SOLE-LEATHER:--Unhairing Hides.
In England, lime is the agent almost universally employed for
loosening the hair, though every tanner admits its deficiencies and
disadvantages. It is hard, however, to recommend a substitute which is
free from the same or greater evils, and lime has one or two valuable
qualities which will make it very difficult to supersede. One of these
is that, though it inevitably causes loss of substance and weight,
it is also impossible, with any reasonable care, totally to destroy
a pack of hides by its use; which is by no means the case with some
of its rivals. Another advantage is that, owing to its very limited
solubility in water, it is a matter of comparatively small consequence
whether much or little is used; and even if the hides are left in a
few days longer than necessary, the mischief, though certain, is only
to be detected by careful and accurate observation. With all other
methods, exact time and quantity are of primary importance, and it is
not easy to get ordinary workmen to pay the necessary attention to
such details. Again, the qualities of lime, its virtues and failings,
have been matter of experience for hundreds of years, and so far as
such experience can teach, we know exactly how to deal with it. A new
method, on the other hand, brings new and unlooked-for difficulties,
and often requires changes in other parts of the process, as well as
in the mere unhairing, to make it successful. As our knowledge of the
chemical and physical changes involved becomes greater, we may look
to overcoming these obstacles more readily; for the power of dealing
successfully with new difficulties constitutes one of the main
advantages of a really scientific knowledge over an empirical one.
Slaked lime is soluble in water at 60° F. (15° C.), to the extent of
1 part in 778. Unlike most substances, it decreases in solubility at
higher temperatures, requiring 972 parts of water at 130° F. (54° C.),
and 1270 parts at 212° F. (100° C.). Its action upon animal tissues
increases rapidly, however, with temperature, though no doubt it is
moderated to some extent by the lessened solubility. Calculating
from Dalton's numbers, pure lime-water at 60° F. (15° C.) contains
1·285 _grm._[R] of CaO per _litre_, and should require 459 _c.c._
of decinormal acid to neutralise it. This estimate in some cases
appears to be slightly too high; e. g. a saturated lime-water from
Carboniferous limestone at 56.5° F. (13° C.) required only 433 _c.c._
of decinormal acid, which equals 1·211 _grm._ of CaO per _litre_, and
this lime-water, kept with excess of lime, gave nearly constant results
for many months together. A magnesian limestone lime-water tested at
the same time required 472 _c.c._ of decinormal acid, confirming the
old observation of tanners, that such lime is stronger than that made
either from chalk or carboniferous limestone. This increased strength
must arise from the presence of some soluble base other than lime, and
may be due to the magnesia, which, however, is very slightly soluble.
Magnesian limestone contains a very large amount of magnesia, and
hence would not go so far as a purer limestone; but as a very large
proportion of the lime ordinarily used is thrown away undissolved, this
is perhaps of little practical moment. (For the chemical examination of
limes, see p. 102).
[Footnote R: 1 _grm._ per _litre_ is very approximately equal to 1 oz.
per cub. ft.]
The action of lime on the hide has already been spoken of to some
extent. It is throughout a solvent one. The hardened cells of the
epidermis swell up and soften, the _rete malpighi_ and the hair-sheaths
are loosened and dissolved, so that, on scraping with a blunt knife,
both come away more or less completely with the hair (constituting
"scud," as some English tanners name it, Ger. _gneist_ or _grund_).
