TANNING PROCESSES
The methods of tanning may be classified as follow: (1) Vegetable
tanning processes; (2) methods of chroming; (3) tawing processes; (4)
oil tannages; (5) formaldehyde tannages; (6) sundry tannages, chiefly
mineral; and (7) combination tannages. The public is chiefly familiar
with types of vegetable tannage in boot sole, bag, and portmanteau
leathers; of chrome tannage, in box calf and glazed kid used for boot
uppers; of the tawing process, in kid glove leather; and of the oil
tannage in wash leather or "chamois."
VEGETABLE TANNING PROCESSES
The methods of tanning with infusions of barks, leaves, and fruits
of trees and plants containing tannin are much more numerous than
they were a decade ago, and tanners have now to pay special attention
to the selection and blending of the materials they use in order to
produce the various qualities of leather required. Formerly, most of
the sole leather was tanned in oak bark liquors, and, in the later
stages of the process, valonia and gambier were added to quicken the
process and give solidity to the leather. This tannage, which may be
described as an oak bark tannage, is still used by a few tanners,
but the wide choice of materials available has brought other tanning
agents into prominence, chiefly because they possess more astringent
properties, and, therefore, tan more quickly than oak bark. Moreover,
the need of other materials than oak bark became a necessity several
years ago, as the supply of the latter would be inadequate to
produce the large quantity of leather now required, even allowing
for the rapid development of mineral tannages. Further, it is amply
proved that a good blend of materials (mixed tannage) produces quite
as durable and a firmer leather than the old oak bark tannage;
unfortunately, there are other blends, occasionally combined with
extraneous weighting materials, which account for the poor quality of
a great deal of modern sole leather.
While the mixed tannage is now popular for sole, belting, harness,
and other heavy leathers, vegetable tanned light skins, such as calf,
goat, and sheep, are in most cases treated with a single material,
sumach being used for a good proportion of them.
Whichever method be used, the first essential is the most suitable
means of leaching the materials, or extracting the tannin. It seems,
however, that this process may be eventually eliminated from the
tannery, for most tanning materials are now converted into extracts,
which only require dissolving in water to prepare the tan liquor. The
manufacture of tanning extracts is quite a separate business, which
is generally, but not always, conducted in factories situated near
the source of the raw materials. There are important extract works in
the Argentine, Paraguay, Canada, the United States, Hungary, North
Germany, Borneo, Smyrna, France, Italy, and England.
[Illustration: FIG. 21
BARK MILL]
Where natural tanning materials are used, either entirely or in
conjunction with extracts, the leaching is done in a series of large
square pits, four of which would suffice for a small yard, while a
very large tanning would need twelve or sixteen. Oak bark, which is
usually delivered to the tannery in strips measuring 3 to 6 ft. in
length, must be chopped or ground into small pieces by machine (Fig.
21). A measured quantity (a certain number of baskets or skeps full)
is placed in the empty pits, which are then filled with water. The
liquor is pumped from these pits, as required, to others in which
the hides are tanned. The hard fruit of myrobalans, which somewhat
resembles nutmegs, is powdered in a disintegrator or special crusher.
[Illustration: FIG. 22
TAN-YARD
("Dri-ped" Tannery)]
[Illustration: FIG. 23
HIDE ROUNDED FOR SOLE LEATHER]
The three principal vegetable tannages are those for sole, dressing,
and light leathers, and it is obvious that a careful selection of
materials is necessary to obtain the desired effects. For this
reason, oak bark, although a good tanning material for dressing
leather, is insufficient to produce a firm sole leather, and,
therefore, even in the so-called pure bark tannages, valonia or other
suitable tannin is used towards the end of the process in order
to increase the solidity and waterproof quality of the leather.
