TANNING MATERIALS
Tanning materials are derived from the vegetable, mineral, and animal
kingdoms.
The vegetable materials used are woods, barks, shrubs, leaves, and
fruits, either in their natural state or in the form of extracts. The
majority of the minerals have a more or less tanning effect on animal
fibres, but the principal are basic chrome salts, formaldehyde, alum
and salt. Titanium, iron, cerium and potassium salts also convert
skins into leather, but are not yet used commercially.
The animal matters that will convert skins into leather consist of
oxidised oils (chamois leather), fats, and brains (crown, Helvetia,
or Preller's leather).
Each of these classes of tanning materials has characteristic effects
which render them easily distinguishable. The use of combinations
of vegetable and mineral tannins has lately increased, and it is
possible that the blending of the two classes of materials may
produce an ideal tannage for certain classes of leather. In fact,
this result is already claimed for a chemically combined tanning
material which, according to the American patent, is prepared by the
following method: 125 lb. of solid quebracho extract is dissolved
in the same weight of hot water and allowed to cool; 16 lb. of
commercial caustic soda dissolved in two or three times its weight of
water is added, and the mixture agitated about half an hour; 150 lb.
of chromium sulphate is then added. In this way an insoluble tannate
of chrome is produced, but, on boiling and agitating, it changes
to a greenish brown colour and forms a sulphotannate of chrome. The
combination of the alum tannage and gambier (a vegetable extract)
has been used successfully for years past. Another combination which
has given good practical results is the tannage with alum and chrome
salts in the manufacture of glove leather.
The vegetable materials containing tannin should be arranged
botanically, but the following classification is simpler for
practical purposes.
NATURAL TANNING MATERIALS
1. _Barks._--Oak, Hemlock, Pine, Fir, Alder, Khaki, Willow, Cork,
Mimosa or Wattle, Babool, Larch, Mangrove, Spruce, Elm, Birch,
Pomegranate, Cebil.
2. _Leaves, Twigs, etc._--Sumach, Mangrove, Mango, Eucalyptus,
Pistacia, Lentiscus.
3. _Roots._--Canaigre, Palmetto.
4. _Fruits._--Myrobalans, Valonia, Divi-Divi, Cascalote, Mangosteen,
Pomegranate, Celavinia, Bablah, Algarobilla.
5. _Excrescences._--Gall Nuts, Chinese Galls, Pistacia Galls,
Tamarisk Galls.
TANNING EXTRACTS
1. _Woods._--Oak, Quebracho, Hemlock, Chestnut, Mimosa, Mangrove,
Spruce.
2. _Barks._--Oak, Wattle or Mimosa, Larch.
3. _Shrubs, Leaves, etc._--Gambier, Cutch, Catechu, Kino, Sumach.
4. _Fruits._--Myrobalans, Valonia.
5. _Roots._--Palmetto.
Of these materials, only about twenty are of much importance
commercially, the principal being oak, chestnut, quebracho, hemlock,
valonia, gambier, myrobalans, mimosa or wattle, sumach, mangrove,
divi-divi, spruce, larch, and babool.
OAK BARK (_quercus robur_) is still an important material, but is
rarely used alone. The bark from English oaks contains from 8-14 per
cent. of tannin (quercitannic acid) as estimated by the impregnation
of a standardised hide powder in a given quantity of the tanning
material in solution. Owing to its weakness in tannin compared with
other materials, oak bark tans very slowly. Used for sole leather,
it would not produce the essential quality of firmness and solidity,
and it is now customary to use a stronger tanning material, such as
valonia, or valonia extract, or gambier in the latter stages of the
process. This is the nearest approach to the pure oak bark tannage of
former days, and, if carefully regulated, is a great improvement on
the old method.
If dressing hides and calf skins required for boot upper leather are
bark-tanned, the tannage is often completed in a sumach liquor, the
object in this case being to lighten the colour so that the leather
can be dyed evenly.
In England, oak bark is harvested in April and May, when the sap
rises in the tree. Rings are cut round the tree soon after it is
felled and the bark is peeled from the tree with a special tool which
is forced between the bark and the wood. It is peeled in narrow
strips about 3 ft. in length, and on delivery to the tannery is
stacked in huge ricks. If harvested in a good, dry condition, the
bark is said to improve with age, although analytical tests have
shown that there is always a certain loss of tannin. An old rick is
much darker in colour than a new one, owing to exposure to the air.
