the evolution of considerable heat, becoming slaked or converted into
hydrate, Ca(OH)₂. This change takes place rapidly and easily when the
lime is light and porous, such as is obtained by the burning of chalk or
good limestone at a low temperature; but if it has been too intensely
heated or “over-burnt,” or contains silicates or other salts which fuse
at the temperature of the kiln, a compact lime is formed which slakes
with difficulty and extreme slowness, thus being lost to the tanner, or
leading to the still more serious result of burning holes in the hides
by the heat produced by slaking in contact with them. It is stated by Le
Chatelier[69] that for dense limes 24-48 hours is frequently required
for complete slaking in the cold, while magnesia is still more
obstinate, months being sometimes necessary for the complete hydration
of hard-burnt samples; and mixtures of lime and magnesia are
intermediate in their character. Slaking is greatly assisted by heat,
even heavily burnt magnesia being hydrated in about six hours at 100° C.
Slaking is also much more rapid in a dilute solution (2 per cent.) of
calcium or magnesium chloride. From these facts it is easy to deduce the
reason why a suitable quantity of water, neither too much nor too
little, is desirable for the rapid and effectual slaking of lime. If too
little is used, the lime is only partially slaked, and it is not easy
for further portions of water to gain access to the interior of the
powdery mass. On the other hand, if it is “drowned” by excess, the
temperature is lowered, the process goes on slowly, and the mass does
not readily fall into powder, and so fails to be utilised in the liming
process. Of all methods of slaking lime, the ordinary one of tipping it
direct into the lime-pits is perhaps the most irrational, leading to the
formation of unslaked lumps which may burn the hides, and which,
together with stones and dirt, rapidly choke the pits with useless
matter. The best process is that adopted by builders and in many
Continental yards, in which a large quantity of lime is slaked in a
shallow tank by throwing on it sufficient water to thoroughly wet it,
and after allowing it to heat and fall for 24 hours, adding enough water
to convert it into a stiff paste. In this form it may be kept for months
without material deterioration. When required for use, a suitable
quantity of the paste is dug out, and well stirred with water in a tub
or tank before running into the pit when the stones and sand remain in
the tank. In this way all nuisance from dust is also avoided. If lime is
stored unslaked, it gradually absorbs moisture from the air, falling,
and soon becoming dusty and difficult to slake completely, while the
traces of carbon dioxide in the air gradually convert it into useless
carbonate.
[69] Bull. de la Soc. d’Encouragement, 1895, x. pp. 52-62; Journ. Soc.
Chem. Ind., 1895, p. 575.
The solubility of lime in water is very limited, and the figures
determined by different chemists do not agree very satisfactorily. The
following table gives the result of determinations made by Mr. A.
Guthrie in the Author’s laboratory, and is probably one of the most
accurate[70]:--
100 c.c. of saturated lime water at 5° C. contain 0·1350 grm. of CaO.
„ „ 10° „ 0·1342 „
„ „ 15° „ 0·1320 „
„ „ 20° „ 0·1293 „
„ „ 25° „ 0·1254 „
„ „ 30° „ 0·1219 „
„ „ 35° „ 0·1161 „
„ „ 40° „ 0·1119 „
„ „ 50° „ 0·0981 „
„ „ 60° „ 0·0879 „
„ „ 70° „ 0·0781 „
„ „ 80° „ 0·0740 „
„ „ 90° „ 0·0696 „
„ „ 100° „ 0·0597 „
[70] Journ. Soc. Chem. Ind., 1901, p. 224.
It will be noticed that unlike that of most substances, the solubility
of lime in water diminishes as the temperature is raised. It is
therefore necessary in employing lime-water as a standard solution to
take care that it is saturated at a constant temperature. The results
given in the above table are those from pure marble lime. Where the
ordinary impure limes from limestone are employed, a somewhat stronger
lime-water is often obtained. This is difficult to explain, but possibly
some double hydrate of lime and magnesia is formed which is more
soluble than either hydrate alone. The results harmonise with the old
belief of tanners that chalk-lime is milder in its action on skin than
that made from less pure limestones. The solubility of any given lime is
easily determined by adding it in excess to water in a stoppered flask,
and shaking frequently until a solution of constant strength is
obtained. A known volume of this solution (which must be clear and free
from undissolved lime) is then titrated with N/10 hydrochloric acid,
using phenolphthalein as the indicator.
Saturated lime-water may be conveniently used as an alkaline standard
solution for many purposes, and if kept on excess of lime is always
caustic, and varies very little in strength at ordinary laboratory
temperatures. The solution is nearly ¹⁄₂₀ normal, but for accurate work
its strength should be exactly determined with N/10 acid. 1 liter of
pure lime-water at 15° C. should require 471·4 c.c. of N/10 acid for
neutralisation.
Lime is much more soluble in sugar solutions than in water. Such
solutions have been used as standard solutions, and sugar has been added
to limes to increase the action on the hides.
The following is the analysis of a lime used in a Leeds tannery, which
was made by Mr. G. W. Flower, B.Sc., in the Leather Industries
Laboratory of the Yorkshire College[71]:--
Per cent.
SiO₂ and insoluble matter 17·70
Fe₂O₃ 6·42
CaO 49·86
CaCO₃ 14·21
CaSO₄ 3·01
CaCl₂ 0·33
MgO 2·09
Organic matter 0·80
Moisture by difference 5·58
------
100·00
[71] Journ. Soc. Chem. Ind., 1901, p. 224.
The sample only contained 31·02 per cent. of available lime, the
remainder being probably combined with the silica. It also contained an
appreciable quantity of iron oxide, which might lodge mechanically in
the pores of the skin and become dissolved in later processes, darkening
the colour of the leather. The lime was also under-burnt, judging from
the amount of carbonate it contained.
For comparison with this, the analysis of a good specimen of
carboniferous-limestone lime from Buxton may be given:--
Per cent.
CaO 91·95
MgO 1·30
CO₂ and moisture 6·75
------
100·00
_Determination of “Available” Lime._--The practical value of lime for
the tanner is easily determined by drawing a sample by breaking off
small pieces from a number of lumps of the bulk, coarsely pulverising
them in a mortar, and then rapidly grinding a portion as fine as
possible, and transferring it at once to a stoppered bottle for
weighing. A portion of this, not exceeding 1 grm., is shaken into a
stoppered liter flask, which is filled up roughly to the mark with hot
and well-boiled distilled water, and allowed to stand for some hours
with occasional shaking. When cold it is filled exactly to the mark with
cold distilled water, well shaken again and allowed to settle, or
rapidly filtered, and 25 or 50 c.c. of the clear liquid withdrawn with a
pipette and titrated with N/10 hydrochloric or sulphuric acid and
phenolphthalein. Each cubic centimeter of N/10 acid equals ·0028 grm.
CaO. It is generally a very mistaken economy to make use of an inferior
lime for tanning purposes, as any saving in cost is discounted by the
larger quantity required, the more frequent cleaning of the pits, and
the danger of stains and of burns from imperfect slaking.
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 mucous or growing layer 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” or “scurf,” 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” of the fibres is dissolved, and
they become split-up into finer fibrils: the fibrils themselves become
first swollen and transparent, and finally corroded, and even dissolved.
A similar swelling of the fibres is produced by both 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.[72] This swelling 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 splitting up the fibre into its
individual fibrils, and so exposing a greater surface to the action of
the liquors. This is advantageous in dressing-leather which is
afterwards tanned in sweet liquors, and which must have the
cement-substance of the fibres dissolved and removed for the sake of
flexibility; and, in the case of sole-leather, it is necessary for sake
of weight and firmness that the hide be plumped at some stage of the
process; but it is probable that this effect is produced with less loss
of substance and solidity by suitable acidity of the tanning-liquors.
Another advantage of lime is that it acts on the fat of the hide,
converting it more or less completely into an insoluble soap,[73] and so
hindering its injurious effects on the after tanning process, and on the
finished leather. If strong acids whether mineral or organic 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, causing darkening or grease spots on the finished leather.
[72] Cp. p. 84.
[73] This has been questioned, but I have satisfied myself it is
correct.
The customary method of liming is simply to lay the hides horizontally
one at once in milk of lime in large pits, taking care that each hide is
completely immersed before the next is put into the pit, so as to ensure
a sufficiency of liquor between them. 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, not only for various classes of
leather, but for the same kinds in different yards. Lime, as we have
seen, is only soluble to the extent of about 1·25 grm. per litre, or (as
1 cub. foot of water weighs about 1000 oz.) say 1¹⁄₄ oz. per cub. foot,
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
or absorbed by the hide; and this is especially the case where, as is
generally customary, the hides are laid flat in pits, so that no
circulation of liquor is possible. Where hides are suspended in
lime-water, which is constantly circulated and kept up to its full
strength by agitation in another vessel with solid lime, they unhair as
quickly as with milk of lime, but the method seems, in the case of lime,
to present no special advantage over the ordinary one, if in the latter
the hides are hauled sufficiently often to keep the lime uniformly
distributed. The case is otherwise in dealing with more soluble
depilatories. Various patents have been taken for methods of liming by
suspending in liquors, but the idea is now public property, and is
largely used on the Continent. It is necessary that the lime which
settles to the bottom of the pit should be agitated and kept in
suspension, which may be effected either by moving the hides on a frame
as in “suspenders” (p. 221), or by agitators acting on the principle of
pumps, and raising the liquor and sludge from the bottom. Such agitators
have been patented in Germany, but had been in use much earlier in the
Author’s tanyard. An agitator on the principle of the screw-propeller of
a steamship, placed near the bottom of the pit, and protected by a
lattice, may also be usefully employed (Fig. 26). Skins are frequently
limed in paddles, or stirred up by blowing air into the pit. The latter
method is neither effective nor economical in power.
[Illustration: FIG. 26.--Suspension Lime-Pit.]
As has been noted, the solubility of lime, and consequently the strength
of the lime-liquor, is diminished by rise of temperature, but its
solvent action on hide-substance is much increased. As a consequence,
the loosening of the hair proceeds much more rapidly in warm limes, but
the hides do not plump well, and become loose, hollow and inclined to
“pipe” in the grain, and to weigh out badly, and for sole leather the
method is therefore in every way disastrous. In the few cases among the
lighter leathers where a decided softening and loosening of the texture
of the skin is required, it is possible that useful advantage may be
taken of this effect; but it would be exceedingly difficult to regulate
the temperature of an ordinary lime-pit with accuracy, and better
results could probably be obtained with suspenders in which the liquor
could be constantly circulated. When limes are very cold, in spite of
the greater strength of solution, the action is very much checked, and
where goods are frozen into pits in severe weather, there is but little
danger of overliming, although the usual time may be much exceeded. It
is generally best to work limes at about the ordinary summer
temperature, and this is better done in winter by warming the limeyard
than by any direct heating of the limes. If lime which has cooled after
slaking is used, the water with which limes are made may safely be
warmed in midwinter to a temperature not exceeding 20° C.
The quantity of lime used by different tanners, and for different sorts
of hides and skins, is very variable, not only according to the effect
which it is desired to produce, and the way in which it is used, but
from the arbitrary fancy of the user, since its limited solubility
renders an excess comparatively innocuous. For sole-leather, the amount
recommended varies from under 1 per cent. to 10 or 12 per cent. on the
green weight of the hide; but probably 2-3 per cent. is all that can be
really utilised, the remainder being wasted. In order, however, to
utilise the whole of the lime, very frequent handling or agitation is
required to ensure its uniform distribution. It must also be borne in
mind that the strength of commercial limes varies from above 80 down to
30 per cent. of available calcium oxide.
Von Schroeder has found that a strength of 6 grams of calcium oxide
(CaO) per liter was sufficient, but, in practice, much more is generally
added. It is also noteworthy that a perfectly fresh milk of lime must be
made much stronger than one which has been used. This is partially due
to the fact that some bacterial action takes place in an old lime and
that ammonia is formed which assists unhairing, in addition to the
effect of the lime itself, and partially because the lime in old liquors
remains in suspension for a much longer time, and is thus more evenly
distributed.
A method of liming, sometimes known as the “Buffalo method,” has been
largely adopted for sole-leather in America, and is now used in many
Continental yards. It consists in a very short liming and the subsequent
use of warm water. The limes are also often sharpened by the addition of
a little sodium sulphide or of some other sulphide. Thus, in one large
yard in the States, the hides for sole-leather (salted “packers”) are
limed for 10 hours only with 2 lb. lime and 2¹⁄₂ oz. of sulphide of
sodium per side, and after lying overnight in water of a temperature of
35-45° C., are easily unhaired. A Continental firm lime two days in weak
fresh limes with a little tank-waste, and then treat with water at 32°
C. for 6-8 hours, when the hides are unhaired and returned to the warm
water for two hours before scudding. All sorts of combinations between
liming and hot water treatment can be employed. The longer and stronger
the liming, the lower temperature or shorter time in the water will
suffice. The method is much to be recommended for firm sole-leather, but
it does not saponify grease or swell the fibres thoroughly, and usually
vitriol is used for the latter purpose in a later stage. The hide goes
into the liquors practically free of lime, and the loss of
hide-substance is much less than in the ordinary method of liming.