The hair itself is very slightly altered, except at its soft and
growing root-bulb, but the true skin is vigorously acted on. The fibres
swell and absorb water, so that the hides become plump and swollen,
and, at the same time, the "cement-substance" (coriin) is dissolved,
the fibres become differentiated into finer fibrils, and the fibrils
themselves become first swollen and transparent, and finally corroded,
and even dissolved. This swelling of the fibres is produced both by
alkalies and acids, and is probably due to weak combinations formed
with the fibre-substance, which have greater affinities for water than
the unaltered hide. It is useful to the tanner, since it renders the
hide easier to "flesh" (i. e. to free from the adhering flesh), on
account of the greater firmness which it gives to the true skin. It
also assists the tanning, by opening up the fibre, and so exposing a
greater surface. This is advantageous in dressing leather which is
afterwards tanned in sweet liquors, and must have the cement-substance
dissolved and removed for the sake of flexibility; and, in the case of
sole-leather, it is necessary for the sake of weight and firmness that
the hide be plumped; but it is probable that the effect is produced
with less loss of substance and solidity by suitable acidity of the
liquors. A more certain advantage of lime is that it acts on the fat of
the hide, converting it more or less completely into an insoluble soap,
and so hindering its injurious effects on the after tanning process,
and on the finished leather. If strong acids are used later on, this
lime soap is decomposed, and the grease is again set free. In sweated
or very low-limed hides this grease is a formidable evil.
The customary method of liming is simply to lay the hides flat in milk
of lime in large pits. Every day, or even twice a day, the hides are
drawn out ("hauled"), and the pit is well plunged up, to distribute
the undissolved lime through the liquor. The hides are then drawn in
again ("set"), care being taken that they are fully spread out. How
much lime is required is doubtful, but owing to its limited solubility,
an excess, if well slaked, is rather wasteful than injurious. Great
differences exist in the quantity of the lime used, the time given,
and the method of working. Lime, as we have seen (p. 140), is only
soluble to the extent of about 1·25 _grm._ per _litre_, or (as 1 cub.
ft. of water weighs about 1000 oz.) say 1-1/4 oz. per cub. ft., or,
in an ordinary lime-pit, not more than 1/4 lb. per hide. Only the
lime in solution acts on the hide, but it is necessary to provide
a surplus of solid lime which dissolves as that in the liquor is
consumed. Jackson Schultz prescribes 1 bush. (56 lb.) of fresh lime to
60-70 hides, and 3-4 days as sufficient time to unhair and plump them;
while a well-known English tanner states that, after working for 6-10
days through a series of old limes, the hides (presumably wet-salted
South Americans) should have 4 days in a fresh lime, made with 3-12
lb. of lime per hide. It is obvious that if the American authority
is right, the English process is wasteful in the extreme, both in
hide-substance and lime. Much depends on the amount of hauling which
the hides receive, and the more frequently they are moved the better.
It is probable, however, that it would be found impossible to unhair
and flesh hides, to suit the English market, in cold limes with the
quantity and time mentioned, and if the limes are steamed, it is quite
likely that the destructive action on the pelt may be even greater than
by the longer and slower process in the cold. Most likely a compromise
between the two is the most desirable, but about 2-4 lb. of lime per
hide, according to weight, should be amply sufficient; while a week
for market hides, and 14 days for heavy salted, will loosen the hair
and plump the pelt as much as is requisite. This is on the supposition
that the limes are kept at a uniform average temperature of about
60° F. (15° C.) in winter and summer. If they are heated to 80°-90°
F. (27°-32° C.), of course much less time is required; but there are
no published experiments showing the relative weights made by the
two processes, and, from the fact that warmed limes are principally
used for descriptions of leather where weight and solidity are not of
primary importance, it may be concluded that, in this direction, the
results are unsatisfactory. Hides do not plump in warm limes.
Another undecided point is whether the best results are obtained by
making fresh limes for every pack, or by strengthening up the old ones.
An old lime becomes charged with decomposing animal matter and with
ammonia, and, within limits, loosens the hair more effectually than a
new one. An experienced tanner states that, by using old limes, better
weights are obtained, but that the leather is thinner than when a fresh
portion of lime is used; and this is quite possible. If, however, the
old lime-liquor be retained too long, it ceases to swell the hides as
it should, and, in warm weather, the liming proper is complicated by a
putrefactive process allied in principle to sweating.