There are two general methods of tanning sole leather, namely,
the pit and the drum tannage. In the first case, the hides are
passed through three series of pits containing gradually increased
strengths of tannin (Fig. 22). These series are technically described
as "suspenders" or "colouring pits," "handlers," and "layers" or
"layaways." To facilitate handling and economise tanning materials,
hides are "rounded" (_i.e._, cut into sections) either before tanning
or after the hides have passed through the suspenders (Fig. 23). The
obvious advantage of the former method is that the offal (shoulders
and bellies) removed can be chrome tanned if required. The suspenders
may consist of any number of pits from six to twenty-four, according
to the size of the tannery. As the name implies, the hides are
suspended in the liquors from poles, which extend across the top
of the pit. The hides are attached to the poles by means of stout
cord or copper hooks. Various mechanical appliances are in use to
supersede the old method of allowing the hides to rest during this
process. These methods save time and tend to produce even coloration,
the latter being a very important point in the early stages of
tanning. Some of the methods advocated, however, are too vigorous for
the hides at this stage, when the chief object should be to preserve
as much gelatine as possible. The best mechanical system is that
which gently raises and lowers the hides in the liquors. Some of the
American tanners use a mechanical contrivance known as the rocker,
which consists of a stout wooden beam rocking from a central pivot,
and with a see-saw movement. The objection to this method is that the
hides do not receive equal treatment, those in the centre receiving
hardly any movement, while others at the ends of the beam are raised
too far out of the liquor. Under these conditions, the colouring of
the leather must be irregular. Another method, invented in England,
consists in suspending the hides from a wooden frame which can be
mechanically moved on wheels.
As the contact of tanning liquors with iron must be avoided to
prevent discoloration (iron and tannin form the basis of inks),
the wheels should be galvanized. This mechanical method is very
satisfactory, for the hides are moved gently and kept in the liquors.
As some tannins oxidise rapidly (_i.e._, darken in colour by exposure
to the air), any process which exposes the hides at this stage for
lengthy periods should be avoided.
[Illustration: FIG. 24
BARKOMETER]
The theory of the vegetable tanning process is not perfectly
understood, and the principal trade chemists are not yet agreed as
to whether the changes are chemical, physical, or both. Many are
inclined to believe that both changes take place, and this view
is probably correct, as a pure tannic acid produces a thin, empty
leather, while a tanning material which contains a fair proportion
of non-tannin matters, yields a full and firm leather. The practical
tanner who adapts his work to the theory that tanning is a process
of feeding the hides with gradually increasing strengths of tannin,
is, in any event, on the right track. The suspender liquors are
always weak in tannin, and are pumped from "handler" liquors
through which packs of hides have already passed. It is important
that these liquors possess a certain degree of acidity, and, if
the natural acids of the tan liquor are too weak, or neutralised
completely by the lime in a pack of hides that has been treated, a
small quantity of acetic, formic, or lactic acid is added. Formic
acid has the additional advantage of making the liquor antiseptic.
It is absolutely necessary that the liquors have acid properties,
otherwise tanning cannot proceed. Acid assists penetration of tannin.
Hyposulphite of soda, formaldehyde, or synthetic tannin may be used
for the same purpose, while these also help to keep the leather
light in colour, owing to their bleaching properties. After passing
through three or four suspender pits, in which the tan liquor is
gradually strengthened, the hides are often rounded and the butts
(Fig. 23) are transferred to the handlers. The first handler liquor
should register about 25 degrees by the barkometer (Fig. 24), an
instrument which registers the density of tan liquors. The handlers
consist of a series of six or eight, and the strength of the liquors
should be increased gradually. After a day's immersion in the first
handler liquor, the hides are hauled out with a sharp two-pronged
hook fixed to the end of a long wooden pole, similar to that used in
the liming process. After they have been piled up for a few hours,
two workmen, one on each side of the pit, place the hides flat in the
next pit. The tan liquor is then run off the first handler to the
suspender and fresh liquor is pumped into the empty pit, which then
becomes the head or strongest liquor of the series. The succeeding
packs will, therefore, follow in rotation: the first going to No.
2 pit, the second to No. 3, and so on. In the last two or three
handler pits, the hides are sometimes dusted with freshly ground
tanning material, such as oak bark, myrobalans, or divi-divi, the
object being to strengthen the tan liquor so that it penetrates the
hides before they are transferred to the layers. The handler liquors,
which are originally sent back from the layers, are also frequently
strengthened by the addition of oakwood, chestnut, or quebracho
extracts, although some tanners only use extracts in the layers.