A point of probably much greater importance than the quantity of lime
used is the length of time during which a lime is worked without change
of liquor. An old lime becomes charged with ammonia and other products
of the action of lime upon the skin, such as tyrosin, leucin
(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 partially altered gelatin are
at the same time precipitated (compare p. 64). Lime has considerable
antiseptic power, and a new lime is practically sterile, but very old
limes, especially in hot weather, often contain large numbers of active
bacteria, which may be seen in the microscope under a good ¹⁄₆-inch
objective. Their presence is always an indication that putrefaction is
going forward, and if their number be very excessive, the leather out of
such limes will generally prove loose, hollow and dull-grained, and in
extreme cases hides may be totally destroyed. 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 hardly probable that in many tanneries the ammonia would
pay for recovery from the lime-liquors, though it could be easily done
by steaming the old limes in suitable vessels, and condensing the
ammoniacal vapours in dilute sulphuric acid. Its quantity rarely exceeds
0·1 per cent. of NH₃. For methods of estimation of ammonia, see
L.I.L.B., p. 30.
Up to a certain point, it is found that old limes unhair much more
readily, and have a greater softening effect than new ones, which is
often advantageous for dressing goods; though for sole leather, where
weight and firmness are of primary importance, the use of stale limes
must be kept within the narrowest limits. In the finer leathers also,
such as kid and moroccos and coloured calf, where a sound and glossy
grain is desired, the effects mentioned are generally better obtained in
other ways, such as by the use of sulphides. On East India kips and
other dried hides, which are difficult to soften, and which have great
power of resistance to the action of lime, old limes are distinctly
useful, but, even there, there are limits which should not be passed.
Probably no lime ought to be allowed to go for more than three months at
the outside limit without at least a partial change of liquor, and the
system of allowing all the limes in a yard to run for twelve months, and
then cleaning them all together, is almost the worst which can be
planned. A very much better way is to clean the limes in regular
rotation, using, if desired, a portion of the old liquor in making the
new lime, so as to avoid a too sudden transition. The old liquor is
valuable, if at all, for the ammonia and organic matter which it
contains, as the amount of lime in solution is not worth considering.
The ammonia considerably increases the solvent and unhairing power,
while swelling the hide less than an equivalent amount of lime. In some
cases it may be desirable to add ammonia artificially for this purpose.
In this case it will be cheaper and more convenient to add it in the
form of ammonium sulphate than as liquid ammonia. If it be desired to
retain ammonia, the lime should be kept covered. Very old limes
containing excess of ammonia and lime, sometimes in hot weather cause a
transparent swelling of the goods, with destruction of the fibrous
texture.[74] The writer has observed a similar phenomenon in very weak
and old limes strengthened with sulphides, in which hide was left
experimentally for several weeks. The principal effect of the dissolved
animal matter is to enable bacteria to thrive in it, which they will not
do in a fresh lime, but putrid limes probably also contain liquefying
ferments produced by the bacteria present (p. 17), and which dissolve
hide. Eitner has published researches on the amount of hide-substance
dissolved by limes,[75] in which he shows that the loss of substance in
liming sufficiently to unhair is materially greater in old limes than in
fresh ones, although during the first two days of liming the new limes
are decidedly the most active. As he remarks, this justifies the wisdom
of the method, now largely adopted, of working limes in shifts, and
beginning the operation in old limes and completing it in fresh ones.
(See also p. 131.)
[74] Gerber, 1884, pp. 150, 184.
[75] Gerber, 1895, pp. 157-9, 169-72.
For details of the analytical methods employed, Eitner’s original paper
must be consulted, but the annexed table (see next page) summarises his
results. The letters heading the columns have the following meanings.
A. Hide substance precipitated by neutralisation of the lime with
carbonic acid.
B. A further precipitate obtained by slight acidification with
hydrochloric acid.
C. Soluble peptones precipitated by hypochlorous acid or mercuric
nitrate.
It is obvious that none of these figures represent the _total_ dissolved
organic matter, and it is to be regretted that this was not determined.
It is, however, fairly safe to assume that the table correctly
represents the relative solubility in the different liquors. In each
case 2 liters of liquor were used for each kilo of green hide. When old
liquors were employed, the hide-substance they originally contained was
determined, and deducted from the final result.
-------+-----------------------+-------+------------------------+-----
Hide | Description of | Days | Hide-substances in |Loss
Used. | Lime Liquor. |Liming.| Grams per Liter. | per
| | | |cent.
| | | | on
| | +-----+-----+-----+------+ Dry
| | | A. | B. | C. |Total.|Pelt.
--+----+-----------------------+-------+-----+-----+-----+------+-----
1|Ox- |Fresh lime 30 grm. per}| 6[76] |1·068|0·324|2·370| 3·762| 2·35
|hide|liter }| | | | | |
| | | | | | | |
2| „ |Ditto | 9 |2·764|0·540|3·624| 6·928| 4·14
| | | | | | | |
3| „ |Fresh lime 30 grm., }| | | | | |
| |¹⁄₂ grm. sulphide of }| 5[76] |0·852|0·172|1·816| 2·840| 1·75
| |sodium per liter }| | | | | |
| | | | | | | |
4| „ |Ditto | 8 |1·240|0·514|3·846| 5·600| 3·36
| | | | | | | |
5| „ |5 weeks old lime, }| | | | | |
| |through which four }| 2 |0·180|0·212|0·988| 1·380| 0·87
| |packs had passed }| | | | | |
| | | | | | | |
6| „ |Ditto | 5[76] |0·868|1·318|3·356| 5·542| 3·46
| | | | | | | |
7| „ |5 months old lime, }| 2 |0·196|0·188|0·864| 1·248| 0·77
| |with sodium sulphide }| | | | | |
| | | | | | | |
8| „ |Ditto | 5[76] |0·928|1·198|3·004| 5·130| 3·06
| | | | | | | |
9|Cow-|Fresh lime as above | 5[76] |1·982|0·413|4·501| 6·896| 4·30
|hide| | | | | | |
| | | | | | | |
10| „ |Ditto | 8 |3·132|0·672|5·741| 9·545| 5·94
| | | | | | | |
11| „ |Fresh lime as above, }| | | | | |
| |and ¹⁄₂ grm. sodium }| 5[76] |1·012|0·403|2·315| 4·730| 2·96
| |sulphide per liter }| | | | | |
| | | | | | | |
12| „ |Ditto | 8 |2·521|0·653|5·026| 8·200| 4·87
| | | | | | | |
13| „ |Old disused lime | 5 |0·344|0·291|2·341| 2·976| 1·84
| | | | | | | |
14| „ |Ditto | 8[76] |2·119|1·697|6·952|10·768| 6·45
| | | | | | | |
15| „ |Used sulphide of }| | | | | |
| |sodium lime 4 weeks }| 5[76] | .. |1·600|1·047| 2·527| 1·58
| |old }| | | | | |
| | | | | | | |
16| „ |Ditto | 8 |0·791|0·519|4·592| 5·892| 3·43
--+----+-----------------------+-------+-----+-----+-----+------+-----
[76] Hides unhaired.
Taking into account the liming necessary for unhairing only, as shown in
the table, it will be noted that the percentage of loss is invariably
greater in old limes than in new ones, and less in limes sharpened with
sulphide of sodium than where lime alone is used. The only exception to
this rule is in No. 15, where a sulphide lime 4 weeks old shows the
least loss of any in the time required for unhairing; and indeed
sulphide limes if kept strengthened with the requisite addition of
sulphide, seem to deteriorate very slowly, No. 8, with a lime 5 months
old, showing a result which may still be considered good. Another point
especially noted by Eitner is the slight action of old limes during the
first stages of liming, as compared with their rapid solvent effect as
the hair becomes loosened. The loss in any case does not appear to be so
great as the advocates of other unhairing processes have often claimed.
If we assume that all the dissolved hide-substance might have made
leather, the worst loss on oxhide only limed to the point of unhairing
amounts to less than 3¹⁄₂ per cent. on the possible total; and it must
be remembered that at least a part of this consists of dissolved
epidermis matter, which could not by any possible method have been
converted into leather. It will be noted in Nos. 2, 4, 10, 12 and 16,
what considerable losses are produced by plumping limes after unhairing,
but it must be borne in mind that, in the case of dressing-leather,
solution of at least a part of the cementing matter is essential to
produce the necessary softness and flexibility. Eitner calculates the
dry pelt-weight from that of the green hide on the assumption, based on
experiment, that 100 parts of the original skin corresponds to 32 parts
of dry pure pelt in green oxhide, 25 parts in green calf-skins, and 56
parts in dried calf-skins. In some of the smaller skins, such as kid
worked for glove leather, where great softness and stretch is required,
the loss is necessarily much greater than in ordinary dressing-leathers,
amounting, in the case of kid, to from 20 to 27 per cent.
The parts taken by the purely chemical activity of the lime, and by the
action of bacteria and bacterial ferments in the unhairing process must
still be regarded as uncertain. The late Professor von Schroeder[77]
carried out a series of experiments on liming and sweating which were
characterised by his usual care and thoroughness, and which tend to
prove that the chemical action is far more important than the bacterial.
He had fresh hides well washed in a tannery immediately after slaughter,
and fleshed. The butts were then cut into pieces of about 10 cm. (4
inches) square, and salted in brine repeatedly changed, and finally
preserved for use in glass jars in saturated salt solution. He found
that when washed free from salt, and placed in a moist chamber at a
temperature of 16° C., the hair was sufficiently loosened by bacterial
action in four to five days. Pieces placed in the moist chamber without
previous removal of the salt only showed signs of sweating after about
ten weeks’ exposure. Liming experiments were made with similar pieces of
salted hide, both after three days’ washing to free them from salt, and
unwashed, and in both cases the pieces unhaired freely in three to four
days. These experiments were varied by using 6, 18 and 30 grms. of lime
per liter of water in which about 200 grms. of hide were placed, but
neither in the washed, nor unwashed portions was there any material
difference in the time required to loosen the hair. Addition of 1 vol.
of used lime-liquor to 3 vols. of water in making up the limes was
equally without perceptible influence, and careful bacteriological
examination of hide and liquors showed that the former was almost
sterilised by the intense salting, and that the lime-liquors were
practically free from bacteria.
[77] Gerberei-Chemie, Berlin, 1898. p. 646.
Von Schroeder’s conclusion that no gain arises from the use of
excessive quantities of lime, so long as the solution is kept saturated,
is fully justified both by experience and scientific reasoning, but his
results with regard to the effect of old liquors and bacteria contradict
the conclusions both of practical tanners and of other scientific
experimenters.
The different effects of old and new limes are too well known to
practical tanners to be discounted by laboratory experiments, even if
they were not confirmed not only by Eitner’s results, but by a
considerable amount of work done in the Author’s laboratory and
elsewhere; while the necessity of bacterial action is at least rendered
probable by the fact that soda solutions, which are completely sterile
to bacteria, fail to unhair hides which have not previously undergone
some putrefaction (see p. 137). In some experiments undertaken at the
suggestion of the Author it was found that a perfectly fresh and
sterilised calf-skin which was not unhaired after ten days’ liming in
sterilised lime-liquor unhaired rapidly on the addition of a bacterial
culture to the lime. It is extremely difficult to exclude bacteria, and
even where perfectly fresh skins treated with chloroform or carbon
disulphide were employed, bacteria were always to be recognised when the
skin was ready for unhairing. Von Schroeder’s work, is, however, so
painstaking and reliable, that these divergent results must be explained
as other than experimental errors. With regard to old liquors, it is
known that ammonia is a powerful aid to the unhairing process, and it is
not certain to what extent the liquors he used were charged with it. It
is also certain that old limes containing much organic matter, support
bacterial life freely, while 25 per cent. of a possibly not very old
liquor would probably be sterilised by the addition of lime and 75 per
cent. water. In order to test the matter fairly under exact tannery
conditions, the lime should have been made up entirely with old
lime-liquor well charged with ammonia and organic matters, instead of
with water. It is also probable that the hides had undergone a
sufficient amount of bacterial change in the tannery before they came
into Von Schroeder’s salt solutions, and it is not at all unlikely that
the salt solution itself exercised some specific effect on the
unhairing. It is also possible that his bacterial cultures were made on
gelatine media unsuitable for the growth of alkaline bacteria, and
therefore gave blank results. Under these circumstances it is scarcely
possible to arrive at any very definite conclusions, and it is obvious
that further experiments on these points are extremely desirable.