Coppice bark from young trees is preferred by tanners, as it is free
from ross and generally contains more tannin than the rough bark.
In view of the modern demand for materials in extract form, English
oak bark would almost certainly be preferred in the form of a
concentrated liquid, if the supply of the raw material was plentiful
within a limited area. During the last few years it has not met with
a ready sale, owing to the large supply of other materials, but it
would doubtless regain some of its former popularity if it were
prepared in the form of an extract containing about 25 per cent.
of tannin. The only oak bark extract on the market is the American
chestnut oak (_quercus prinus_).
OAK WOOD is very largely used for the manufacture of tanning extract,
especially in Hungary and Canada. The extract contains from 24-28
per cent. of tannin, and is extensively used in the tannage of heavy
leathers, as it strengthens the liquors and hastens the process,
while keeping the quality of the leather at a high standard.
VALONIA (_quercus aegilops_) is the acorn cups of an oak tree which
grows abundantly in Asia Minor and the Greek Archipelago. No other
part but the acorn cups is exported. The harvest in Asia Minor takes
place in August, when the fruit ripens and the cups can be easily
beaten from the trees. They are left to dry on the ground and are
then sent to stores in seaport towns, and principally to Smyrna. The
drying is still further completed in spacious warehouses, where the
cups are spread out and turned over until fermentation ceases. During
this process the acorns shrink and are rejected. The cups should be
perfectly dried and very hard before export. The Turkish valonia
contains from 30 to 35 per cent. of tannin, and is of much better
quality than the Greek, which is usually harvested before it is ripe,
and, therefore, contains the acorn. As the acorn has practically no
tannin value, the Greek valonia contains only 25 to 28 per cent.
tannin. There are several grades of valonia, the best going to
Russia, Austria, and Italy. English tanners seem to prefer the lower
qualities at present, probably because the price is much less than
that of the best grade.
The beard of the valonia cup is much richer in tannin than the shell,
and, as several of the spines become detached during the storage of
the material, there is always a certain quantity of beard (_trillo_)
on offer. This may contain up to 42 per cent. of tannin, but its
price is usually the same as that of ordinary valonia. Of late years,
very large quantities of valonia have been made into extract at two
works in Smyrna. The production of extract will no doubt increase,
with a corresponding reduction in the export of the raw material.
The great advantages of the extract over the natural cup are its
superior strength of tannin (60 to 65 per cent.), easier solubility,
uniformity of quality, lower cost per unit of tan, and guaranteed
purity.
Valonia is well adapted for the tannage of sole leather in
conjunction with oak bark, for it deposits a heavy bloom (ellagic
acid), imparts weight and solidity, and increases the resistance of
the leather to moisture.
THE CHESTNUT TREE (_castanea vesca_) probably provides the next
tanning material of importance. This must not be confused with the
chestnut oak, an American tree which also yields a very useful
tannin. The chestnut is indigenous to the South of France and Italy,
where the forests have been considerably reduced in size to meet the
great demand for this popular tanning material. The greater part of
the denuded forests have not been replanted with the chestnut, as
the land has been put under cultivation whenever possible. A further
depletion has been caused by the ravages of an insect, which turns
the interior of the wood quite black and renders it unfit for
tanning purposes. It will, naturally, be several years before the
supply is exhausted, even if no reafforestation is undertaken. As
it is the most important tanning material grown in France, and the
chestnuts are used as a food, steps may be taken to cultivate the
trees on areas unsuitable for agriculture. Liquid chestnut extract
contains from 30 to 32 per cent. of tannin, and, when decolorised,
gives a light brown colour to the leather. It is rarely, if ever,
used alone, but generally in conjunction with quebracho, valonia or
myrobalan extracts.
THE CHESTNUT OAK TREE is indigenous to America and the wood yields a
very good tanning extract, containing up to 30 per cent. of tannin.
This material is the principal tanning agent used in America, where
the tannages are roughly classified in three sections: (1) oak, (2)
hemlock, and (3) the union (_i.e._, a mixture of hemlock and oak).