Gambier is a useful material at this stage, as it keeps the liquors
mellow and plumps the leather. In the layers, the last series of tan
pits, the hides are treated with very strong extracts, and are also
heavily dusted with such natural tanning materials as give firmness
and solidity to the leather. For this purpose, nothing is better
than valonia, or valonia extract, which deposits a large quantity of
bloom, or ellagic acid. Where a mixed tannage of natural materials
is used, the density of the liquors may reach 50 degrees barkometer,
but if strong extracts be added, the barkometer may exceed 100
degrees. The relation between the layers and handlers in regard to
the disposal of the used liquors is similar to that between handlers
and suspenders; the first layer used is sent to the last handler.
By the English method, the leather is not, as a rule, transferred
immediately from one pit to another, as it is found preferable to lay
the hides in pile for a few hours, by which means the great weight
helps to force the tannin through them. By draining the hides, they
are in good condition for the absorption of the next tan liquor.
The English method of handling is too slow for most of the American
tanneries, where the greater proportion of the hides are cut straight
down the centre to form "sides," as they are technically known. In
America, the pieces of leather, whether hides, butts, bellies, or
shoulders, are often tied together and transferred from one pit
to another by means of a skeleton reel, worked either by hand or
mechanically. The latest type of reel is a great improvement on the
old reel. The cost of manual labour in many English tanneries is
very great, in view of the fact that the hides have to pass through
many pits, and the work of transferring the hides from one pit to
another is done entirely by hand. It seems that there is plenty of
scope for mechanical ingenuity in overcoming this difficulty, and it
may yet be found possible to construct a perforated brass platform,
connected with a crane, by which the whole of the hides could be
lifted out in a second, left to drain for half an hour or more, and
then bodily transferred to, and lowered in, the next pit. Naturally,
it is easier, and quite within the bounds of practicability, to raise
the hides from suspenders by mechanical power, as it would not be
difficult to fix a stout cross-beam to the pulley from which the
hides are suspended. The leather can be safely treated with very
astringent tannin in the last series of pits, and the use of strong
extracts is, therefore, a common practice at that stage, not only to
give solidity and firmness to the leather, but to increase its weight.
It would be quite fatal to the production of good leather if strong
tannins were used in the early stages of the process, because the
surface on both sides would tan quickly, the grain of the hide would
be badly drawn owing to the sensitiveness of gelatine to astringent
solutions, and it would be impossible to make the tannin penetrate
the hide. The final product would be a half-tanned leather which
would be extremely brittle and poor in quality; in fact, quite
unsuitable for sole leather. Despite the well-known axiom that the
vegetable tannage (but not the mineral) must proceed slowly and
gradually in the early stages of the process if the hides are to be
properly tanned, the modern tendency is to speed up the tanning, with
the result that, in some cases, the so-called sole leather is really
not fit for the purpose for which it is intended. The root of the
evil is the desperate attempt made by many boot manufacturers to
produce lower-priced boots than those of their competitors.
Taking into account the development of leather trades chemistry, it
is not surprising that the problem of reducing the time required to
complete the old processes of tanning has received much attention.
The results have been successful in some instances and have certainly
proved that very good sole leather can be made without leaving the
hides in the pits for a year or longer.
It has also been shown that the absorption of tannin can be
accelerated by treating the hides, before putting them in the
suspenders, with a solution of acid (lactic or butyric for
preference), or hyposulphite of soda, or synthetic tannin. The latter
penetrates the hides in a few hours without contracting the grain,
increases the solubility of the vegetable tannins subsequently
used, and helps to keep the leather light and uniform in colour.
It is of no use alone for heavy leathers, however, as it lacks the
physical properties necessary to make the leather firm and resistant
to water. The use of "soluble" oils in the tan liquors is another
late innovation which has given good results. The term "soluble" in
connection with oils merely means that they are made miscible with
water by treating them with acids. Solubilised cod oil might be
specially useful, as it has tanning properties and would increase the
wearing and waterproof qualities of the leather. The chief objects
of using oil in tanning, however, are to increase the weight of the
leather and permit the use of strong liquors.