_Sodium and Potassium Hydrates._--From the earliest antiquity,
wood-ashes, consisting mainly of potassium carbonate, have been used for
unhairing, either alone or in conjunction with lime, and indeed the
German name of the process (_Aeschern_) is derived from the fact. In
more recent times, caustic soda, either ready formed, or causticised on
the spot by the addition of lime, has often been recommended as a
substitute for lime. Its action is very similar to lime, but, from its
greater solubility, is far more powerful, and probably this has hitherto
formed one of the greatest obstacles to its use, since a solution of the
strength of lime-water is almost immediately exhausted, while a much
stronger one is too violent in its action on the hides. Experiments made
in the Author’s laboratory show that caustic soda, in solutions of the
same strength as lime-water, dissolve considerably less hide substance
than the latter, but it is more antiseptic than lime, and does not
unhair readily without the aid of bacterial action (cp. p. 137). It also
swells more violently, and it is difficult to keep the grain smooth and
unwrinkled.
Caustic soda has the great advantage that from its solubility, and that
of its carbonates in water, it is much more easily and completely
removed by washing than is the case with lime. It has been successfully
applied in some instances to soften skins of which the texture is
naturally too compact for moroccos and the softer leathers; and is
usefully employed in softening dried goods (p. 115). Where caustic soda
is required merely to “sharpen” limes, it is best added in the form of
sodium carbonate (soda-ash or crystals), which are causticised by the
lime in the pits. One-quarter or one-half per cent. on the weight of
hides added in this way decidedly increases the plumping power of the
lime. It may be noted that in the use of sodium sulphide in conjunction
with lime, caustic soda is one of the products of its decomposition,[78]
and is probably one great cause of the difference of effect of this
material for sharpening limes as compared with red arsenic.
[78] This has been denied, but is probably correct, though the actual
reaction is not easy to prove analytically; but the effect on the hide
is practically what is stated.
An indirect method of liming has recently been patented by Messrs.
Payne and Pullman of Godalming,[79] which is of both scientific and
practical interest. From the difficult solubility of lime, and the
consequently weak solutions which must be employed, the ordinary process
of liming is a slow one. Caustic soda, however, can be used in much
stronger solutions without producing injury to the hide, or larger
solution of hide substance, and from its great diffusibility, it
penetrates very rapidly. Used alone, however, the hide becomes too much
swollen for most purposes, and for certain classes of leather at least
(e.g. buff and chamois leather) the presence of a portion of lime in the
hide appears to be necessary for successful work. If a hide which has
been swollen with caustic soda be afterwards treated with a solution of
calcium chloride, double decomposition takes place, and caustic lime is
formed actually in the interior of the fibre of the hide, while the
sodium unites with the chlorine to form common salt. Both solutions may
be used in any convenient way, and by the employment of drums, the whole
liming process may be accomplished in five or six hours. It is found,
however, that perfectly fresh hides treated in this way cannot be
unhaired, and the explanation appears to be that in the ordinary liming
process, the epidermis is made soluble by the joint action of bacterial
ferments and of the alkaline solutions. If sodium sulphide be added to
the caustic soda used for unhairing, the goods will unhair without the
use of putrefactive means, but the process is difficult to manage
without destruction of the hair, and Messrs. Pullman now recommend that
all hides or skins for unhairing by their process should be soaked for
forty-eight hours in winter, and twenty-four hours in summer in a really
putrid stale soak. This necessity constitutes for very many purposes a
serious weakness in the method, as putrid soaking is always extremely
dangerous to the grain of the hide, and especially so in hot weather.
For certain purposes, however, advantage may be taken of the fact that
the hide or skin can be fully limed by Pullman’s process and the fibres
swollen so as to be prepared for tanning without any loosening of the
hair, and the Author has seen deerskins which have been treated in this
way, on which the hair was perfectly firm, while they possessed a
softness and fulness which could not be attained without liming.
[79] Eng. Pat. 2873, 1898.
Messrs. Pullman now recommend that the treatment with their solutions
should take place in pits, in preference to drums or paddles, and that
the caustic soda should not exceed a strength of one pound in ten
gallons (1 per cent.). The hides or calf-skins remain in this for about
forty-eight hours, during which they are once drawn and returned, by
which time, if the putrid soaking has been properly done, the hair
should be fully loosened. The hides are then drained for two hours, and
passed into another pit containing a solution of calcium chloride, which
should be slightly stronger than the caustic soda, say of about one and
a half pounds per ten gallons. The goods remain in this for about
forty-eight hours, during which they are drawn once, and are then well
washed in soft water (free from temporary hardness) in which they may be
kept for some time without injury. As both the caustic soda and the
calcium chloride solutions are quite sterile to ordinary putrefactive
bacteria, both can be used for an almost unlimited time, and they are
conveniently kept up to strength by the addition of strong
stock-solutions. These may be made of a sp. gr. of 1·4 (80 deg. Tw.)
which gives a strength of about 5¹⁄₂ lb. of caustic soda and 5³⁄₄ lb. of
calcium chloride per gallon.
In addition to the advantage of considerable saving of time, the effects
can be much more easily regulated than in ordinary liming, and the
amount of soda (and subsequently of lime) absorbed by the hide can be
exactly determined by titration of the liquors. Grease is better removed
than by ordinary liming, as soda-soaps are soluble in water, but if this
result is to be obtained, the soap must be worked out before passing
into the calcium chloride solution, which would otherwise convert it
into an insoluble lime-soap. A great gain in many districts is that the
process yields practically no effluents and no lime slab, both of which
are frequently very difficult to dispose of. The serious disadvantages
of the stale soaking, however, have already been mentioned.
In place of applying the caustic soda first, and the calcium chloride
subsequently, hides may be first treated with calcium chloride solution,
and then with caustic soda, or the caustic soda may be applied to the
flesh side of the hide by painting. These modifications are covered by
Messrs. Pullman’s patent, but they are willing to grant licences for
experiments at a nominal fee.
_Alkaline carbonates_ are much milder in their action on hide than the
corresponding hydrates, and although they will unhair hides, in absence
of lime, their action is somewhat uncertain and slow. “Polysulphin”
(Polysulphin Co., Keynsham) owes its unhairing power principally to the
sodium carbonate, and not to the small traces of sulphur compounds which
it contains.
Sodium carbonate occurs in commerce in three forms: “soda ash,” a more
or less pure dry sodium carbonate; “soda crystals,” or washing soda,
Na₂CO₃·10Aq, containing 62·95 per cent. of water of crystallisation, and
efflorescing in the air; and Gaskell and Deacon’s “crystal soda,”
Na₂CO₃·1Aq, containing only 14·5 per cent. of water of crystallisation.
It must be remembered that where carbonate is used in conjunction with
lime it becomes causticised and converted into NaOH.
_Sulphides._--The practice of using realgar, or red sulphide of arsenic
(Ger. _Rusma_) as an addition to limes for fine leathers is one of
considerable antiquity. It has the property of loosening the hair and
epidermis structures with less solution of cement-substance than lime
alone, and hence produces a leather of fuller and closer texture. It
will, however, be convenient to defer the consideration of this agent
till after that of some of the more modern and simpler substitutes, such
as the sulphides of sodium and calcium. Sulphides of the alkalies and
alkaline earths, if used in strong solution, say 5 per cent. or upwards,
have the effect of very rapidly reducing the harder keratin-structures,
such as hair and wool, to a pulp, attacking first the interior cells, so
that the hair crumples up like a string of sausages, and in a few hours,
or even, with very strong solution, in a few minutes, the whole mass
becomes so completely disintegrated that it can be swept off the hide
with a broom, or washed off in a tumbler. At the same time, the action
on the substance of the hide, and especially on the cementing substance,
is very slight, though the grain is swollen and temporarily rendered
somewhat tender. On the other hand, when used in weak solutions, say ¹⁄₄
per cent. and under, in conjunction with lime, the hair is but little
injured, while the hair-roots and dirt are rapidly loosened, and results
are obtained very similar to those with arsenic.
_Sodium Sulphide_ (Na₂S·9OH₂).[80]--For the methods of valuation and
determination of sodium sulphide, see L.I.L.B., p. 28.
[80] In the Laboratory Book the water of crystallisation is given as
10 Aq. Later researches show that pure crystals of the commercial
sulphide only contain 9 Aq., or 67·5 per cent. of water.
Hides suspended in solutions of sulphide of sodium of 2 to 3 per cent.
strength unhair rapidly.
For the commoner classes of sole-leather, hair is frequently removed by
painting on the hair side with a 15°-28° Tw. (30-40 per cent.) solution
of (crystallised) sulphide of sodium thickened with lime, applied with a
fibre-brush, and folding the hide in cushions in a damp place, or
packing in a tub. The hair is reduced to paste in a few hours. The same
effect is produced by drawing the hides through a similar solution
without lime, of which sufficient is retained by the hair to destroy it.
The workmen must be provided with indiarubber gloves to prevent the
caustic effect of the solution on the skin and nails. Skins and lighter
hides are conveniently unhaired by painting the mixture on the flesh
side, when it will loosen the hair or wool in a few hours without
destroying it.
For dressing-leathers and the finer sorts of sole it is best employed as
an addition to ordinary limes to the extent of ¹⁄₄-¹⁄₂ per cent. on the
weight of the hides or skins, when the hair is loosened more rapidly
than with lime alone, and with less loss of hide substance.
Good samples of sulphide of sodium consist of pale-brown, almost
colourless crystals, containing 28 to 32 per cent. of dry sodium
sulphide, which readily deliquesce on exposure to air. Fused sodium
sulphide can now be obtained, which contains nearly twice as much actual
sulphide as the crystalline form. The dark green colour possessed by
many samples of sodium sulphide is due to the presence of iron sulphide.
If carefully used no serious harm can accrue from its presence. If
allowed to stand a short time in solution the iron sulphide will settle
out.
_Calcium sulphydrate_, Ca(SH)₂, sometimes called Böttger’s _Grünkalk_,
is a powerful depilatory, while it has probably less destructive action
on the hide-fibre than even the sulphide of sodium, and would no doubt
be largely used but for its unstable character. It is probably the
principal active product produced by the use of sulphide of arsenic in
conjunction with lime, though it is possible that a sulpharsenite may be
formed. It may be produced by passing hydrogen sulphide (SH₂), into milk
of lime. According to von Schroeder, it is _not_ formed by the reaction
of sodium sulphide on lime solutions (see note, p. 136). It may be
obtained crystallised, and is soluble in water, but is decomposed on
boiling. The sulphide, CaS, is insoluble in water, but by the action of
steam under pressure it is said to be converted into a mixture of
equivalent parts of hydrate and sulphydrate. It may also be dissolved in
a solution of hydrogen sulphide, forming a solution of sulphydrate. In
this way it might be formed on a large scale from the “tank waste” of
the Leblanc soda process.
Gas-lime is principally active on account of the calcium sulphide which
it contains, but is very variable in its strength, as both sulphydrate
and sulphide are decomposed by the carbon dioxide always present in the
gas, forming carbonates. Lime has nearly gone out of use for purifying
gas, its place being now taken by iron oxide, but formerly gas-lime was
a good deal used for unwooling the small lambskins used for the commoner
sort of glove-kid, usually by painting a cream of it on the flesh side,
but sometimes by immersing in a strong solution, which of course
destroyed the wool. Its place is now taken by a solution of sodium
sulphide of 15°-18° Tw. (approximately 30-35 per cent. crystals),
thickened with lime to a soupy consistence, the use of which is much to
be recommended for unwooling sheep-skins.
The tank-waste from the Leblanc process, consisting principally of
calcium sulphide, is, when fresh, quite insoluble, and has no depilatory
powers; but when exposed to air and moisture, decompositions take place,
resulting in the formation of sulphydrates and polysulphides, which form
a solution which has been the subject of several patents for
unhairing.[81] Polysulphides alone have probably no unhairing effect,
but in conjunction with lime, sulphydrates are formed which rapidly
loosen the hair. This fact was the basis of an ingenious and effective
unhairing process used very many years ago by Mr. John Muir, of Beith,
who, after liming for 24 hours in the usual way, submitted the hides to
a pretty strong solution of weathered tank waste for 24 hours, and
finally to water for 24 hours, to remove the surplus lime and sulphides.