American tanners also use other materials to a smaller extent,
chiefly for blending with the principal tannins. Quebracho and spruce
extracts are specially favoured.
QUEBRACHO COLORADO is a tree indigenous to South America, the best
wood for tanning purposes being found in the Gran Chaco district in
the north of Argentina, and in Uruguay. The wood contains from 17 to
22 per cent. of tannin, and is so hard and heavy that it sinks in
water. In fact, its name is derived from two Portuguese words meaning
"axe-breaker."
After felling the trees, they are cut up into logs about 4 ft. in
length and either exported in this state to Hamburg, Havre, and
Liverpool, or sent to the numerous factories in close proximity to
the forests to be made into extract, in which an enormous trade has
of late years been developed. Very little of the natural material
is now used, as, even after cutting the wood into chips, the tannin
is extracted only with great difficulty, whereas the extract can be
treated with sulphites, alkalies, or neradol (the artificial tannin)
to render it easily soluble, besides which the concentration of the
material raises the percentage of its tannin to 65 or even 70 per
cent. Owing, perhaps, to faulty preparation, this tanning extract was
not well received at first, but it is now among the principal tannins
and increases in importance every year.
MYROBALANS is the unripe fruit of an Indian tree (_terminalia
chebula_) and contains from 35 to 40 per cent. of tannin which gives
a light colour to leather. This material is useful both for light
and heavy leathers, but is generally used in admixture with other
tannins. It deposits much bloom (ellagic acid) and is largely used
for brightening the dark colour produced by other tannins. A large
quantity of this material is now made into extracts, which are
more convenient to handle and more uniform in strength of tannin.
Natural myrobalans have the appearance of shrivelled nutmegs, except
that they are yellowish in colour; they are very hard and require
a special milling machine to reduce them to powder. The quality of
myrobalan nuts varies in different districts, the best being Bhimlies
and Jubbalpores.
SUMACH is a valuable tanning material, and is used for a large
proportion of the light and fancy leathers. It is a small bush plant
which grows in Italy, Spain, Southern France, America, and Algeria,
but of the numerous varieties the Sicilian (_rhus coriaria_) is by
far the most important. Sumach is one of the few materials cultivated
on an extensive scale; most tanning materials are derived from
natural sources and, chiefly owing to the length of time before trees
reach maturity, it would not be a paying proposition to cultivate
them. The sumach shrubs are propagated from small cuttings and
the leaves can be picked at the end of the first year, but it is
better to allow the shrubs to become more firmly established before
stripping them. The leaves are dried and sometimes exported whole,
especially for the use of the silk manufacturers in Lyons; but
they are more often ground to a fine powder. All sumachs should be
ventilated to remove foreign matters and all traces of iron, which
would cause dark bluish stains on the leather. "Ventilation" is
effected by passing currents of air, preferably with a fan, through a
narrow room, when the pure sumach is sent forward, while the heavier
particles of dirt and small pieces of wood remain behind. Sometimes
the process is repeated, and the best brands of sumach are generally
described as "pure, extra ventilated." As far as possible, the male
plants (_mascolino sommacco_) are cultivated in Sicily, where the
best sumach is grown. Female sumach (_femminello sommacco_), grown in
parts of Italy, is weaker in tannin than the male, but is rarely sold
separately. The serious amount of adulteration formerly practised
by the admixture of inferior plants, and particularly of lentisco
(_pistacia lentisco_) led to the Italian Government taking strong
action a few years ago, and it is now possible, for a very small sum,
to have any consignment inspected and analysed by the Government.
Lentisco is now sold separately and is used for common work.
Sumach has been successfully introduced into Australia, but its
development is retarded owing to difficulties of labour, which render
competition with the European product almost impossible. An inferior
sumach (_rhus glabra_) is grown in America, chiefly in the State of
Virginia. It contains from 15 to 20 per cent. of tannin, and produces
a darker coloured leather than Sicilian, the best qualities of which
contain 27 to 30 per cent. of tannin.
A useful test for finding out if a sumach has been adulterated is to
treat a small quantity with strong nitric acid, which destroys the
structure of the leaves. The mass is washed and neutralised with an
alkali, when the appearance of the midrib and veins of the leaves of
the common adulterants are easily recognised.