The tanning process outlined is the oak bark tannage, which is now
supplanted in many yards by the mixed tannage, in which various
tannins, chiefly exotic, are skilfully blended to produce the kind of
leather wanted. Each tannin seems to have special characteristics,
although the materials are classified into only three groups, namely:
pyrogallol, catechol, and mixed (pyrogallol and catechol) tannins.
Generally, the best mixed tannages for solid leather consist of a
combination of both pyrogallol and catechol tannins. Reviewing the
characteristics of some of the principal tanning materials, it may
be said that oak bark produces a nice, fawn-coloured leather of
strong texture, but tans slowly; valonia makes the leather solid,
durable, and waterproof; myrobalans quicken the process and lighten
the objectionable colour of other tannins; divi-divi and algarobilla
are very rich in tannin, and are, therefore, useful in later stages
of the process; gambier mellows the astringency of other liquors
and plumps the leather; while most of the tanning extracts on the
market penetrate the hides much more quickly, often give a lighter
colour than that produced by solutions of the natural materials, and
expedite the process considerably.
The most popular tanning materials are: oakwood, quebracho, chestnut,
valonia, myrobalan, mangrove, mimosa, hemlock and spruce extracts,
oak bark, valonia, gambier and sumach. The following are some of the
combinations in use--
_For Sole Leather._
1. Oak bark and valonia, or valonia extract.
2. Oak bark, valonia, and gambier.
3. Oak bark, quebracho extract, and myrobalan extract.
4. Chestnut, quebracho and valonia extracts.
5. Chestnut and oakwood extracts.
6. Oakwood and quebracho extracts, and divi-divi or algarobilla.
7. Quebracho extract, myrobalans, and valonia.
8. Quebracho, mangrove, and valonia extracts.
9. Oakwood, mimosa, and valonia extracts.
10. Hemlock and oak extracts (American union tannage).
_For Dressing Leathers._
1. Oak bark and sumach.
2. Oakwood and quebracho extracts, and sumach.
3. Synthetic tannin and oakwood extract.
4. Synthetic tannin and myrobalans.
5. Gambier and sumach.
6. Wattle bark and myrobalans.
Sumach is often used alone for some classes of goat, sheep, and calf
skins. Synthetic tannins can also be used alone for light leathers.
Many other variations may be tabulated, for nearly every tanner has
his own recipe, having found by experience the blend of tannage that
best suits his trade.
Apart from these innumerable combinations, the only method whereby
the vegetable tanning process may be hastened is the mechanical, for
which purpose either the paddle (Fig. 25) or the drum (Fig. 13) is
used. The former consists of a wheel constructed of wooden shelves
which, when in motion, dip a little way into the tan liquor in a
vat, and so keep it in constant circulation. The drum is fitted
inside with wooden shelves or pegs, which carry some of the hides
or skins to the top of the drum at each revolution. Were it not for
these shelves, the tannage would probably be irregular or otherwise
unsatisfactory, as the hides would always be heaped together at the
bottom of the drum. The paddles and drum are more often used for
light than for heavy leather, as they not only have the effect of
making the leather loose on the grain, but also make it soft and
supple, characteristics which are not required in most of the heavy
leathers.