The sulphydrates formed in the hide attacked the hair-roots with little
injury to the hair itself, and the hides contained so little lime that
they could be tanned for dressing without bating, and made about 10 per
cent. more weight than those treated in the ordinary way. Some trouble
was occasioned by stains caused by impurities in the tank-waste.
[81] Squire, E. P., 756, 1855; Claus, E. P., 1906, 1855.
A somewhat similar unhairing mixture to that obtained from tank-waste,
which is now seldom to be got, was patented by Prof. Lufkin,[82] who
mixed equal parts of sulphur and soda-ash with a little water till
combined, and then added 8 to 10 parts of lime, slaked and still hot.
Schultz[83] states that such a mixture containing 10 lb. of sulphur,
will unhair fifty hides in the same way, and in about the same time as
an ordinary lime, the pelt being little plumped and easily reduced
without bating by a few minutes’ wheeling in warm water. By boiling lime
and sulphur with water a yellow solution is obtained which can be used
in the same way as that from the tank-waste. A further quantity of water
can be boiled on the same materials, more lime and sulphur being added
as required. Polysulphides appear to have a marked effect in preventing
plumping.
[82] Eng. Pat. 2053, 1860.
[83] ‘Leather Manufacture,’ p. 35.
_Barium sulphydrate_ has been put on the market experimentally as an
unhairing agent, in the form of a strong solution containing yellow
polysulphides, and which deposits crystals of sulphydrate in cold
weather. It is more stable than calcium sulphydrate, but, on the whole,
does not seem to present any advantages over sodium sulphide.
_Realgar or red sulphide of arsenic_, As₂S₂, is made by fusing arsenious
acid and sulphur. (Orpiment is As₂S₃, but its action is different from
that of realgar.) Mixed with lime it produces calcium sulphydrate and
possibly hyposulpharsenite. To produce a rapid and complete reaction it
must be mixed with hot lime, and the hotter the mixture is made the more
powerful is its unhairing action. Milder forms may be made by mixing
cold, or with the aid of hot water only. It is used with great advantage
in conjunction with lime in varying proportions for unhairing lamb- and
kid-skins for glove-kid and other fine leathers, to which it gives the
necessary stretch and softness and cleanness of grain, without the
loosening of texture and loss of hide-substance which would be caused by
an equivalent amount of ordinary liming. For glove-kid about 0·1-0·3 per
cent. of realgar and 5 per cent. of lime is used, reckoned on the green
weight of the skin.
For painting the flesh side of calf- and lamb-skins 1 part of realgar
is mixed with 10 parts of hot lime, made into a paste with water. Calf
will unhair in 8 or 10 hours.
“Inoffensive” unhairing solution contains a large quantity of arsenic
sulphide apparently dissolved in caustic soda, although Moret’s original
patent claimed the use of wool-sweat potash only!
W. R. Earp[84] has suggested the use of compounds of sulphur and arsenic
(thio-arsenates, thio-arsenites, etc.), in 5 per cent. alkaline
solution. He prefers to add the compounds to the ordinary lime-liquors,
or to manufacture them _in situ_ by adding the proper quantities of
arsenious or arsenic acid mixed with one-third of its weight of sulphur
to a solution of an alkaline sulphide in lime-liquor. The pelt is not
bated or drenched in the ordinary way, but, after unhairing, is passed
directly into the tanning liquor to which sulphurous acid has been
previously added.
[84] Eng Pat., No. 2052, Feb. 12, 1886.
There is more danger of injury to the hide from the very prolonged
action of weak solutions of sulphides, which tend ultimately to destroy
the structure and reduce the fibre to a gelatinous condition, than there
is from too concentrated solutions. No danger need, however, be
apprehended in the course of any ordinary liming. Arsenical limes are
not suited for tainted skins, and they should not be made so strong as
to destroy the hair or wool.
For methods of analysis of both old and new lime-liquors, see L.I.L.B.,
pp. 27 to 34.
Whichever method of loosening the hair be adopted, the actual removal
must be effected by placing the hide on a sloping beam with a convex
surface, and then scraping it with a blunt two-handled knife (Fig. 27),
the workman pushing the hair downward and away from himself. The beam
may be either of cast iron or of wood, usually covered with zinc to
increase its wearing capacity. The hides after being removed from the
lime-pits, are allowed to drain for half an hour or so before the hair
is removed, and immediately this operation has been completed, they
should be placed in soft water. It is of great importance that the limed
hides should not be exposed to the air longer than is absolutely
necessary for the removal of the hair, as the carbonic acid present in
the atmosphere quickly carbonates any lime contained in the surface of
the skin, forming chalk, and leading to uneven tanning at a later stage.
When hide has been insufficiently limed it is often easy to remove the
longer hair but excessively difficult to get rid of the short
under-growth of the young hairs, which even in properly limed skins can
often only be removed by shaving them with a sharp handknife. This
difficulty is caused partly by the small resistance which the short
hairs offer to the unhairing-knife, and partly by their being more
deeply rooted in the skin than the older hairs (see p. 49).
[Illustration: FIG. 27.--Unhairing (Penketh Tannery).]
Various machines have been devised to accomplish the removal of the
hair, but owing to the rapidity with which it may be worked off by hand,
and the fact that the work is not difficult, no machine has as yet come
into general use. Hand-work has the further advantage that in those
portions of skin where the hair is tighter than usual it may be removed
by greater pressure of the knife or by hand-shaving, whereas after goods
have been unhaired by machine they must always be examined and any
patches of hair removed by hand on the beam. The edges invariably
require to be gone over by hand.
Several machines with spiral knives have been introduced for the
purpose. That made by the Vaughn Company (Peabody, Mass.) for fleshing
is one of the most satisfactory for unhairing, though any other machine
of a similar type, and provided with spiral knife-blades, purposely kept
blunt, may be used. The Leidgen unhairing machine, shown in Figs. 28 and
29, is one of the latest and most ingenious.[85]
[85] E. H. Munkwitz, Milwaukee.
[Illustration: FIG. 28.--Leidgen Unhairing Machine.]
Occasionally goods are unhaired by fulling in the “stocks”; but it is
very doubtful whether the saving in labour is not more than counteracted
by the loss of weight caused by submitting the hide, while its gelatin
is in a partially dissolved condition, to such rough usage.
The use of the wash-wheel (see pp. 111, 118) for the same purpose is
much more satisfactory, and may be profitably employed for common
goods, especially when the hair has been loosened by painting with a
sulphide mixture.
After being unhaired, the hides are “fleshed” on the beam. This work,
which consists in removing any small pieces of flesh and fat left by the
butcher on the inner side of the skin, should be carefully and
thoroughly done; but the closeness of the fleshing required is dependent
on the purpose to which the hides or skins are to be applied.
[Illustration: FIG. 29.--Leidgen Unhairing Machine.]
It is necessary not only to remove those portions of fat which are
easily visible, but also to force out that contained in the loose
areolar tissue. The form of knife used in England in fleshing is shown
in Fig. 30. It differs from the one used for unhairing in being
somewhat broader and heavier, and both its edges are sharp, so that
where the flesh is too tight to remove by mere friction of the knife, it
may be actually cut away by holding the knife almost flat on the beam,
and using the convex sharp edge. The strokes in cutting must not be too
broad, or, from the convexity of the beam, the substance of the hide
will be cut into in the middle, or flesh will be left at the edges of
the stroke. This difficulty is avoided by the flexible knife commonly
used in Germany, but in other ways its work is less rapid and effective.
[Illustration: FIG. 30.--Fleshing.]
Machines have long been used for fleshing and scudding light goods, such
as lamb-, kid-, and goat-skins, and their use for fleshing dressing
hides has now become very general in the United States, and is gradually
gaining ground in England. The type of machine used for these heavier
leathers, varies considerably from that used for light skins, but the
general principle is the same. In most cases the working tool of the
machine is a cylinder with spiral blades, which are generally arranged
right-handed on one half, and left handed on the other, so as not only
to scrape the hide in the direction in which the cylinder works, but
also to extend it sideways. Much of the efficiency of these machines
depends on the exact adjustment of the pitch of the spiral, and in the
Vaughn machine, which is probably most in practical use, the blades are
so arranged as to form two intersecting spirals, one of steeper pitch
than the other. The great difference in the machines for skins and for
heavy work, consists in the means adopted to support the skin, and to
carry it under the spiral blades.
[Illustration: FIG. 31.--Jones Fleshing Machine.]
In the machine invented by the late J. Meredith Jones, the skins are
supported upon an india-rubber blanket stretched over two rollers, so
that the knife-cylinder works on that part of the blanket which is
between them, by which great elasticity is obtained, and this machine
has proved most successful in treating delicate skins. In some other
forms of machine, cylinders thickly covered with rubber have been
substituted for this arrangement. The Jones machine is shown in Fig. 31.
For heavy hides the Vaughn machine is most generally used, and may be
taken as the type of the rest, as the Vaughn Company certainly
originated the semi-cylindrical “beam,” which forms a very important
feature. Its construction will be seen from Fig. 32.
[Illustration: FIG. 32.--Vaughn Fleshing Machine, front view.]
It will be easily noticed that if a hide be thrown over the
half-cylinder so that one half hangs outside it, and the other half
falls in its hollow, and it be then rotated, the hide is first caught
firmly by a spring-clamp, which has been supported above the edge of the
half-cylinder by blocks attached to the frame. As the edge rises, it
lifts this clamp off the blocks, and thus carries the hide under the
spiral knife-cylinder. The blades of this spiral knife-cylinder are
ground to a sharp rectangular edge, and partly scrape and partly cut the
loose tissue of the flesh. When the half-cylinder has made a
semi-revolution, it returns to its original position, and the sizes of
the driving pulleys are so arranged that the cylinder travels downwards
more rapidly than it rises, in order to economise time, though in both
cases the hide is worked upon by the knife-spiral which is rotated at a
still higher speed. The hide is of course turned on the beam-cylinder
and the other half is similarly fleshed. The beam-cylinder reverses
automatically, or may be reversed by hand, and its nearness to the
spiral knife is also under control. It is usually covered with a thick
sheet of rubber.
It is obvious that machines of this type can not only be used for
fleshing, but for unhairing and scudding, by the substitution of
suitable knife-cylinders, and in the case of light skins, cylinders
fitted with slates are frequently employed for the latter operation. The
slate for the purpose must be of a peculiarly fine and even grain, and
is mostly obtained from a single quarry in Wales. The Vaughn machine is
frequently used in America for fleshing hides after soaking but before
they go into the limes, and much is to be said in favour of this method,
as the removal of the flesh permits even and uniform action of the lime.
It is, however, a distinct disadvantage to the method that the flesh
appears rough-looking after tanning, and the method is most suitable in
conjunction with the American system of splitting the tanned leather.
In the production of sole-leather, fleshing machines have not as yet
come into very general use. This may be accounted for by the fact that
if used before liming a rough flesh is produced, which is unsightly on
sole-leather, and which cannot well be afterwards improved, while
something of the same objection attaches to fleshing after liming, with
the added disadvantage that the hide is too much pressed, and is not
easy to plump again, so as to make a satisfactory sole-leather.
In America, both sole- and dressing-leathers are usually tanned in
sides, the hide being cut down the centre of the back. In England, the
hide is usually “rounded” for sole-leather into “butts” or “bends” and
“offal,” as shown in Fig. 33. The rounding is done by hand with a sharp
knife on a table, and in some of the best tanneries frames made of wood
or metal are employed, to mark the sizes required. The chief advantage
of rounding before tanning is that the different parts of the hide can
be differently tanned, and appropriated to the purposes for which they
are most suitable. The offal is now frequently split and worked up for
light leather, or in other cases is tanned with a cheaper and more rapid
tannage than the butts.
[Illustration: FIG. 33.--Diagram of Hide.]
Dressing leather is more frequently rounded after tanning, according to
the purposes for which it may be required.
CHAPTER XIII.
_DELIMING, BATING, PUERING AND DRENCHING._
Although lime is in many respects the most useful and satisfactory means
of loosening hair from hides and skins, it is of the greatest importance
that it should be completely removed when it has done its work, since
its action on tannins is most injurious, and it is often harmful in
tawing. For soft leathers it is also necessary that the skin should be
brought from a swollen to a soft and flaccid condition.
In practice this is mainly accomplished for dressing leathers by bating,
puering and drenching; while sole-leather and strap-butts are only too
frequently left to chance, and to the natural acidity of the tanning
liquors.
_Bating_ consists in handling, or steeping the goods in a weak,
fermenting infusion of pigeon- or hen-dung for a time usually extending
over some days, and is applied to the heavier classes of dressing
leather, such as “common” and shaved hides, kips and calf-skins.