Sumach is not so much used for shoe upper leather as it formerly
was, but it is the best tanning agent for many kinds of fancy light
leathers, such as bookbinding, calf, and skivers (the grain of split
sheep skins), moroccos, furniture leather, etc. It is also less
subject to the action of the air and gaslight than any other tanning
material, and is strongly recommended for tanning purposes by a
special committee appointed by the Society of Arts to enquire into
causes of the rapid decay of leather bindings. Sumach is very useful
for brightening up the colour of leather tanned with darker tannins,
and is frequently used for improving the colour of both dressing and
sole leather. By itself, it yields an almost white leather which
affords a good foundation for the most delicate shades.
GAMBIER or TERRA JAPONICA (_uncaria gambir_) is a crude extract of
a shrub indigenous to the Malay Peninsula. Nearly the whole of the
production is shipped from Singapore. The leaves and twigs are boiled
in an iron vessel, and when the mass has become syrupy it is strained
through a rough sieve into a shallow tub, where it is cooled. The
liquor is stirred while cooling and rapidly thickens. Before it sets,
it is cut into 1 in. cubes and thoroughly dried. Good qualities
contain from 50-65 per cent. of tannin. An inferior product, called
"block gambier," is made by allowing the syrupy mass to set in large
blocks weighing about 2 cwt. each. These are packed in coarse
matting. The strength of tannin varies from 30 to 40 per cent.
Gambier is a good tanning material, but its use has been declining
for some years past owing to its being frequently adulterated with
sago and other farinaceous plants. However, a pure gambier extract,
manufactured on the latest scientific principles, has been placed on
the market, and there will undoubtedly be a revival of the use of
this valuable tannin. This pure gambier is prepared at Asahan, in
Sumatra, and is guaranteed to contain a minimum of 38 per cent. of
tannin.
Gambier can be used to advantage in keeping up the strength of bark
liquors in the tannage of sole leather and hastening the process,
while it may be used alone for the tannage of boot upper leather
and dressing hides. It produces an exceptionally mellow and plump
leather. It is preferable, however, to complete a gambier tannage
with a little oak wood or quebracho extract, in order to fix the
tannin principle of gambier, which, perhaps on account of its
viscosity, does not readily combine with the fibres of the skin.
MANGROVE or MANGLE, a tree found on the coasts of several tropical
countries, yields a useful bark for tanning purposes. At low tide,
these trees show their great arched roots standing high above the
ground. The best varieties, the _ceriops_ species, are found in the
East Indies and Bengal, and the bark of these is said to contain
sometimes nearly 40 per cent. of tannin. Other varieties contain from
15 to 25 per cent. The bark is generally made into a solid extract,
or "cutch," in which form it contains more than 60 per cent. of
tannin, It is useful to blend with other materials, such as oak wood,
chestnut, and quebracho extracts, but used by itself it imparts a
strong reddish colour to the leather. Some of the Indian varieties
are used as dyeing materials, and act as a satisfactory mordant in
dyeing leather a dark shade.
MIMOSA or WATTLE trees, which belong to the _acacia_ species,
yield bark rich in tannin. Australia is the native country of
several varieties, including the Black Wattle (_acacia pycnantha_),
the Golden Wattle (_A. longifolia_), and the Green Wattle (_A.
decurrens_).
The bark contains from 20 to 45 per cent. of tannin. The cultivation
of wattle in Australia seems to have declined, owing to the high cost
of labour and inability to compete with the mimosa bark imported from
South Africa, where it is cultivated on a very large scale and where
labour conditions are more favourable for the growers, as is clearly
shown by the fact that the wattle growers in Australia successfully
petitioned the Government a short time ago to place a duty of £1 10s.
per ton on the imported bark.
The introduction of the industry into South Africa was quite an
interesting adventure. A Mr. Vanderplank brought the seeds from
Australia to England about seventy years ago, and afterwards took
them to South Africa, where, in recognition of certain services a
few months after his arrival, he was granted a farm by the Dutch
Government. He then planted the seeds of the black wattle, which grew
so well that it was only a question of developing the industry. It
was some years before any African bark was exported, and only £11
worth was shipped in 1886. In 1911, the exports had risen to £288,000.