[Illustration: FIG. 25
PADDLE VAT]
However, the drum is now largely used on the Continent for the
tannage of sole leather. A great saving in the cost of production
is thereby effected, but the leather, although of satisfactory
appearance, lacks the durability and waterproof quality of pit-tanned
leather. The process is much simpler than the pit method, and less
room is required. There are only two stages of tanning: (1) by
placing the hides in colouring pits or suspenders, in which the
hides are nearly struck through with tannin; and (2) running them
in slowly-revolving drums containing solutions of tannin which are
gradually strengthened until the process is finished. There can be
no doubt that the tannin is rapidly forced between the fibres of
the hide by mechanical action, but it is not so firmly combined as
that slowly absorbed by the hide in the pit method. This can be
proved by placing two pieces of weighed leather--one tanned in pits
and the other in a drum--for twenty-four hours, drying the leather
and reweighing it, when it will be found that the drum-tanned
leather has lost the greater percentage of weight. To obviate this
disadvantage as far as possible, use is made of special oils, which
serve to fix the tannin more firmly between the fibres and render the
leather more resistant to water. Other frequent drawbacks of drum
tanning are looseness of grain and lack of substance. In sole leather
tanning, the former is modified to some extent by extra pressure in
rolling the leather during the finishing operations; while the latter
difficulty is sometimes overcome by swelling the leather with acid
and then fixing the swollen condition of the hide by treating it
with a weak solution of formaldehyde. This chemical also has tanning
properties, so that the process is hastened; but leather prepared in
this way cannot be as durable as that produced by a natural tannage.
It seems that the aid of the engineer is necessary to overcome the
difficulty of looseness of the grain caused by the severe pounding of
the hides in the drums. The constant circulation of the tan liquor is
required, but the hides should only be subjected to gentle motion.
It is true there are tanning drums on the market which are said to
obviate all the difficulty found in making sole leather in ordinary
drums fitted with shelves or pegs, but a drum that meets the ideal
conditions for the production of a solid yet flexible sole leather
has not yet been invented. It seems that a kind of inner framework,
to which the hides can be attached and which rotates much more slowly
than the main drum, may solve the problem. The drum tannage permits
the use of an excessive quantity of tannin, which, of course, adds to
the weight of the leather. By the drum method of tanning, heavy hides
can be tanned in two days after leaving the colouring pits; lighter
hides are, naturally, tanned in less time, proportionate to their
substance. The pit method occupies any time from one to fifteen
months, although nowadays very few hides are left in the tan pits for
a year.
The methods of tanning just described relate chiefly to sole
leather, but there is an enormous production of leather known as
dressing hides, which are tanned, dried in the rough state, and
sent to curriers or leather dressers for finishing. These hides are
used for numerous purposes, including bags, portmanteaus, harness,
saddlery, straps, belting, and boot uppers. The tannage of dressing
hides differs slightly from that of sole leather; the liquors must
be mellower and contain less insoluble matter, in order to obtain
the necessary pliability, and a good, clear colour on the grain.
A satisfactory tannage is obtained by treating the hides in oak
bark liquors, which, in the later stages of the process, may be
strengthened with oak wood, or myrobalan extract, or pure gambier,
and completing the process in a tepid bath of sumach, which clears
and lightens the colour. The drum is more suitable for the tannage of
dressing hides than it is for sole leather. A quick method of drum
tanning would be to treat the hides first in a 5 per cent. solution
of neradol, the artificial tannin, and then complete the process
with oakwood, chestnut, or quebracho extracts, or even in mixtures
of these extracts. In this way, fairly good leather could be made in
about two days. Neradol prevents the drawn grain and dark coloration
that would result from the use of vegetable extracts alone.
In the case of light skins, such as calf, goat, and sheep, the
method of vegetable tannage again differs from those just described,
although there is a fair quantity of calf skins tanned with oak bark,
especially those used in the shoe trade. The tendency, however, is
to complete the process rapidly by using extracts, such as oakwood,
quebracho, or mimosa. A very good tannage for the production of a
mellow and plump leather is that of pure gambier, the colour produced
forming a good ground for brown shades. Where light, fancy colours
are required on the finished leather, this tannage must be completed
by placing the skins in a tepid bath of sumach.
A large proportion of the vegetable-tanned sheep and goat skins
is produced by sumach alone, which was adjudged by the Commission
appointed a few years ago by the Royal Society of Arts to investigate
the cause of decay in bookbinding leather to be the best tanning
material and the one least affected by exterior conditions, such as
gas, sunlight, air and dust.
Many sheep skins are split into two sections by machine before
tanning, the top portion, known as the grain, being tanned in sumach,
and described as "skivers." The under section, the side near the
carcase, is known in the trade as a "lining," and is usually made
into the so-called "chamois" leather by means of the oxidation of
fish oils.