Beside considerable quantities of ammonia, old limes contain tyrosin,
leucin or amidocaproic acid, and some caproic acid, the disagreeable
goaty odour of which is very obvious on acidifying an old lime-liquor
with sulphuric acid, by which considerable quantities of a partially
altered gelatin are at the same time precipitated. Very old limes,
especially in hot weather, often contain active bacteria, which may be
seen in the microscope under a good 1/4-in. objective. Their presence
is always an indication that putrefaction is going forward, and leather
out of such limes will generally prove loose and hollow-grained.
Spherical concretions of calcium carbonate may also be seen under
the microscope, resembling on a smaller scale those found in Permian
limestone, and caused perhaps in both cases by crystallisation from
a liquid containing much organic matter. It is probable that in many
tanneries the ammonia would pay for recovery from the lime-liquors,
which would be easily done by steaming the old limes in suitable
vessels, and condensing the ammoniacal vapours in dilute sulphuric
acid. (Some appliances suitable for this purpose are described in
the Journal of the Soc. of Chem. Industry, iii. 630.) For methods of
estimation of ammonia, see p. 103.
Several variations in the above-described method of liming have been
proposed. The hides may be suspended on laths, or by strings attached
to pegs or notches, and the liquor agitated by plunging in place of
hauling. Probably this is an actual improvement, especially if some
mechanical agitating contrivance be substituted for hand plunging. It
has, however, the drawback that much room is required, though this may
be, to some extent, compensated by the hides liming more quickly. The
method has been long in use in America, and had been tried in several
places in England before the patent of Messrs. Conyers and Pullein was
obtained. Two other American labour-saving methods in connection with
liming may be mentioned here. One is to have the liming-vat double
the ordinary size, and, instead of hauling the hides, to simply draw
them from one side to the other by two strings, which are attached to
the fore and hind shank of each hide, either by sharp iron hooks or
by loops. The strings are looped over iron rods at the four corners
of the pit, or have simple knots, which are placed in notches sawn in
wood. Of course, while the hides are at one side of the pit, the other
side may be plunged or warmed. The other method (Fig. 24) is to have
a spindle sunk below the surface of the liquor, and with discs A, at
each end, to which the hides or sides are attached by hooks set round
the edges. The hides are turned over by revolving the spindle with a
handspike inserted in the holes C, at the ends of the cross-arms B, and
the whole spindle is also capable of being raised and lowered in the
liquor, in the slot D. In Germany, hides are frequently suspended on
laths radiating from a central upright revolving spindle in a round vat
(_Drehkalk_).
An American plan, sometimes known as the "Buffalo method," is described
by Jackson Schultz. The hide is prepared in the usual way, and is then
thrown into a strong lime for 8-10 hours, when it is taken out and
immersed in water heated up to 110° F. (43° C.), in which it remains
24-48 hours. The warm water soaks, softens, and swells the roots of
the hair, and much the same result is obtained as in "scalding" pigs.
So little lime really permeates the inner fibre that, after a slight
wheeling, the hides may be thrown into cold water, and allowed to cool
and plump, preparatory to taking their places in the handlers. The
process is strongly recommended for sole-leather, particularly where
great firmness of fibre is desired. The tanner who tries it must be
satisfied if he gets 20-30 sides a man unhaired and fully ready for the
liquor per diem. Of course this process may be varied to any extent by
giving more liming, and less hot water, and this is frequently done in
America. About 3-4 days' cold liming in good limes, and with hauling
if possible twice daily, followed by 12-24 hours in water at 86°-95°
F. (30°-35° C.), which should be changed at least once, will give good
results. The hides are of course less plump than usual, but if properly
managed in the handlers will swell well in the tan-house. Grease is
obviously less thoroughly "killed" than in the ordinary method, and
especial care must be used that the hides are well worked on the beam,
both on grain and flesh. In this method, and indeed in all liming
processes, much is gained if the fat can be fleshed off green.
[Illustration: Fig. 24.]
