed, are a few of the points
requiring attention and consideration.
For furs which are only to be dressed, a simple tan like the pickle will
suffice in most cases. Special instances, such as the rabbit and mole
already mentioned, and a few other furs are tanned by the alum method. The
pickle is undoubtedly the cheapest and simplest method of tanning skins,
and yields a soft, white leather which is permanent as long as it is kept
dry. If it is put into water, about 25% of the salt contained within
the pelt dissolves out, and the acid present swells up the tissues. If
the skin is dried in this condition, it will come out hard and brittle,
tending to crack very easily. By treating the leather before drying with a
strong salt solution, a good deal of the extracted salt will be replaced,
and on drying and stretching, it will work out soft. Skins tanned by
the “Schrot-beize” are affected by water in quite the same manner as the
pickled skins.
The alum tan gives a leather similar to that produced by the pickle, but
with the advantage that the skins possess greater stretch and flexibility.
In its resistance to water, the alum-tanned pelt is quite as susceptible
as the other. As a general rule, the skin absorbs about 6% of its weight
of alum from the tanning solution, but gives up three-quarters of this
when it is soaked in water, producing on drying, a hard, stiff leather.
The chrome tan is especially impervious to water, easily resisting
temperatures of 80° C., and even boiling water. It is employed to only
a limited extent on account of the special effort and care required
to obtain satisfactory results, also because the pelt acquires a pale
blue-green color which is not desired on dressed skins. The chamois
tan, and some of the combinations of the formaldehyde tan with the other
methods, give very soft, flexible leathers which possess a sufficiently
great resistance to the effects of water and heat.
In tests made to determine the best working temperatures for dyeing
skins dressed by the salt-acid tan, and for skins dressed by the chamois
process, some very interesting facts were brought out. These two tans
were chosen because they represent opposite extremes, the salt-acid tan
usually giving the poorest results, and the chamois tan giving the best
results in practise in dyeing. Other methods, except the chrome, range
between these two. The procedure in these experiments was to treat the
skins at ordinary temperatures in water, or dilute solutions of the
various chemicals and dyes usually employed in dyeing, and then heat these
solutions until the leather just began to shrink and shrivel up. This
point, called the shrinking point (S.P.), gave the temperature to which
the skins could be subjected in the given solution without danger to the
pelt. (The experiments and observations were made by Erich Schlottauer,
while director of a large German fur dressing and dyeing plant).
The first observation made was that different furs tanned by the same
process were affected differently in the same solutions. Thus in plain
water, three furs, all tanned by the acid-salt tan, had shrinking points
varying by several degrees; similarly with two different furs tanned by
the chamois process, there was a variation in the shrinking point of two
degrees. The explanation of this discrepancy among the different skins may
be that there was a slight difference in the conditions under which they
were tanned, experiments showing that a maximum difference of 4° C. may
exist among skins tanned by the same process, but not under the same or
identical circumstances. Another reason for the variation may be the fact,
that some skins are more greasy than others, and are thus more resistant
to the effects of water or of some chemicals. The furs with the higher
shrinking points in water were those which naturally are more greasy than
the others.
Weak solutions of acids tend slightly to lower the shrinking point, while
weak solutions of alkalies appreciably raise it, in both chamois-tanned
and salt-acid-tanned skins. Solutions of dyes and mordants as a general
rule increase the resistance of the skin to heat, varying quantities of
these substances having no, or little different effects on the shrinking
points. Previous treatment of the leather with some oil considerably
raises the shrinking point of the pelt. Formaldehyde effects a great
increase of the resistance of the skins to heat, especially with
chamois-tanned furs. The experiments in this case were made by first
treating the skins in the weak formaldehyde solution, and then determining
the shrinking point in plain water.
Two skins, both dressed by the “Schrot-beize,” a Persian lamb and an
astrachan, after dyeing had shrinking points almost 10 degrees higher than
when undyed. The extra tannage which the skins received from the tannins
used in the dye mixtures for these furs, accounts for this increased
resistance to heat.