Other noteworthy vegetable tannages are those used in the production
of Russia leather, and a large proportion of East India leather.
Real Russia leather, of which the raw material consists of small
native hides and calf skins, has a characteristic and pleasant odour,
which is derived from the birch and willow barks used in the tanning
process. Birch bark contains an essential oil, which is permanently
fixed on the fibres of the leather during the process of tanning.
This leather is only produced in Russia, and chiefly in one large
tannery, although imitations are made in Great Britain, America, and
Germany. These are produced by the use of ordinary tanning materials,
and the scent is applied, either during or after the dyeing process
by the addition of birch tar oil, which is made by the distillation
of birch bark. In some respects, for example, in brilliance of
finish, smoothness of grain, and freedom from defects, the imitation
is better than the real, but the latter has the great advantage
that its perfume is of superior fragrance and permanent, whereas
the imitation leather only retains the odour for about a year. It
is somewhat remarkable, in view of the good demand that exists for
the leather, especially in England and Germany, that no firm outside
Russia has thought it worth while to produce the genuine article. The
tannage would be particularly serviceable for bookbinding leather, as
the oil of the birch has both insecticidal and antiseptic properties.
The principal vegetable tannage used for Indian leather, namely,
the bark of the _acacia arabica_, known to the natives as babul,
or babool, has quite a contrary effect, for it contains a large
quantity of red colouring matter, which is incorporated with the
leather in the tanning process, and although it shows very little
in the rough-tanned leather, it is apt to darken if the finished
leather is exposed to strong light for a long period. This oxidation
is accompanied by a gradual weakening of the fibres of the leather,
which is, therefore, quite unsuitable for bookbinding. Fortunately,
Indian tanners are making rapid progress in using other tanning
materials, a good number of which are found in India.
Although the bark of the _acacia arabica_ is not altogether
satisfactory, the pods of the same tree, which are commercially known
as "bablah" and contain nearly twice as much tannin as the bark,
produce a very light-coloured, almost white, leather, and it is
asserted that this material is a valuable substitute for sumach.
MINERAL TANNAGES
By far the most important of the mineral tannages is the chrome
process, the merits and demerits of which have not only been
freely discussed in the trade, but also in the lay press. Fanciful
theories of the poisonous character of this kind of leather have
been published from time to time in the daily press. Such absurd
statements as that prussic acid and mercury are used in the
manufacture of chrome leather hardly need refuting, as, even if they
could convert skins into leather, the cost would prohibit their use.
The only poisonous acid used in one of the many chrome processes is
chromic acid, but this is converted into the oxide of chromium in a
second bath and is, therefore, made quite innocuous.
The chrome tannage is effected either by the one-bath method or the
two-bath. In the former case, the tanning agent, either a basic
chromium sulphate or chloride, is present in the one liquor used; in
the latter method, the hides or skins are impregnated with a solution
of chromic acid, which is reduced to chromic oxide in a second bath
consisting of sulphurous acid and a small quantity of free sulphur.
The properties of leather produced by the two-bath process render it
especially suitable for vulcanising on rubber; hence its large use
for non-skidding bands for motor tyres.
There are several recipes for making one-bath liquors. A favourite
mixture consists of chrome alum and sodium carbonate (common soda).
Another method consists of reducing a solution of chromic acid with
glucose or grape-sugar. This liquor has a greater plumping effect
on the leather than the chrome alum liquor has. A third process of
making a one-bath liquor combines the use of bichromate of potash and
chrome alum, which, when dissolved, is converted into a basic chrome
salt by means of a reducing agent. The one-bath liquor can be easily
and safely applied to hides and skins, and is used in much the same
manner as a vegetable tan liquor, beginning with a weak solution and
gradually increasing the strength until the process is completed.
The two-bath method needs great care, as a slight difference in the
proportions of the ingredients used may alter the character of the
leather produced.
The formula now largely used is practically the same as that of the
original patentee, Augustus Schultz, an American chemist. The first
bath, the chromic acid solution, is made by treating bichromate
of potash or soda with hydrochloric, sulphuric, or formic acids.