On the Continent and in America, the prevalent mode of loosening the
hair, at least for sole-leather purposes, is called "sweating," and
consists in inducing an incipient putrefaction, which attacks the soft
parts of the epidermis and root-sheaths, before materially injuring
the hide-substance proper. The old European method of "warm-sweating"
consisted simply in laying the hides in pile, and, if necessary, in
supplying heat by covering them with fermenting tan; but as this crude
and dangerous process is everywhere being supplanted by the American
plan, where sweating at all is adhered to, it is not necessary to
do more than describe the latter. This is called "cold sweating,"
but really consists in hanging the hides in a moist chamber, kept at
a uniform temperature of 60°-70° F. (15°-21° C.); or in some cases
slightly warmer.
The "sweating-pit" now in use is sometimes of wood, but usually
consists of a building of brick or stone, protected from changes of
temperature, both above, and at the sides, by thick banks of soil or
spent tan. If soil be used, it will form an excellent bed for vines,
&c., which are fertilised by the ammonia penetrating from below,
which is evolved in large quantities and which assists the unhairing
process by its action on the epidermis.[S] Though called a "pit,"
it is undesirable that it should be actually below the level of the
ground, but should be arranged so that the hides can be wheeled in
and out in barrows. It is lighted and ventilated by a lantern roof
above a central passage, and should be divided into chambers, each
capable of suspending a pack of hides. By means of sprinklers above and
steam-pipes below, the chambers may be cooled or warmed, as required,
and the air kept so moist that globules of condensed water collect on
all parts of the hides, which are suspended from tenterhooks.
[Footnote S: Hides have been unhaired by the action of gaseous ammonia
alone, but the method does not seem suited for technical use.]
The process is principally used in America for dried hides, but may
be employed either for wet or dry salted, after complete removal of
the salt. It is imperatively necessary that dried hides should be
completely softened before sweating. As the sweating process advances
more rapidly in the upper than in the lower part of the pit, and as
the thick portions are more resistant than the thin ones, the hides,
after about 3 days' sweating, require constant attention in changing
their positions, and in checking the forward ones by taking down and
laying in piles on the bottom of the pit.
The usual treatment for sweated hides, when the hair is sufficiently
loosened, is to throw them into the stocks, and work out in this way
the slime and most of the hair. This has the disadvantage of working
out too much of the dissolved gelatin, and of fulling the hair so
firmly into the flesh, that it is difficult again to remove it. To
overcome these evils, some American tanners now pass the hides, after
sweating, through a weak lime. This, to a great extent, prevents
the hair fixing itself in the flesh, and tends to counteract the
injurious effect of the vitriol (which is almost invariably used in
plumping sweat stock) on the colour of the leather. By this process,
10,000 Texas and New Orleans wet-salted hides gave an average yield
of leather of 73 per cent. on their green weight, and the leather was
excellent in quality (Schultz). If sweated or very lightly limed hides
are imperfectly worked on the grain, greasy spots are apt to remain,
which will not colour in the liquors ("white spots"). These may be made
to colour by scraping and working the grain with a knife, or by the
application of a solution of soda or soda ash, and would probably be
avoided by the use of soda ash in the soaks on greasy parcels of hides.
It must be clearly understood that all sweating depends on partial
putrefaction. This is proved both by the plentiful production of
ammonia in the pits, and by the fact that antiseptics, such as salt
or carbolic acid, entirely prevent sweating till they are removed.
Although the process undoubtedly has advantages, and especially so in
the treatment of dried hides, it is an open question whether it gives
the extreme gains over liming in weight and firmness, which are claimed
by some of its advocates.