_Puering_ is a very similar process, applied to the finer and lighter
skins, such as glove- and glacé-kids and moroccos, in which dog-dung is
substituted for that of birds, and, as the mixture is used warm and the
skins are thin, the process is generally complete in a few hours at
most. Neither bating nor puering are very effective in removing lime,
and seem to act principally by some direct effect of the bacterial
products on the swelling of the pelt.
_Drenching_ is occasionally used (e.g. on calf-kid) as a substitute for
bating or puering, but more frequently follows the latter, and serves to
cleanse and slightly plump the skins before tanning, and complete the
removal of lime. The drench-liquor is an infusion of bran made with hot
water, and allowed to ferment under the influence of special bacteria,
which are always present in vats used for the purpose, and which
develop lactic and acetic acids.
It will be noted that all these methods are fermentative, and their
effect is not simply the chemical one of removing the lime, but the
bacterial action leads also to solution of the cementing substance of
the hide-fibres, and produces a marked softening effect on the leather,
together with considerable loss of hide-substance. In the manufacture of
the softer leathers this effect is generally desired, and no process
would be satisfactory which did not produce it; but in other cases, such
as harness- and strap-butts, firmer and heavier weighing leathers would
be preferred, if it were known how to make them. The putrefactive
processes would be gladly relinquished, if satisfactory substitutes
could be found, not only on account of their offensive character, but
because of their uncertainty and danger to the goods; and even if lime
only were removed, the necessary softness could often be obtained by
appropriate liming and tanning.
It will be best, therefore, to deal first with the purely chemical
methods which aim only at removal of lime, before considering those
involving bacterial action. Unfortunately, the chemical problem is not
so simple as it might at first sight appear. The alkaline lime clings
obstinately to the hide-fibre, and can only be removed very slowly, if
at all, by mere washing. On the other hand, the use of any excess of
strong acid is absolutely precluded, because of its powerful swelling
effect on the pelt, in the tanning of which it would prove even more
injurious than the lime, making dark-coloured and brittle, or tender,
leather. This effect is not to be avoided by the use of even very dilute
solutions of strong acids, since the affinity of hide-fibre for them is
so strong that it will abstract practically all the acid from even a
decinormal solution, leaving it quite neutral. What is required is an
acid of extremely weak affinities, forming soluble lime salts, and
obtainable at a low cost; or, on the other hand, a salt of some weak
base which could be displaced by lime, and which would not act
injuriously on the pelt. With certain precautions, and in special cases,
however, the stronger acids may be used successfully.
In the cases of sole- and belting-leather no softening is desired, and
formerly tanners usually contented themselves with a very perfunctory
washing in water, trusting to the acids present in the liquors to
complete the removal of the lime. Even pure distilled water effects this
removal very slowly and imperfectly, owing to the strong attraction of
the lime for the fibre; and if “temporary hard” water is used, the lime
present in the hide combines with that present in the water and is
precipitated as chalk in the surface of the hide. This may be prevented
by previously adding a small quantity of lime or lime-liquor to the
water before use to soften it (see p. 95); but unless this is very
carefully done, the free lime present in the water prevents it from
removing any from the hide. The safest way is not to add lime direct to
the water, but to change the latter gradually, so as to allow the lime
already present to soften the new portion of water.
A much more efficient method is to suspend the butts in water to which
small portions of diluted acid are successively added till the lime is
nearly, but not quite, neutralised. If carefully used, sulphuric
acid[86] is perhaps as good as any, but, of course, any excess will
spoil the colour or “buff” of the leather.
[86] The use of sulphuric acid for this purpose was patented by H.
Belcher of Wantage (No. 14,943), but was used some years previously in
several tanneries known by the author.
Acetic, formic, and lactic acids are safer than sulphuric, but are
somewhat costly, and must not be used in appreciable excess. Crude
pyroligneous acid may be used, and it has a considerable antiseptic
effect owing to the phenols, etc., which it contains. Hydrochloric acid
is not suitable for sole-leather, on account of the bad effect of
chlorides on plumping. Sulphurous acid[87] is perhaps the best, and its
acid properties are so weak that slight excess does little harm, but the
neutral calcium sulphite is insoluble, and to actually dissolve the lime
the hydric sulphite must be formed, which can only occur in presence of
excess of the acid. Unless such excess is used, the colour of the pelt
in the early liquors is apt to be somewhat greyish. Probably a very good
method would be to suspend the butts in a solution of sulphurous or some
other acid of about N/20 strength, sufficiently long to remove all lime
from the surface and slightly to plump it but not to penetrate to the
centre of the hide, which should then be suspended in water until any
excess of acid had been taken up by the unneutralised lime still present
in the middle of the butt, which at the end of the operation should be
rather alkaline than acid. The course of this, or any other bating
operation can be followed by cutting the hide, and moistening the cut
surface with alcoholic solution of phenolphthalein, which is turned red,
or pink, by the least trace of free lime.
[87] Manufacture of sulphurous acid, see p. 24; testing, see L.I.L.B.,
p. 37.
In using mineral acids it is of great importance that they should be
perfectly free from iron, and that the vat employed should contain no
iron which could become dissolved, since, if present in the bating
liquid, it is sure to be fixed by the hide, especially if the quantity
of acid used is insufficient to neutralise the whole of the lime.
Besides the direct use of mineral acid which has been described,
sulphuric, or still better, oxalic acid may be very advantageously
employed in precipitating lime from used bating liquids containing weak
organic acids, or other lime solvents, so as to restore their original
activity. Not only is the bate economised by being used repeatedly, but
some of the organic products dissolved from the hide have themselves
considerable power of removing lime. Putrefaction should not be allowed
to take place; but many of the organic acids which have been proposed
for bating belong to the aromatic series, and have considerable
antiseptic power. Where organic acids are employed, the presence of
their neutral lime-salts in the liquor, resulting from previous
operations, will reduce the swelling action of the acid on the skin,
without diminishing its power of removing lime (cp. p. 81).
In place of sulphuric acid, some tanners have employed a material
advertised under the name of “boral.” This substance consists simply of
sodium anhydrosulphate melted up with about one-seventh of its weight of
boric acid, the quantity of which is, however, too small to have
appreciable influence as an antiseptic, while it is said to form
insoluble borates with the lime present, which are sometimes a source of
subsequent trouble.
There is no reason why ordinary sodium bisulphate should not be used for
the purpose, and its action is more mild than that of sulphuric acid
itself, but great care must be taken that no nitric acid is present, as
is frequently the case in the crude product obtained in the manufacture
of nitric acid from sodium nitrate, and known in commerce as
“nitre-cake.” The presence of a trace of sodium chloride would not be
disadvantageous for dressing leather, but would tend to prevent
plumpness in sole. Paessler and Appelius[88] have recently shown that
raw hide absorbs sulphuric acid from sodium bisulphate, leaving the
neutral sulphate in solution.
[88] ‘Wissenschaftlich-Technische Beilage des Ledermarkt,’ 1901, p.
107.
Boric (boracic) acid, though used to a slight extent for a number of
years past, has recently come much into favour as a deliming agent, for
which purpose it is in many respects particularly suitable. Sole-leather
may be improved in colour by giving a short bath in 1¹⁄₂-2 per cent.
boric acid solution to remove surface-lime. In this case the acid is
best applied just before the hide enters the suspenders. Boric acid may
also be suitably employed on hides which have been bated. It then acts
as a drench and removes traces of lime still left in the hides, so that
the liquors have a more even effect on them. Experience has shown that
the skins should never be allowed to lie for any length of time in the
boric acid solution in a motionless condition, as this tends to produce
patches of partially delimed skin, which cause irregular colour. It is
best to keep the skins in fairly constant motion in a paddle or by
frequent handling. Boric acid has considerable influence in preventing
drawn grain in the early liquors, but if it gets into the forward
liquors it renders the leather loose and light (cp. p. 229, and L.I.L.B.
p. 37).
Borax has also been suggested as a deliming agent, and as it is
chemically an acid salt, it has naturally some deliming effect, but it
cannot compare with boric acid in either price or efficiency.
Both boric acid and borax are antiseptics (see p. 25).
In the employment of either sulphuric, boric, or any other acid forming
calcium salts of limited solubility, it must be borne in mind that if
the solution is repeatedly re-strengthened, it will become saturated
with the lime-salt, and although the acid will still combine with the
lime and render it neutral, it will no longer remove it from the hide.
Under these conditions, sulphuric acid may cause the deposition of
crystalline calcium sulphate in minute nodules between the fibres.
Calcium borate may be similarly deposited, and has the further
disadvantage of becoming decomposed by the tanning liquors, which form
dark compounds with the lime. In using sulphuric acid alone it is
therefore best to renew the water each time. When it is used in
conjunction with some other acid, forming very soluble lime salts, this
danger is not to be apprehended, while oxalic acid precipitates the lime
almost completely from the solution.
It is to be borne in mind that in all cases of using acids, any
carbonate of lime present on the pit sides or elsewhere will be
decomposed, and the carbonic acid will become dissolved in the liquor,
and unless acid is used in sufficient quantity to remove the whole of
the lime, may tend to fix the remainder as carbonate. In the case of
dressing leather there is less danger of this, as warm water is
generally used, in which little carbonic acid dissolves. It is probable
that some of the coal-tar acids which have been advertised for bating
dressing leather might be advantageously employed for sole. Hauff’s
“anticalcium” (see pp. 29, 163), would appear to be very suitable for
this purpose, and if the liquor were regenerated by the addition of
sufficient sulphuric acid to neutralise the lime dissolved from the
hide, might be used repeatedly, and would not then prove expensive;
while its sterilising power would be very advantageous to the proper
swelling of the butts in the handlers, since nothing tends to check
plumping so much as putrefactive action.
Turning from sole to dressing leather, mineral acids are very
successfully employed for “pulling down,” the goods being thrown into a
paddle containing warm water of about 30°-35° C., and the calculated
quantity of sulphuric or hydrochloric acid, previously largely diluted
with water, is then added in two or three successive portions at
intervals of perhaps ten minutes. The acid must in no case be sufficient
to neutralise quite the whole of the lime. Goods treated in this way can
be further bated, puered, or drenched as required by the ordinary
methods, if they are not sufficiently soft. If too much acid has been
used, and the skins show signs of swelling, they may be brought down by
the addition of a little ammonia, borax, or even soda.
In many cases the addition of salt in small quantity to the acid liquor
will tend to deplete the hides, and at the same time prevent any
injurious action of the acid. Ammonium chloride may also be used with
advantage (see p. 159). A solution containing about 15 per cent. of salt
and 0·3 per cent. of sulphuric acid, with some molasses, has been a good
deal used in the States as a bate, and seems to answer well on some
classes of goods, but the acid and salt are apt, ultimately, to find
their way into the liquors and destroy tannin. The process is well
suited for chrome-leather, and may also be usefully applied in cases
where goods have become “wind-blasted” or otherwise impregnated with
carbonate of lime, since in presence of salt the acid can be used in
sufficient excess to dissolve the carbonate. Vegetable acids may, of
course, be used in conjunction with salt in the same way. The salt does
not neutralise the acid, but simply controls the swelling of the skin,
and if acid has been used in any material excess, the first part of the
tanning must be done in salted liquors, or the acid neutralised with
ammonia, sodium carbonate, or chalk, previous to tanning, as, otherwise,
the goods will plump up in the liquors, and be tender when tanned (cp.
p. 91).
Lactic acid has recently come largely into use as a deliming agent. It
is best known as the acid which gives a characteristic taste to sour
milk, and is the chief product of the lactic ferment. It may be very
successfully used for neutralising the lime left in the skins after the
depilation, but, if used in excess, it tends to plump or swell the
leather very strongly, being one of the best plumping agents known. When
used for deliming, a solution of 2 lbs. in 100 gallons is very suitable.
It may, in many cases, be substituted for the bran-drench with
advantage, and is much more rapid and less dangerous in hot weather, but
the effect is not in all respects identical.[89]
[89] On the manufacture of lactic acid by fermentation, see Claflin,
Journ. Soc. Chem. Ind., 1897, p. 516. Campbell states that practically
pure cultures of the lactic bacteria are obtained by continued culture
in milk. These cultures employed as a ferment for drenches have given
good results in the Yorkshire College Experimental Tannery.
When lactic acid is used for bating, or drenching, the operation should
always be conducted in a paddle, and the liquid works more
satisfactorily if it is at a temperature of 30-35° C. As regards cost,
it will be found that in practice it is not appreciably more expensive
than dung or bran. About an hour’s paddling will generally suffice, if
the right quantity of acid has been used, but in some cases it is best
to add the acid in several portions and take more time.