Bichromate of potash and hydrochloric acid (commonly known as
muriatic acid, or spirits of salts) are commonly used, and in the
proportion of 5 per cent. and 2-1/2 per cent. of the weight of the
drained pelts (5 lb. of bichromate and 2-1/2 lb. of acid for 100 lb.
of pelts). The chemical reaction is represented by the following
equation--
K_{2}Cr_{2}O_{7} + 2HCl = 2KCl + 2CrO_{3} + H_{2}O
Bichromate of potash + Hydrochloric Acid
= Potassium chloride + chromic acid + water.
This process is most conveniently carried out in the paddle-vat
(Fig. 25), which, in this case, should be fitted with a wooden cover
to exclude light, since the colour of the chromic acid liquor is
affected by strong light. Some chrome tanners prefer to use the drum
tumbler (Fig. 13), but the pounding of the skins by this method is
apt to make the grain loose. Whichever method be adopted, the hides
or skins should be horsed up to drain for several hours before
transferring them to the second bath. It is important that they be
placed grain to grain and smoothed out, as creases and air bubbles
between the skins become fixed in the second liquor, and depreciate
the value of the finished leather. To avoid this danger, many tanners
pass the hides or skins through a striking-out machine under light
pressure. Another detail of importance is to cover the chromed skins
with canvas or matting to keep them from the light. Although the
skins are preserved by chromic acid, they are not made into leather,
for in this condition they would dry quite horny. Hence, it is
undesirable that the hides should be dried at this stage and sold
as leather, although such a proceeding has been attempted. Further,
chromic acid is an irritant poison which may cause an eruption on
the hands and arms of workers handling the hides in this solution,
unless they are protected by rubber gloves, or by coating the hands
with a mixture of vaseline and lanoline. The second bath consists of
chemicals which reduce the chromic acid to the oxide, which is quite
inert, so that there is no danger whatever of contracting a poisoned
foot as the result of wearing chrome leather. The chemicals largely
used for the second bath are hyposulphite of soda and hydrochloric
acid. Sodium sulphite may also be used without the addition of acid.
The skins change from an orange colour to a pale, bluish-grey tint,
but the process is not completed until a cross section shows that
the colour has changed right through the skin. A suitable proportion
is 10 per cent. "hypo" and 2-1/2 per cent. acid (28° Twaddell)
calculated on the weight of the drained pelts. Calf, goat, and sheep
skins are usually treated in the drum, but hides, especially if a
certain degree of firmness is required, are preferably run in the
paddle-vat. The reaction of the chemicals in the second bath are
somewhat complicated, but the principal point is the reduction of the
chromic acid (CrO_{3}) to chromic oxide (Cr_{2}O_{3}).
The sulphurous acid produced acts as the reducing agent, but is not
freely liberated until towards the end of the process, the skins
first changing into a dirty brown colour which gradually gives place
to a beautiful pale-bluish tint. The last stages of the process are
also marked by the formation of free sulphur, which aids materially
in softening the leather, and giving the two-bath chrome tanned
leather its characteristic rubbery texture. The vats in which this
process is carried out should always be fitted with lids to confine
the strong sulphurous fumes. By adding an excess of "hypo," the skins
can be bleached until they are nearly white. This has no harmful
effect on the leather, but makes it softer, though somewhat looser
on the grain. When completely tanned, the skins are horsed up again,
left to drain for at least twenty-four hours, and are then ready for
dressing and finishing.
The recipes for the one-bath process are numerous, but it is becoming
a common practice for tanners to buy the liquors or extracts already
prepared in chemical works, which are, naturally, better fitted
up for the production of a more uniform material than it would be
possible to make in most tanneries.
COMBINATION TANNAGES
Combination tannages have steadily grown in favour during the last
few years, and will probably have an important bearing on future
methods of tanning. Among those found practicable are: (1) Vegetable
and chrome; (2) vegetable and alum; (3) alum and chrome; (4)
synthetic and natural tannins; (5) synthetic and chrome; (6) alum and
synthetic; (7) formaldehyde and chrome; (8) chrome and iron.