An unhairing process, largely coming into use on the Continent, depends
on the action of alkaline sulphides, and particularly sodium sulphide,
upon the hair. While all the methods already spoken of involve the
softening and destruction of the hair-sheaths, either by lime or
by putrefaction, the sulphides are peculiar in attacking the hair
itself; when strong, they disintegrate it rapidly and completely into
a sort of paste. From very early times to the present day, arsenic
sulphide ("rusma") mixed with lime has been used in unhairing skins for
glove-leather and similar purposes. About 1840, Böttger concluded that
the efficacy of arsenic sulphide was due simply to the sulphydrate of
lime formed by combination of the sulphur with the lime, and proposed
lime sulphydrate, formed by passing sulphuretted hydrogen into milk of
lime, as a substitute for the poisonous and expensive arsenic compound.
It proved a most effective depilatory, but has never obtained much hold
in practice. This is probably due to the fact that it will not keep,
oxidising rapidly on exposure to the air; hence it must be prepared
as it is required, which is both troublesome and expensive. A minor
objection is the unpleasant smell of sulphuretted hydrogen, which is
inseparable from its use.
It was proposed to replace it by sodium sulphide, which, though at
first said to be only effective when mixed with lime, so as to produce
calcic sulphide, has since proved a powerful depilatory alone. Its use
has been greatly extended on the one hand by its production on a large
scale, and in the crystallised form (at first by reduction of sulphate
by heating with small coal), and on the other, by the great interest
which Wilhelm Eitner, the able director of the Austrian Imperial
Research Station for the Leather Trades, has taken in its introduction.
The substance, as manufactured by De Haen, of List, Hanover, is in
small crystals, coloured deep greenish-black, by iron sulphide, which
must have been held in suspension at the time of crystallisation.
If the salt be dissolved in water, and the solution be allowed to
stand, this is gradually deposited as a black sediment, leaving the
supernatant liquor perfectly clear and colourless. Sodium sulphide is
now manufactured from tank waste in a much purer form by Schaffner
and Helbig's process, of which Messrs. Gamble of St. Helens are sole
licencees. The crystallised salt is SNa_{2}10Aq, and therefore contains
69·8 per cent. of water.
For sole-leather, the method recommended by Eitner is to dissolve 4-5
lb. of sulphide per gal. of water, making the solution into a thin
paste (of soupy consistence) with lime or pipe-clay. This is spread
liberally on the hair side of the hides, one man pouring it down the
middle of the hide from a pail, while another, with a mop or cane
broom, rubs it into every part. The hide is then folded into a cushion,
and in 15-20 hours will be ready for unhairing, the hair being reduced
to a paste. In the writer's experience, the concentrated solution here
prescribed will completely destroy all hair wetted with it in 2-3
hours, and if left on longer, will produce bluish patches, and render
the grain very tender. The hides should be thrown into water before
unhairing, to enable them to plump, and to wash off the sulphide,
which is very caustic, attacking the skin and nails of the workmen.
There is no doubt that this process gives good weight, and tough and
solid leather; but there are several difficulties attending its use.
Unless the mopping is done with great care, it will fail to completely
destroy the hair, and the patches of short hair left are very difficult
to remove. The expense of the material and the loss of hair are also
important considerations. The hides are rather difficult to flesh,
unless previously plumped by a light liming, and it is necessary to
swell them with acid or sour liquor in the tanhouse, as the sulphide
has but little plumping effect.