Wattle trees can be grown on soil that is unsuitable for agriculture,
and there is every prospect of the industry expanding in South
Africa, where a factory has lately been established for the purpose
of converting the bark into an extract, which, it is said, will
contain between 50 and 60 per cent. of tannin. By far the greater
proportion of wattle bark is still exported in the natural form,
ground or chopped, and packed into bags weighing about 1 cwt. each.
Before the European War nearly the whole of the bark was shipped to
Hamburg, English tanners taking very little interest in it, although
it was largely used by German tanners. But since the supplies of
the materials favoured by British tanners have become somewhat
restricted, attention has been drawn to the value of mimosa bark.
DIVI-DIVI (_caesalpinia coriaria_) is the dried pods of a Central
American tree. It has also been successfully cultivated in India. The
pods are rich in tannin, containing anything from 40-50 per cent.,
but its value is discounted by its liability to fermentation, which,
however, may be checked to some extent by the use of antiseptics,
such as carbolic acid, formaldehyde, or by the addition of synthetic
tannin, neradol. If this tendency to fermentation and oxidation of
the colouring matter could be checked completely, divi-divi would
be a valuable material, as it makes a firm leather of good colour.
When dried, the pods curl up in the shape of the letters S and C. The
tannin is found in the husks of the pod. The seeds, which contain
no tannin, are so hard that it has not yet been found profitable
to extract the oil from them. Very similar tanning materials to
divi-divi are cascalote, indigenous to Mexico, and algarobilla
(_caesalpinia brevifolia_) which grows in Chili. Cascalote is chiefly
used by Mexican tanners and is rarely exported. Algarobilla is not
available in large quantities, otherwise it would be largely used in
Great Britain, as it does not ferment so readily as divi-divi, and is
even richer in tannin.
CELAVINIA (also spelt celavina and cevalina) has been on the English
market since 1905, but has only lately been sold in large quantities.
The scarcity of some of the popular tanning materials since the
outbreak of the European War resulted in enquiries for materials that
were very little known, and celavinia has proved worthy of attention.
It consists of the seed pods of the tree _caesalpinia tinctoria_,
which grows abundantly in certain districts of Central and South
America. The pod is from 4 to 6 in. long and is flaky when dried. It
contains 30 to 32 per cent, of tannin of the pyrogallol class, and
gives a very light-coloured and almost white leather. It is the only
pyrogallol tannin which does not deposit bloom, or ellagic acid, on
the leather. It may be used as a substitute for sumach in tanning,
but has not the same bleaching effect in the retanning process. A
tanning extract of celavinia would be useful for some classes of
light leathers, where paleness of tint is important. It is difficult
to make a second extraction of tannin in the case of the natural
material, as, after the first extraction, it forms a soft pulp,
through which water will not easily percolate.
HEMLOCK (_abies canadiensis_) is an important tanning material, both
the bark and the wood being extensively used in America. The wood is
now generally converted into extracts in factories built near the
principal forests. Of late years, this extract has been imported into
the United Kingdom in fairly large quantities, in order to produce a
cheap red sole leather to compete with the American hemlock-tanned
leather. It contains only about 25 per cent. of tannin, but its value
is increased by its contents of insoluble non-tannins, which give
weight and solidity to the leather. Hemlock really gives a strong,
durable leather, but in America the practice of using artificial
weighing materials, such as glucose and Epsom salts, with a reduced
quantity of tanning material, has considerably lowered the value of
this leather.
In addition to the materials described, there are several of minor
importance which can only be briefly mentioned.
LARCH BARK is obtained from the tree _larix Europea_, which is found
in Scotland and North Europe. It contains 10-12 per cent. of tannin,
which gives a light colour and pleasant odour to leather. Scotch
basils (sheep skins) are tanned with this bark.
BIRCH BARK, from the white birch, _betula alba_, is another aromatic
tanning material. It contains only about 5 per cent. of tannin,
and is, therefore, generally used with other tanning materials.
It contains a tar which imparts an agreeable scent to the leather
that protects it from the ravages of insects. In conjunction with
willow bark (_salix arenaria_) it is used in the tanning process
for the real Russia leather. An oil containing the scent can be
extracted from the birch bark by dry distillation, and this extract
is sometimes used during the dyeing process, in the manufacture of
imitation Russia leather, which, however, only retains the scent for
a few months, whereas the real Russia leather has a permanent odour.