The estimation of the amount of lactic acid in the commercial article
may be carried out by diluting exactly 9 grms. with about ten times its
volume of water, and then titrating it with normal caustic soda as
described in L.I.L.B., p. 16, for acetic acid. As each c.c. of normal
alkali is equivalent to ·090 grm. of lactic acid it will represent one
per cent. of real lactic acid in the sample. If other acids are present,
they are of course included. Commercial lactic acid is usually of about
50 per cent.
It is important that the lactic acid should be free from iron, a dilute
solution should give no blue coloration on addition of either potassium
ferrocyanide or ferricyanide. Acid perfectly free from iron is now
easily obtained.
Formic acid in 60 per cent. solution, formed synthetically by the
combination of carbon monoxide with caustic soda and the subsequent
decomposition of the sodium formate so produced, has recently been
brought into commerce at a cheap rate, and will probably form a
satisfactory substitute for acetic acid in the deliming of hides and
many other technical operations.
Instead of acids, many neutral salts may be used to neutralise lime, and
in sole-leather, it is not generally disadvantageous to leave the lime
in the hide, so long as it is in an insoluble and fixed condition, and
combined with an acid which cannot be displaced by tannin. Thus
phosphates, or oxalates of sodium or ammonium will convert the lime into
insoluble phosphate, or oxalate, setting free sodium- or
ammonium-hydrate which form soluble tannates and other salts which are
easily washed out of the hide. Zinc sulphate will form sulphate of lime
and zinc oxide in the hide, and seems worth further experiment for
sole-leather, but must be free from iron. Alum, or sulphate of alumina,
would similarly form calcium sulphate and alumina, but the tanning
effect of alumina salts is too great to admit of their general use for
bating. Ammonium sulphate will form calcium sulphate with liberation of
ammonia.
For dressing leather, the use of ammonium chloride would be still more
advantageous, and it is a powerful bating material, converting the lime
into calcium chloride with the evolution of ammonia, which has but
little plumping power, and which is easily washed out. Ammonium chloride
has been very successfully used in calf-kid manufacture as a preparation
for drenching, instead of puering, which was formerly in vogue. As,
however, only about ³⁄₄ oz. per dozen skins was employed, the cleansing
must have mainly depended on the warm water with which it was used, and
the free ammonia evolved.
The use of ammonium chloride as a bate was patented by Zollickoffer in
1838.
A bating liquor which was proposed by the writer, and which has been
used with some success on harness-leather, is made up with a ¹⁄₄ lb. of
good white ammonium chloride (sal ammoniac) and a ¹⁄₄ lb. of Boakes’
“metabisulphite of soda” per hide, and for successive packs sufficient
sulphuric acid to neutralise the ammonia formed, together with a small
quantity of metabisulphite and ammonium chloride to restore that carried
out by the hides is added. It is probable that this would also answer
well for deliming sole-leather as it entirely removes lime without
pulling down the hides much, and they would remain still plumper if
ammonium sulphate were substituted for ammonium chloride, while the
sulphuric acid might be safely increased till the liquor was but
slightly alkaline when the bating was finished. About 2-4 oz. of good
white oil of vitriol is required per hide, but the exact quantity will
depend on the mode of liming, and the amount of washing the hides
receive before going into the bate, and can therefore be only
ascertained by experience. As no free sulphuric acid can exist in the
liquor so long as the quantity of metabisulphite is maintained, there is
no practical danger of spoiling the leather if the acid be in slight
excess. The quantities given may in most cases be advantageously
diminished, since it is not always advisable in practice to remove the
whole of the lime, which in small quantity renders tannage and
penetration of the liquor much more rapid, either by acting as a mordant
to the tannin, or by temporarily neutralising it and diminishing its
astringent action on the hide-fibre.
Turning to dressing leather, we find that the use of cold water alone
has been practically abandoned in this country, though the finest French
calf is produced by repeated soakings in cold water with alternate
workings over the beam, sometimes extending to nine or more. In this
case, from the lengthened exposure to waters which are only gradually
renewed it is probable that putrefactive action takes place, and that a
sort of bating is effected by the decomposing products of the hide
itself; in fact, in many French yards, bran-drenches have been
introduced to supplement the action of the water alone. Waters differ
greatly in their power of removing lime from skin. Slightly acid and
peaty waters, and those in general which contain much organic matter,
are much more powerful in reducing than those which are purer (cp. p.
107).
Warm water has much more effect in removing lime than cold, since the
heat lessens the risk of dissolved carbonic acid, and seems to have a
direct depleting effect on the pelt. A good tumbling in warm soft water
will remove a great deal of lime, and is an excellent preparation for
bating, but heat must be used cautiously, and should never exceed
30°-35° C.; some skins, such as seals, being very readily tendered by
its action, while others, especially sheep-skins, will stand a
comparatively high temperature.
The use of a solution of carbonic acid for removing lime has been
patented by Nesbitt,[90] who takes advantage of the fact that calcium
carbonate is soluble in excess of carbonic acid (p. 94). The gas, which
he generates, as for soda water, by the action of acids on chalk, or
limestone, is received in a gasholder, and forced by a compressing pump
into the vessel containing the hides, which is preferably a rotating
drum lined with copper, and capable of bearing a pressure of about three
atmospheres. The invention excited considerable interest on its
introduction, as the gas is, certainly, quite uninjurious to the hides,
and it was claimed that it enabled the grease and dirt to be better
removed than by the ordinary methods. Further experience has shown,
however, that the removal of the lime is far from complete, since, for
success, it is not only necessary to bring it into solution, but to wash
it out with carbonic acid solution under pressure, as on exposure to the
air, solutions of lime in excess of carbonic acid rapidly deposit
calcium carbonate. At the present time, the only tannery in which to my
knowledge the process is in use is that of Messrs. Mossop and Garland,
of Capetown, who state that it answers very well for harness-leather
when a pure lime made by calcining sea-shells is used for liming, but is
not satisfactory with ordinary stone lime. It is difficult to account
for this on chemical grounds. Gluestuff may be treated very
satisfactorily by simply blowing carbon dioxide, or washed and cooled
lime-kiln- or furnace-gases, into an open pit in which the material is
kept agitated. In this case, however, there is no need for the actual
removal of the lime, so long as it is carbonated and its caustic
character destroyed. Carbonic acid does not decompose lime-soap, and
hence sets free no fatty acids, which, together with grease, are the
main cause of the turbidity of glue, and the process therefore yields a
more brilliant though darker coloured glue than does treatment with
sulphurous acid.
[90] Eng. Pats. 7744 and 12,681, 1886.
Several acids of the aromatic series have been from time to time
recommended as deliming agents, and generally possess the merit of
acting at the same time as powerful antiseptics. In this connection it
may be well to mention the solution of 1 per cent. of phenol and 2 per
cent. of boric acid used by Dr. Parker and the writer for preparing and
preserving skins for colour tests (L.I.L.B., p. 133). This answers very
well as a bate even when much diluted, and may be rendered cheap enough
for use in practice by the employment of a good commercial carbolic acid
instead of pure phenol, and the use of sulphuric acid to remove lime
from the solution and render it capable of repeated employment. The
carbolic acid should not be too dark in colour, and should be carefully
dissolved, or “carbolic” stains will result.
“Cresotinic acid,” a mixture of impure acids obtained from cresols in
the same way as salicylic acid is manufactured from pure phenol, was
introduced as a bate and unhairing and deliming agent by J. Hauff, of
Feuerbach.[91] He also claims the use of hydrochloric acid to liberate
the acid after it has been combined with lime in the deliming process.
It is only soluble to the extent of about 1 in 800 of water, so that,
even if used in excess, no dangerously strong solution is formed, but it
has a tendency to slightly swell, and somewhat harden, the hides or
skins, so that it is perhaps more suitable for sole than dressing
leather. It has also powerful disinfectant properties (see p. 29).[92]
[91] Eng. Pat. 14,889, 1888.
[92] Compare also Journ. Soc. Chem. Ind., 1889, p. 954.
Hauff states that a solution of 18 lb. of cresotinic acid in 500 gallons
of water at 30° C. will bate one lot of 50 heavy hides, and that the
same liquor may be used continuously, by adding 4-5 lb. more cresotinic
acid for each successive 50 hides. For bating glove-leather, Hauff
recommends the use of 5 kilos. cresotinic acid dissolved in 1000 liters
of warm water for every 500 kilos. of wet skins, to which is added
ammonia nearly sufficient to neutralise the cresotinic acid, leaving the
solution still slightly acid to litmus paper; and he also advises the
addition of 5 kilos. of ammonium chloride or sulphate. The goods are
paddled in this solution for about half an hour.
“Oxynaphthoic acid,” the corresponding mixed acids of the naphthols (p.
30), has also been patented by Hauff as a bate, since cresotinic acid
sometimes acts too powerfully on light skins.[93] He mentions that
mixtures of this and cresotinic acid, or salicylic acid, may also be
used. Oxynaphthoic acid requires for its solution 20,000 to 30,000 parts
of water.
[93] Eng. Pats. 10,110 and 12,521. Journ. Soc. Chem. Ind., 1889, pp.
124, 809; 1890, p. 85.
A mixture of the α and β mono- and di-sulphonic acids of naphthalene has
also been patented for bating,[94] under the name of “Acrilene bating
and puering acid.” 150 calf-skins, weighing 880 lb., were pured in a 3
per cent. solution of the α acid, and gave 266 lb. of leather as against
255 lb. from a lot of similar weight treated with hen-dung, and this
gain was more than maintained on stuffing, while the shoulders were
plumper and fuller. This patent appears to anticipate a part of Hauff’s
claim mentioned in the next paragraph.
[94] Burns and Hull, Eng. Pat. 8096, 1891; Journ. Soc. Chem. Ind.,
1892, p. 48.
More recently Hauff has patented, under the name of “anticalcium,” a
mixture of impure sulphonic acids of various cresols and hydrocarbons.
This is cheaper than cresotinic acid, and like it, possesses
considerable antiseptic powers. One-half to one-quarter per cent.
solution will keep hides uninjured for a considerable time, but at this
strength it plumps considerably, and seems more suitable as a deliming
agent for sole-leather than as a bate for dressing-leather, though it
may replace drenching. No doubt, by the use of warm water, and avoidance
of excess of acid, skins could be pulled down satisfactorily, or the
plumping could be controlled by addition of salt, but the disinfectant
powers of the acid would render further treatment with an ordinary bate
or puer very difficult.[95]
[95] J. Hauff, Eng. Pat. 22,546, 1894; Journ. Soc. Chem. Ind., 1895,
p. 170, Gerber, 1895, p. 133.
The “C. T. Bate,” manufactured by the Martin Dennis Chrome Company, is
of a very similar character; and is in the form of a greyish crystalline
paste, consisting mainly of sulphonic acids of naphthalene and probably
other hydrocarbons. It is very possibly made by sulphonating
coal-creasote oils, which contain much naphthalene and phenanthrene. The
following directions are given by the company for its use.
“1. After unhairing and fleshing from the lime, the skins should be
thoroughly washed with water (preferably warm) so as to remove as much
lime as possible.
2. If, in the liming process, the sulphide of sodium is used in
combination with the lime, it will render the lime more soluble and
therefore more easily removed with water.
3. The more completely the skins are cleansed with warm water the less
will be the quantity of bate required.
4. After washing, the skins should be thoroughly worked on the beam,
especially on the grain.
5. A solution of C. T. Bate is now prepared in the proportion of from
one-half pound to one pound of bate in 100 gallons of warm water (90°
F.). In making the solution do not have the water over 140° F. Under no
circumstances boil it.
6. If the hides or skins have been treated as above indicated, one pound
of bate should be sufficient for 400 pounds wet hide, washed from the
limes. The hides or skins are placed in the bating solution and worked
for an hour. They are then allowed to rest in the solution with
occasional stirring for some hours or over night.
7. The length of time that the bating should continue will depend upon
the degree of softness and pliability required in the leather. For
instance, for sole-leather fifteen minutes is sufficient; for satin
leather thirty minutes; for glove-leathers four to six hours or even
longer.
8. On removing the skins from the bating solution it is sometimes
desirable, especially for the finer grades of leather, to wash them in
warm water and again work them over the beam. They are then ready to be
placed in the tanning liquors.
9. In preparing the bating solution for the second pack, draw down the
old solution one-third and replace with fresh water; then add in
solution just one-half the quantity of bate used at first, and so on
with each succeeding pack.