Another method, which is much cheaper in labour and easier in
execution, is to suspend in a solution of sodium sulphide, containing
3/4 lb. a hide or upwards; the hide should unhair in 24 hours. Very
weak solutions loosen the hair, without destroying it; but it is
always weakened, as the specific action of the sulphides is on the
hair itself. After or before unhairing, the hides may receive a
light liming, to plump them, or lime may be added to the solution of
sulphide, which by forming calcium sulphide, and liberating caustic
soda, considerably increases the unhairing and plumping effect. The
pit may be several times strengthened for successive packs, but
the loosened hair must be fished out, or it will quickly spoil the
solution. When hides have been suspended in sodium sulphide solution,
the hair is very quickly loosened by a short liming. Squire, Claus,
and J. Palmer have all taken out patents for the use of tank-waste
as a depilatory. It consists of impure calcium sulphides, and when
brought into the form of soluble sulphydrate, either by boiling in
water, or by the oxidising action of the air, it will unhair hides. The
conversion is, however, very imperfect in either case, and its action
is uncertain and slow; while the iron present is apt to cause unsightly
stains. It is probable that the weights obtained may somewhat exceed
those by liming. Palmer employs sulphuric acid to plump the hide and
remove stains, and then reduces it by a bate of whiting and water. He
claims that this prepares the hide for rapid and heavy tanning, but
the swelling and subsequent reduction almost certainly entail loss of
weight and quality, and to get good results the bate should at most
only be allowed to have a superficial effect. Professor Lufkin proposed
the use of a mixture of various sulphides of lime and soda, formed by
mixing 10 lb. each of soda ash and sulphur, kneading to a paste with a
little moist slaked and then mixing _warm_ in a cask with 80 lb. stone
lime slaked to a paste. This quantity will unhair 50 hides in the same
way and in about the same time as an ordinary lime. The pelt is not
much plumped and is easily reduced by a few minutes' wheeling in warm
water. (J. S. Schultz.)
Various other depilatories have been proposed, but as they have not
come into general use, brief mention of the most important will
suffice. Anderson, in 1871, patented the use of wood-charcoal, applied
in a similar manner to lime in the ordinary process. The hair is
probably loosened simply by putrefaction, as in sweating, while the
charcoal acts as a deodoriser, very little smell being produced, and
the action proceeding with considerable uniformity. John Palmer has
patented a process for unhairing, in which the hides are alternately
steeped in water and exposed to the air till the hair loosens. In this,
very similar principles to those of the charcoal method are involved.
Caustic potash and soda will loosen hair, but seem to have no decided
advantage over lime, though it is quite possible that in skilful
hands good results might be obtained. They are more costly, and their
corroding action on the hide-substance is more powerful, but they form
soluble soaps with the grease of the hide. Unless used in very dilute
solution, the pelt is so swollen as to fix the hair, and the leather is
dark-coloured and spongy. Soda-ash or crystals (sodic carbonate) may be
used to strengthen ordinary limes, in which caustic soda is formed. The
time of liming is shortened, the hides are more swollen, and the grease
is better "killed" than when lime alone is used. The patent for Moret's
"Inoffensive" claimed the use of the carbonate or caustic potash formed
from calcined wool-washings, for unhairing. This is more costly than,
and has no advantage over soda. I am not aware whether "Inoffensive,"
as now sold, has other constituents.
Whatever method of loosening the hair may be adopted, the next step
is to remove it by mechanical means. This is usually accomplished by
throwing the hide over a sloping beam, and scraping it with a blunt
two-handled knife (Fig. 25), the workman pushing the hair downwards
and away from him. The beam is now usually made of metal. The knife
employed is also shown at C, Fig. 26.
When a hide is lightly limed, it is often easy to remove the long
hair, but excessively difficult to get rid of the short under-coat
of young hairs, which are found in spring, and which can sometimes
only be removed by the dangerous expedient of shaving with a sharp
knife. The reason of this difficulty is obvious: not only do the short
hairs offer very little hold to the unhairing knife, but, as has been
explained in describing the anatomical structure of the skin, their
roots are actually deeper seated than those of the old hairs they
replace. Several attempts have been made to unhair by machinery, but
so far without such success as to lead to their general adoption. The
fleshing-machine invented by Garric and Terson, and manufactured in
this country by T. Haley and Co., of Bramley (Fig. 27), is furnished
with a special wheel for unhairing. An American machine for the
purpose, invented by J. W. Macdonald, and said to be capable of
unhairing 800 sides a day, is shown in Fig. 28.
[Illustration: Fig. 25.]
[Illustration: Fig. 26.]
[Illustration: Fig. 27.]
[Illustration: Fig. 28.]