CANAIGRE (_rumex hymenosepalum_) is the tuberous root of a dock
plant indigenous to Mexico and the Southern States of America. It is
fairly rich in tannin (25-30 per cent.) and yields a moderately firm
leather. It contains too large a proportion of starch, however, and
cannot be described as a really satisfactory tannin. Moreover, it is
not harvested economically and the only way to make a satisfactory
tannin of it is to convert it into extract and remove the starchy
matters near the source of supply, if anyone dare take the risk of
establishing a factory in Mexico.
BABLAH or BABOOL (_acacia arabica_ or _acacia vera_) grows in India,
Egypt, and the Sudan. The bark of the babool tree is one of the
principal tanning materials used in India for hides, calf, and sheep
skins, which are sent in large quantities to Great Britain in a
rough-tanned state and dressed there. It contains 15-20 per cent. of
tannin, which readily oxidises in the leather in contact with light,
turning into a bright pink colour. It also seems to weaken the fibres
of animal tissues, and, for that reason, babool-tanned leather was
condemned by the Society of Arts Commission on Bookbinding.
The pods contain from 20-30 per cent. of tannin and give a mellow and
plump leather similar to that produced by gambier. The bleaching of
the material is troublesome, and it is probable that its use would be
increased if the tannin were prepared in the form of extract.
CUTCH is a crude extract made from the Indian tree _acacia catechu_.
This is the real cutch, as distinguished from the mangrove "cutch."
It is very rich in tannin (50-60 per cent.), but contains a large
proportion of insoluble matter and is, therefore, very little used
for tanning. It is well adapted for the dyeing of dark colours or
black with mineral strikers, such as chrome and iron salts; but
its chief use is for tanning fishermen's nets, which it renders
waterproof.
Commercial tannic acid, used for medicinal purposes, is prepared from
galls or excrescences on oak trees growing in Asia Minor. These galls
are caused by an insect (_cynips_) puncturing the small branches
and producing abnormal growth in the perforated parts. The acid is
gallotannic, which, if used for making leather, would produce a soft,
spongy, and nearly white leather. This tanning material is used a
little by Near Eastern tanners, but the result is unsatisfactory.
CLASSIFICATION OF TANNING MATERIALS
Tanning materials are divided into two main classes: (1) Pyrogallol,
(2) Catechol. The pyrogallol tannins give a bluish-black colour,
and the catechol tannins a greenish-black, with iron salts. Bromine
water does not precipitate pyrogallols, but causes a precipitate with
catechols. Pyrogallols yield ellagic acid (called "bloom" in the
trade), which improves the waterproof qualities of leather. On the
other hand, catechols contain a large proportion of insoluble reds,
or phlobaphenes, which are deposited between the fibres and thus give
solidity to the leather. Pyrogallol tannins give a light coloured,
soft leather, and can be used alone satisfactorily; but heavy
leathers, such as sole and belting, need a blend of both kinds of
tannin. The pyrogallol tannins include sumach, chestnut, myrobalans,
divi-divi, oakwood, algarobilla, chestnut oak, willow, and galls.
The catechols include quebracho, gambier, hemlock, mimosa or wattle,
mangrove, larch, birch, canaigre, and cutch. Oak bark and valonia
contain some of the properties of both classes of tannins.
There are sub-divisions of these two classes, arranged according to
chemical tests, by which one tanning material can be distinguished
from another.
TANNING EXTRACTS
The manufacture of extracts from vegetable tanning materials has
increased so rapidly of late years that the process of tanning has
undergone radical changes; and, whereas the tanner was limited to
three or four materials thirty years ago, he now has the choice of
about twenty good materials. These tannins can be suitably blended
to produce first-class leather. The processes in extract manufacture
are few. The wood, bark, or fruit is broken up into small pieces
and macerated in hot or cold water. The concentration of the liquid
is done in vacuum, or in an evaporator. The latest method is to
treat the tan liquor in a "triple-effect" evaporator (Fig. 4),
the object of using three compartments being to economise in steam.