10. When fresh white limes are used toward the end of the liming
process, and a manure bate is deemed necessary to reduce the harshness
of grain caused by the fresh lime, it is very beneficial to give the
skins from the manure bate a drench of C. T. Bate, thereby arresting
the bacterial action of the manure bate, preserving the grain, besides
cleansing, bleaching and neutralising the skins preparatory to placing
them in the tanning liquors.
11. Again, when it is considered desirable to use a manure bate, it is
good practice to treat the skins as above indicated (down to item No.
7), and then place them in the manure bate. By this previous treatment
the antiseptic action of the C. T. Bate tends to arrest the destructive
bacterial action of the manure bate, thereby lessening the risk of
damage to the grain. In all cases where the value of the leather is
dependent on the quality and perfection of the grain, this is an
important advantage to gain.”
All these coal tar “bates” are rather suitable to replace drenching than
bating or puering, as their effect is mainly that of removing lime. From
their antiseptic character they are very useful in stopping the effects
of putrefaction, and preventing ferments being carried into the tanning
liquors, and skins may safely be kept at least for some days in weak
solutions, but any necessary fermentive puering or bating should usually
be done before and not after their use.
A writer in the ‘Gerber,’ 1875, p. 279, recommends the use of dilute
solution of sulphide of sodium as a bating agent. Possibly it removes
lime as sulphydrate, and the writer named seems to have obtained good
results with glove lamb-skins. In experiments made at the Yorkshire
College, a solution of 4 grm. per litre used on 40 grm. of pelt was
found to plump it considerably, but probably a much weaker solution
might be sufficient and more satisfactory. Polysulphides, such as “liver
of sulphur,” or the yellow solution obtained by boiling dilute sodium
sulphide or sodium hydrate solution with excess of sulphur, have great
power of “bringing down” the pelt, and seem well worthy of experiment as
bating agents.
In India, the pods of the babool (_Acacia arabica_) are much used as a
bate, the infusion being allowed to ferment. In their dry state they
contain about 12 per cent. of an easily changeable tannin, which does
not precipitate lime-water, and which by fermentation is very probably
converted into gallic acid. The use of gallic acid itself as a bate has
been patented by Albert Hull,[96] and would undoubtedly accomplish the
removal of the lime if used in sufficient quantity; but as he only uses
a solution of 25 mgr. per litre (one part in 40,000) any effect must be
mainly due to the washing with water. Gallic acid forms dark oxidation
products with lime.
[96] Eng. Pat. 14,595, 1889.
Of the fermentive methods of removing lime, “drenching” with fermenting
bran-infusions is the simplest in theory, and has been very carefully
investigated by Mr. J. T. Wood.[97] It will, therefore, be convenient to
consider this process first, although it is frequently employed as a
means of cleansing and slightly plumping the skin after the lime has
been removed by puering or bating. In calf-kid manufacture, however, it
is now used without previous puering, and in some other cases it is
substituted for the use of dung bates. The most important of the active
ferments are two species of bacteria, named by Wood _Bacterium furfuris_
α and β, which are very similar in their form and action (see L.I.L.B.,
p. 264), but produce a somewhat better fermentation together than
separately. They are shown in Figs. 34 and 35.
[97] Journ. Soc. Chem. Ind., 1890, p. 27; 1893, p. 422; 1897, p. 510;
Brit. Assoc. Rep., 1893, p. 723.
[Illustration: FIG. 34.--_Bacterium furfuris_ α.]
[Illustration: FIG. 35.--_Bacterium furfuris_ β.]
Neither species has any direct action on the hide substance, but
ferments the glucose produced by the action of the cerealin of the bran
on the starch which is present. A considerable quantity of hydrogen,
with carbon dioxide, nitrogen and small quantities of hydrogen sulphide,
are produced during the fermentation, together with lactic and acetic,
and traces of formic and butyric acids and amines. Active drenches
contain 1-3 grm. of mixed acids per liter, to which they owe their
action, a perfectly satisfactory drenching being produced by an
artificial drench containing 0·5 grm. of glacial acetic acid and 1 grm.
of lactic acid (sp. gr. 1·210) per liter in which the skins were worked
for 1¹⁄₂-2 hours, while 12-16 hours would have been required in the
ordinary drench. An experimental drench gave the following results on
analysis:--
Formic acid 0·0306 grm. per litre
Acetic acid 0·2402 „
Butyric acid 0·0134 „
Lactic acid 0·7907 „
------
Total 1·0749 „
It is probable that other organisms are capable of producing similar
fermentations, and it is not certain that in all tanneries the same
ferments are present. Mr. A. N. Palmer states that at the Cambrian
Leather Works at Wrexham, he has been unable to detect lactic acid in
the drenches, all the acids present being of the acetic series.
The drench-ferments investigated by Wood are incapable of attacking or
injuring the hide, and, in his opinion, when the skin is attacked, it is
generally due to putrefactive and gelatine-liquefying organisms
introduced from the bates, or from the air in hot sultry weather.
Drenching takes place most safely and satisfactorily at temperatures not
exceeding 30°-35° C., when the process is usually complete in 12-24
hours. In hot sultry weather a butyric fermentation of an active
character sometimes suddenly takes the place of the normal one (Ger.
_Umschlagen_), the skins swell rapidly, become translucent (_glasig_)
and finally dissolve to a jelly. If tanned in the swollen condition,
tender and useless leather results, and the injury, once begun, proceeds
with alarming rapidity, skins being sometimes completely ruined in a few
hours. Prompt action is therefore necessary, and the first step to take
is to add salt, which checks the fermentation, and acts in the same way
as in the pickling process, controlling the action of the acid, and
producing a sort of tawing. Such skins will yield sound leather, though
the grain is apt to be somewhat drawn. If the skins can be immediately
got out of the drench, the acid may be neutralised by the cautious
addition of ammonia, soda, or whitening to the water in which they are
placed, preferably in a paddle, and if they are insufficiently drenched
they may then be paddled in tepid water, though this is hardly likely to
be needed, as the effect of the acid is to remove the lime very
completely. The objection to the use of whitening, which otherwise is
the safest and best material to employ for removing acid from pelt, is
that it is apt to become mechanically fixed in the grain, and, thus, to
produce bad colour with vegetable tans. For white or chrome leather it
would do no harm. Precautions to prevent the recurrence of the injury
are to keep the temperature of the drench low, and to free the bran from
flour by washing in two or three cold waters, before adding to it the
hot water with which the actual drench-liquor is made, since the flour,
or at least its starch, is the source from which the butyric acid, as
well as the lactic, is formed. In cold weather, where drenching is
proceeding in a normal way, the flour is useful, since it is the natural
nutriment of the drench-ferment; and, in England, flour is frequently
added purposely to the bran to increase the activity of the drench. To
retain the flour, the bran may be washed first with boiling water, which
gelatinises the starch and makes it adhere to the bran, and, according
to Eitner, removes a sticky fatlike matter from it, and fits it better
to remove the fat of the skin. After soaking in hot water for two hours,
it is washed in several cold waters and infused at about 40° C. for
use.[98] Many tanners use the bran without previous washing, but if much
flour is present it rises to the top with the gas evolved by the
fermentation, and forms a pasty mass on the skins, which interferes with
even drenching.
[98] Gerber, 1882, p. 246.
The quantity of bran used in ordinary drenching is very variable, but
about 4 parts per 1000 of water used and from 5 to 10 per cent. on the
weight of pelt may be taken as an average quantity, more being
frequently employed. The temperature may vary from 10° up to about
30°-35° C., and the time inversely from days or weeks down to two or
three hours, according to the temperature of the drench, the amount of
ferment present, and the thickness and character of the skins. The skins
are usually thrown into the freshly prepared drench, to which a few
pailfuls of old drench-liquor is frequently added as a ferment.
Fermentation soon sets in, and the gas evolved causes the skins to float
to the surface; this is called the “working” of the drench. Thin skins
may be sufficiently drenched after once rising, while thick ones require
to be put down two or three times. A certain sign of sufficient
drenching is the appearance of small blisters on the grain, caused by
the evolution of gas in the substance of the skin. When these are seen
the drenching should be at once discontinued, as otherwise the blisters
will increase in number and burst through the grain, causing minute
holes or “pricks” (one of the many forms of the complaint called in
German _Pikiren_ or _Piquieren_). When a bubble of air is enclosed in a
fold of the sufficiently drenched skin and pressed, it raises the grain
without actually separating it from the substance of the skin. The
properly drenched skin also falls easily in folds when held between the
hands either lengthways or crossways, and if thin, the skin tightly
stretched over the hand shows grains of bran underneath it as little
lumps, round which the skin clings to the hand. The drenched skin should
not be transparent, but white and soft; and when pressed should retain
the mark of the finger. Some experience is required to determine
certainly the point of sufficient drenching, which, of course, varies
with the character of the skins, and the kind of leather which is to be
produced; and the feel of the skin to a practised hand is one of the
most important criteria.
A writer in the ‘Gerber’[99] divides drenching into three
classes--“sweet,” “alcoholic” and “sour.” Sweet drenching is done in a
bath of tepid bran-water, made by infusing in hot water and drawing the
clear liquor off the bran, which settles to the bottom. The skins are
only allowed to remain in 2-3 hours, or not long enough for fermentation
to set in. The process is only suited for very thin or soft skins, which
will not stand any further loosening. The use of bran-water has the
advantage of saving the labour of “branning,” or removing adhering bran
with the knife on the beam, but it is doubtful if unfermented bran has
much actual effect. Bran-water can, however, be used for drenching by
fermentation, and for small glove-lamb has largely superseded the older
method. The mechanical action of the bran in cleansing the pelt is
however often useful. In sour drenching the bran is allowed to steep and
soften in cold water for many hours, and boiling water is then added
till the temperature is raised to 75° C., and it is allowed to infuse
with frequent stirring for some hours, and after cooling to 45° a
considerable quantity of old drench-liquor is added as a ferment. If the
drench is used warm (30°-35°, or, in cold weather, even 40° C.), the
skins only remain in 1-3 hours, but if cold the drenching can be
extended over a period of 2-3 days, the skins being frequently handled.
This modification is suitable for glacé-kid and the harder sorts of
skins, but glove-lamb are always treated by the warm and rapid process.
What the writer in the ‘Gerber’ describes as the “alcoholic” bran-drench
is probably the method of fermentation investigated by Mr. Wood, in
which ordinary inflammable gases, but no alcohol, are produced.
[99] Gerber, 1888, p. 257.
A normal drench plumps the goods slightly, but if it contains much of
the putrid ferments carried in from the bate or puer the skins fall in
it as they would do in a bate. To increase this effect, putrid
soak-liquor is sometimes added to the drench, but with doubtful
advantage.
In drench-liquors the total acidity may be determined by titration with
lime-water or N/10 caustic soda, with phenolphthalein as indicator; and
the volatile acids may be distilled off as described under the analysis
of tanning liquors (L.I.L.B., p. 126). For more complete methods of
analysis the reader is referred to Messrs. Wood and Willcox’s paper on
the “Nature of Bran Fermentation.”[100]
[100] Journ. Soc. Chem. Ind., 1893, p. 422.
Drenches are said to “work” somewhat better if made with water
containing nitrates, and this is quite probable; but the necessary
nitrogen can easily be supplied if required by the addition of a very
small quantity of saltpetre.
Wood is of the opinion that the ferments found in bran do not originate
in the drench itself, but come from the bated skins, as the
drench-bacteria soon die out without finishing the fermentation, and
constant renewing of the nutrient material is necessary (cp. p. 18).
_Bating and puering_, though differing practically in many ways, are
identical in theory, and most of what follows applies to both of them.
The action is much more complex than that of the drench, involving both
chemical reactions and those of organised and unorganised ferments, and
it is a matter of no little difficulty to say what proportion of the
observed effect should be ascribed to each of these agencies.
Formerly, the principal effect was attributed to organic salts of
ammonia and its homologues, and to amido-acids which combine with lime.
Phosphoric acid is also present, and if any exists in the form of
soluble salts, it will combine with lime, and render it insoluble and
inactive. It is probable, however, that most if not all the phosphoric
acid is already in the form of tricalcium phosphate, and therefore
without effect.
It is now, however, recognised that the effects of these chemicals are
of no importance as compared with the products of bacterial action, and
the researches of J. T. Wood have cleared up much that was until
recently quite inexplicable.[101]
[101] Journ. Soc. Chem. Ind., 1894, p. 218; 1895, p. 449; 1898, pp.
856, 1010; 1899, pp. 117, 990.