When the hair is very thoroughly loosened, as by sweating, or
destroyed, as by sodium sulphide, it is not uncommon to work it off by
friction in the stocks; but it is very doubtful whether the saving of
labour is not more than compensated by the loss of weight, consequent
upon submitting the hide while its gelatin is in a partially dissolved
condition, to such rough usage.
[Illustration: Fig. 29.]
After unhairing, the loose flesh and fat are removed from the inner
side of the hide by a sharp-edged knife E (Fig. 26), partly by brushing
or scraping, partly by paring. It is necessary not only to cut off the
visible adhering fat, but to work the hide well, so as to force out
that contained in the loose areolar tissue, which would not only impede
tanning, but is liable to soak completely through the hide, producing
most unsightly blotches. Several machines have been introduced to
supersede hand-fleshing, but with only partial success. One of the
best is Garric and Terson's machine (Fig. 27), which gives a very
level flesh, free from galls, and without so much loss of weight, but
scarcely so clean as desirable, while the saving in labour is not
great. Molinier's machine (Fig. 29), and that of Jones and Rocke, are
well adapted for skins, but hardly capable of fleshing an entire hide.
All these machines are very similar in principle, the working parts
consisting of drums with oblique or spiral knives.
When unhaired and fleshed, the hides intended for sole-leather are, in
England, almost invariably "rounded," or separated into (1) "butts,"
which are the best and thickest parts, and receive the most solid
tannage, and (2) "offal," which is thinner, and for which a cheaper and
more rapid tannage is sufficient. Fig. 30 shows the customary division.
Frequently the butt is divided down the centre, and the halves are then
called "bends." A piece called a "middle" is sometimes taken between
the butt and the shoulder.
[Illustration: Fig. 30.]
After rounding, it is necessary to get rid of the lime, as completely
as possible, before taking into the tan-house. For this purpose,
the butts are usually suspended in fresh water for 12-24 hours, and
frequently shaken up in it to remove adhering lime and dirt. If
the water is hard, it is best to add to it, before putting in the
butts, a few pailfuls of clear lime-water, to precipitate the lime
bicarbonate,[T] which would otherwise cause a deposit of chalk on the
surface of the butts; this would not only make the grain harsh, but
afterwards, by combining with the tannin of the liquors, would cause
bad colour. For the same reasons, it is important that limey hides
should be as little exposed to the air as possible, as the latter
always contains a small amount of carbonic acid, which renders the lime
insoluble.
[Footnote T: Lime softens water containing lime bicarbonate in
solution by combining with half the carbonic acid, when the whole is
precipitated as normal carbonate or chalk. CaO + CaCO_{3} . H_{2}CO_{3}
= 2CaCO_{3} + OH_{2}. This is Clark's process. See also p. 84.]
This suspension in water is frequently considered sufficient for
sole-leather, but it removes the lime very imperfectly. In olden days,
it was customary not only to wash the hides much more thoroughly in
water, but to "scud" them (i.e. work them over with a blunt knife),
to remove lime, and the detritus of hair-roots and fat-glands, and
this should never be omitted from sole-leather treatment where bright
colour and clean buff are desired. Some tanners go so far as to bate
best butts slightly with hen-dung, but with such treatment firmness
and weight are lost. Washing in weak solution of sugar, or ammonic
chloride or sulphate, or of sulphuric, or hydrochloric acid, may be
adopted. It is essential to use acids nearly free from iron, as it may
be precipitated on the butts and give a bluish colour in the liquors,
and the acid must be of such a strength as neither to allow the iron
to be precipitated, nor, on the other hand, perceptibly to plump the
butts, which in this stage would endanger buff and colour. 100 _cc._
may neutralise 15-20 _cc._ of lime-water for this purpose. Hydrochloric
acid and chlorides have a tendency to prevent plumping, and are
therefore better adapted for dressing than for sole leather. Great
care must also be taken to prevent putrefaction, or the use of putrid
solutions, if firmness and plumpness are desired.