This apparatus makes liquid extracts; where a powdered or crystal
extract is required, the liquid is afterwards treated in a vacuum
drying apparatus. Some extracts contain a lot of colouring matter and
insoluble substances. To overcome this defect, they are clarified
with blood albumen, sulphites, casein, or acetate of lead.
[Illustration: FIG. 4.--TRIPLE-EFFECT EVAPORATOR (Blair, Campbell
& McLean, Ltd.)]
SYNTHETIC TANNINS
The discovery of a synthetic tannin, in 1911, by Dr. Stiasny, who
was then an assistant in the Leather Industries Department of Leeds
University, created a great deal of interest in the leather trade,
and can certainly be regarded as a triumph of the application of
chemistry to industry. It was thought at first that these tannins
might play as important a part in the leather trade as the synthetic
dye-stuffs have, but it is now generally believed that this will
not be the case. Their use is likely to be as an aid to tanning,
rather than as a complete tanning agent, although it has been found
practicable to use them for one or two classes of light skins, where
it is essential that the colour of the leather should be nearly white.
The original patent was taken out in September, 1911, in Austria,
the native country of the inventor, but the patent rights have also
been protected in other countries, while the manufacture and sale of
the product passed into the hands of a large German dye firm, who
have since taken out several patents for other synthetic tannins.
The original tannin is produced by treating a sulphonated phenol
with formaldehyde. A patent had been taken out several years before
for the use of formaldehyde in tanning, but this expired early in
1911; many patents had also been granted for sulphonated phenol
preparations, chiefly as disinfectants, but it was left to Dr.
Stiasny to discover the value of combining the two chemicals by the
process of condensation. The preparation, first known as "neradol,"
is now made in England under licence, but, since the war, its cost
has been more than doubled, so that it is unlikely to replace natural
tanning materials to any great extent.
Formaldehyde itself has certain properties which are harmful to
leather, and it must be used with extreme caution and in small
quantities to avoid these defects. It is used in the leather trade
chiefly for keeping up the substance of hides during the tanning
process, and thus causing a rapid absorption of tannin. At the
beginning of the tanning process, the liquors are acidified,
preferably with a weak organic acid, which causes the fibres of
the hides to distend. It is at this stage that the formaldehyde is
used. This method, which cannot improve, but may easily damage, the
leather, is almost entirely confined to the Continent. Formaldehyde
has a hardening and tanning effect on animal tissues, and leather
treated by this process often has inferior wearing qualities.
In the case of the artificial tannin, this property of formaldehyde
is to a large extent modified by the chemical reaction with a
sulphonated phenol, the addition of which also increases its tanning
effect; but, while the tannage is very rapid, especially in the case
of light skins, leather produced solely by means of artificial tannin
has a slight tendency to dryness after being in stock for some time.
No doubt, however, methods will be found to overcome initial
difficulties, and various liquors will be prepared to meet different
requirements. The question of price remains the deciding factor as
to its use on an extensive scale, for, while it is so high, natural
tanning materials will be preferred, except in a few special cases.
According to the hide-powder method of tannin analysis, Neradol
contains about 30 per cent. of tannin, but there are many natural
tannin extracts containing nearly double the amount of tannin at
nearly half the price of the artificial tannin; the latter, however,
has a more rapid action and also produces a nearly white leather.
Another advantage of Neradol is that it prevents drawn grain, so
that, if raw hides be treated with a small quantity before tanning,
stronger vegetable tan liquors may be safely used to hasten the
process. In the manufacture of sole leather, for example, limed
hides, after washing in water, may be suspended in a solution of
Neradol containing 3 to 5 lb. per 100 gal. of water for twelve to
twenty-four hours. This quantity produces a slight tanning effect,
the hides are thoroughly delimed, and strong vegetable tan liquors
may then be used to complete the tannage without the grain of the
hide being drawn or dark in colour, as would be the case if the hides
were not first treated with the artificial tannin.
Neradol is also said to be an effective bleaching agent for tanned
leather in a 5 per cent. solution for a few hours, without any loss
of weight.
This property may also be utilised in the production of chrome
leather, where a whiter colour is required than that produced by
the ordinary chrome tannage. For this purpose it may be used in a
pickling liquor before the one-bath process, or in the second bath of
the two-bath tannage.
For dressing wool and fur skins, the synthetic tannins are much
superior to the alum and salt process.