The following tables give the observed figures in the different
experiments:
TABLE I
+-------------------+------+------+------+
| | A | B | C |
| | S.P. | S.P. | S.P. |
+-------------------+------+------+------+
|_Salt-acid Tan_ | C. | C. | C. |
|Australian Opossum | 46° | 58° | 45° |
|Marmot | 45° | 50° | 42° |
|Skunk | 47° | 56° | 43° |
| | | | |
|_Chamois Tan_ | | | |
|Mink | 52° | 61° | 45° |
|Muskrat | 50° | 58° | 42° |
+-------------------+------+------+------+
A--Water
B--Water plus 1% Ammonia (s.g. 0.910)
C--Water plus 1% Sulphuric acid (66°
Beaumé)
TABLE II
+-------------------+------+------+------+
| | A | B | C |
| | S.P. | S.P. | S.P. |
+-------------------+------+------+------+
|_Salt-acid Tan_ | C. | C. | C. |
|Australian Opossum | 53° | 52° | 54° |
| | | | |
|_Chamois Tan_ | | | |
|Mink | 59° | 59° | 59° |
+-------------------+------+------+------+
A--1000 c.c. water plus 40 c.c.
Peroxide plus 5 c.c. ammonia
B--500 c.c. water plus 2 grams Ursol D
(Para-phenylene-diamine)
C--500 c.c. water plus 5 grams Ursol D
TABLE III
+-------------------+------+------+------+------+
| | A | B | C | D |
| | S.P. | S.P. | S.P. | S.P. |
+-------------------+------+------+------+------+
|_Salt-acid Tan_ | C. | C. | C. | C. |
|Australian Opossum | 51° | 51° | 53° | 56° |
| | | | | |
|_Chamois_ | | | | |
|Mink | 59° | 59° | 61° | 62° |
+-------------------+------+------+------+------+
A--500 c.c. water plus 5 grams ground
nut-galls
B--300 c.c. water plus 2 grams pyrogallic
acid
C--500 c.c. water plus 2 grams potassium
bichromate
D--Water, after treating leather with
rapeseed oil
TABLE IV
+-------------------+------+------+------+------+
| | A | B | C | D |
| | S.P. | S.P. | S.P. | S.P. |
+-------------------+------+------+------+------+
|_Salt-acid Tan_ | C. | C. | C. | C. |
|Australian Opossum | 49° | 49° | 55° | 50° |
| | | | | |
|_Chamois Tan_ | | | | |
|Mink | 59° | 67° | 69° | 70° |
+-------------------+------+------+------+------+
A--500 c.c. water plus 5 c.c. formaldehyde
for 1 hour
B--500 c.c. water plus 5 c.c. formaldehyde
for 12 hours
C--500 c.c. water plus 10 c.c. formaldehyde
for 3 hours
D--As in C, but treated with 500 c.c. water
plus 5 c.c. ammonia, instead of water
alone.
TABLE V
+-------------+------+------+
| | A | B |
| | S.P. | S.P. |
+-------------+------+------+
| | C. | C. |
|Persian Lamb | 44° | 54° |
|Astrachan | 47° | 55° |
+-------------+------+------+
A--Before dyeing
B--After dyeing
As a result of these experiments it may be concluded that the maximum
temperature for drying salt-acid tanned skins should be 40° C., while
for chamois tanned skins the temperature may be permitted to reach 45° C.
without any danger of the leather being affected. Moreover, in the case
of pickled skins, the matter of extraction of the tanning agent, as well
as that of the leather becoming “burned” may be effectively counteracted
by brushing some oil or fat on to the leather side before dyeing the pelt.
The shrinking points of skins dressed by the various tanning methods are
constant within certain limits, depending on the nature of the skin and on
the conditions of tanning, and it is possible by observing the shrinking
point, in conjunction with other characteristics of a given pelt, to
determine what method of tanning was used.