Much effect has been ascribed to the digestive ferments, such as pepsin
and trypsin, which are present in fresh dung. It is known that the
animal organism secretes these in considerable excess of its
requirements, but it is doubtful whether any exist undecomposed, even in
fresh dung; though they are apparently more resistant to putrefaction
and decomposition than would _a priori_ have been expected of such
complex organic compounds, and there is therefore a possibility of their
existence in the dung, even as it comes to be used in the tannery. Both
pepsin and trypsin are enzymes (see p. 16), and belong to the great
class of albuminoids. They are soluble in water, but insoluble in
alcohol, and hence are precipitated by the addition of the latter to
their solution, but are not altered by it, and regain their activity on
solution in water. By heat they are coagulated and decomposed, and their
activity permanently destroyed.
_Pepsin_ is the active principle of the secretion of the glands of the
stomach, and large quantities are prepared for medical use as an aid to
digestion from the stomachs of pigs. Pepsin only acts in slightly acid
solution, and, though fresh bate liquor is slightly acid to litmus, it
speedily becomes alkaline from the lime of the skins and the ammonia
present, so that the action of pepsin in a bate can only be a very
limited one. Wood[102] compared the action of a 1 per cent. solution of
pepsin, acidified with 0·2 per cent. of hydrochloric acid, with that of
a dogs’ dung puer liquor, both at the temperature of 40° C. At the end
of one hour the skin in the pepsin-solution was considerably fallen, but
that in the puer-solution was almost dissolved. Since the solution here
employed was much stronger than is likely to occur in practice, and the
conditions much more favourable to its action, it may be assumed that
the practical effect of traces of pepsin in the bate may be neglected.
[102] Journ. Soc. Chem. Ind., 1894, p. 220.
_Trypsin_ or _pancreatin_[103] if present, is more likely to have an
effect, since it is active in neutral and in alkaline solutions. It is
the product of the pancreas, and is largely concerned in intestinal
digestion. Chemically it much resembles pepsin, but is more resistant to
heat, retaining its power of digestion after heating to a temperature of
160° C. in a dry condition. Its warmed solution dissolves fibrin almost
instantly, and in large quantity, and peptonises gelatin and hide-fibre,
so as to render them soluble in water. Wood found that a 1 per cent.
solution of pancreatin acted far more rapidly than a solution of pepsin
of equal strength. At 40° C. in neutral solution, the skin fell rapidly,
and the action continued even in the cold. In 15 hours the liquid was
swarming with minute bacteria. At the suggestion of the Author, the
experiment was therefore repeated, with the addition of 15 per cent. of
chloroform, which prevented the development of bacteria, while it did
not stop the action of the pancreatin. The skin fell as before, but in
neither case had it the peculiar touch of puered skin, nor were the
characteristics of the leather produced from it the same. We may
therefore conclude that, though trypsin may contribute to the action of
the bate or puer, it can only do so in a minor degree, and that the
principal effect of the bate or puer is due to other causes. It is
certain, however, that fresh bird-dung, and probably that of all
animals, contains ferments capable of liquefying gelatin. An instance of
this is found in the observation, common in glue manufacture, that if
the dropping of a sparrow falls on a cooler full of solidified gelatine
size, it will liquefy a track quite down to the bottom of the cooler.
Trypsin, or at least the secretion of the pancreas, as well as the gall
from the liver, have great power of wetting and emulsifying fats, and
this has possibly something to do with the action of the bate in
enabling the skins to be cleansed of fat.
[103] Loc. cit. and Beilstein, iii. p. 1308, 2nd ed.
Bacterial fermentation and its products are however the main factor in
the action of puers and bates, and on this subject we owe most of our
knowledge to the work of J. T. Wood, since, though Popp and Becker have
worked over much of the same ground, they have not nearly so freely
published their results.
Wood showed that a fresh puer liquor, even when boiled for half an hour
and so freed from living organisms and albuminoid ferments, has still
considerable action on a limed skin, though much less than the unboiled
puer. He found that this action was principally due to amines and their
compounds with organic acids, which removed lime, but did not remove the
interfibrillary substance or give the proper feel of puered skin. A very
similar result was obtained with aniline (phenyl-amine) hydrochloride in
1 per cent. solution.
A considerable variety of bacteria from dung and other sources were
cultivated in various media and their puering power tested, but though
greater than that of the unorganised chemical compounds such as amine
salts and organic acids, it was in no case equal to that of an ordinary
puer, or sufficient for practical use. When, however, a small quantity
of the amine salts obtained from the puer were added to a mixed
bacterial culture the effect on the skin was almost as rapid and
considerable as with an actual puer.
In order to determine whether the puering effect was due to the direct
action of the bacteria or to their enzyme-products, the latter were
separated from a filtered puer solution by adding it to a large volume
of 98 per cent. alcohol in which the enzymes are insoluble. When
redissolved in water, they had a decided puering effect, and a solution
of 0·5 grm. of the mixed enzymes and 0·5 grm. of the mixed amine
hydrochlorides in 100 c.c. of water at 350° C. brought down a piece of
limed sheep-skin in thirty minutes exactly like a puer. The action is
therefore dependent on the mutual action of the enzymes and amine salts,
but as the separation of these would be too costly for practical use,
and the puering proved more effectual when they were formed in contact
with the skin by active bacteria, Wood adopted the method of preparing a
suitable sterilised nutritive liquid, which was inoculated before use
with a mixed culture of suitable bacteria. For laboratory purposes a
suitable culture-medium was obtained by digesting 10 grm. of gelatine
with 5 grm. of lactic acid (reckoned water-free) and 100 c.c. of water
for three hours in a closed vessel on the water-bath. The resultant
solution was neutralised with sodium carbonate and diluted to 1 litre
with addition of a small quantity of potassium phosphate.
The bacteria of fresh dog-dung were not found to possess a satisfactory
puering effect, but those from dung which had been fermented a month (as
in practice) gave a result nearly equal to actual puer. A still better
result was obtained by a mixed culture from the roots of wool loosened
by sweating. The bacteria were principally of two species, of which
neither separately was capable of satisfactory puering; but which
together acted more rapidly than an actual puer. These bacteria do not
liquefy gelatine.
During the course of his experiments, Wood found that filtered puer
solutions were less active than turbid ones and that their activity was
increased even by the addition of inert substances, such as kaolin.
Wood attributes the differences in action between dog-dung and bird-dung
not only to different bacteria, but to the fact that in the latter case
the urinary products, and especially uric acid are contained in the
dung.
From the results of these and similar researches, Wood in England, and
Popp and Becker in Germany succeeded in producing a practical artificial
puer, which they now manufacture in conjunction under the name of
“Erodin.”
“Erodin” consists of a solid nutrient medium and a liquid “pure culture”
of the bacteria necessary to effect the required bating or puering.
The following are the directions for working with erodin bate, as
supplied by the manufacturers:--
“For 100 lb. of wet skin washed ready for bating, about 1 lb. of erodin
is required. Or in the metric system, 1 kilo. wet skin requires about 10
grm. erodin. The strength or concentration of the bate must not fall
below 3 grm. per litre of bate liquor, i.e. ¹⁄₂ oz. per gallon.
For preparing the bate a sufficiently large cask or tub carefully
_cleaned_ and steamed out is placed near the bating paddle. The cask
should be fitted with a steam pipe easily screwed on and off, and also
furnished with a _clean_ cover.
The requisite quantity of erodin is weighed out and put into the tub
with fifty times its weight of water, and the whole brought up to a
temperature reaching but not exceeding 40° C. (104° F.) by direct
admission of steam, thoroughly stirred, and the pure culture of
_Bacillus erodiens_ added to the mixture. The temperature must not be
allowed to fall below 25°C. (87° F.), and a little steam should be
admitted first thing in the morning, again at noon, and in the evening,
to bring the temperature up to 40° C. (104° F.).
A practical mode of procedure is as follows:--On Friday make up and
start fermenting twice as much erodin as will be required for a day’s
work. This is allowed to remain under the above-mentioned conditions
until Monday. On Monday half the amount will be used for bating; this is
replaced by an equivalent amount of fresh erodin powder, dissolved in
fifty times its weight of water, which is added to the already fermented
erodin in the tub. Proceed in this way each day until the following
Friday, when there will be left in the tub sufficient erodin for one
day. This is put into a smaller tub for use on Saturday, and the cycle
of operation begun again.
One pure culture of _Bacillus erodiens_ should be used for every 11 lb.
(5 kilos.) erodin powder or less quantity.
Suppose the amount of erodin required for a day’s work to be 11 lb. (5
kilos.), then on Friday 22 lb. (10 kilos.) erodin must be mashed as
above described in 110 galls. (500 litres) water, 2 pure cultures added,
and allowed to ferment until Monday.
On Monday half of this is used, and to the remainder 11 lb. (5 kilos.)
erodin and 55 galls. (250 litres) water is added. This is repeated on
Tuesday, Wednesday and Thursday; and on Friday half is used and the
remainder put into a separate cask for use on Saturday, and in the
mashing cask a fresh quantity of 22 lb. (10 kilos.) erodin with 110
galls. (500 litres) water is made up for use next week.[104]
[104] Mr. Wood has found that in many cases it is unnecessary to start
afresh at the end of each week, but that additional quantities of
erodin solution with the accompanying bacterial culture may be added
continuously to the stock-tub as required. In puering, the
concentrated solution from the tub may be diluted with 4 to 6 times
its volume of warm water. The diluted liquor should usually only be
used for one pack of skins.
On Saturday the remainder of the old mash is used up.
In case this mode of procedure is for any reason not suited to the
conditions of work, erodin may be used by making up every day a fresh
quantity with fifty times its weight of water, adding the pure culture,
and allowing it to ferment three days before use.” In some cases the
solution may be used for several consecutive packs, merely adding water
and a small quantity of erodin without a new culture.
Erodin is being used most successfully in several large works both in
England and abroad, and on calf-skins and sheep-skins has proved quite
as effective and much safer than dog-dung; the skins coming out clean
and free from stains. It has been a good deal used in the experimental
tannery of the Yorkshire College, and has proved a satisfactory
substitute for puer, but with the present bacterial cultures can only be
employed warm, and does not answer used cold like the ordinary
pigeon-dung bate. No doubt a suitable bacterial medium and culture can
be found for cold bating, which for thicker leathers is often preferable
to puering, and experiments in this direction are being undertaken.
From the multiplicity of germs present, and the adaptability of the dung
infusion as a nutrient medium for any putrefactive organisms which may
gain access to it, the bating and puering process is necessarily a
dangerous one for the goods, always leading to loss of weight, and, if
the process is carried on too long, to the more or less complete
destruction of the skins. Loss of weight, however, in greater or lesser
degree is inevitable, and indeed necessary where a soft leather is to be
produced. If the skins are allowed to lie in the bate or puer liquor,
mud, containing organisms, and zooglœa-forms of bacteria settle in the
folds, and produce marbled markings, streaks and lines by the
destruction of the grain surface (hyaline layer). Black or bluish stains
are also often produced, known as bate-stains, and either due to
bacterial pigments, or in some cases, to the action of evolved hydrogen
sulphide on iron present from salting or other sources. Frequent change
of position is therefore necessary, especially when the liquor is active
from being used at a high temperature, but it does not seem to be
desirable to keep the skins in constant motion, and if puering is done
in a paddle, it should only be run at intervals.
T. Palmer[105] determined in experiments on pigeon-dung bates that there
is considerable loss of nitrogen during the process, and recommended
bating in pits from which the air was excluded as much as possible, both
as effecting a considerable economy in the dung, and in excluding false
ferments, which, he concludes, are mostly aerobic. It is not improbable
that the method is advantageous, since it has been shown by Roscoe and
Scudder that liquefaction of gelatin only takes place in presence of
oxygen, and its partial exclusion would therefore lessen the risk of
overbating, and consequent damage and loss of weight.
[105] Leather Trade Circular, 22nd Sept., 1891; 1887, p. 667; and
Sanford, Journ. Soc. Chem. Ind., 1893, p. 530.
Starting from the presumption that bating and puering are, in the main,
bacterial processes, more or less successful attempts had been made
previous to those of Wood, Popp and Becker, to substitute other
fermenting substances for dung; and probably these efforts failed in
many cases, not so much because they were wrong in principle, as from
want of knowledge of the necessary details, such as the use of proper
ferments, and the provision of suitable culture-media. Guano, prepared
horse-flesh, urine, yeast, and fermenting vegetables have all been
tried. A solution of glucose or treacle of about 10 per cent., to which
3 per cent. of pasty dog-puer is added about a week before use, was
tried many years since in a morocco-factory, at the suggestion of the
writer, as at least a partial substitute for puer, and is still in use
there. The mixture keeps for some time in an active state, and is added
to the puer liquors in the same way and in approximately the same
proportions as the dung paste. Similar in principle is the solid bate
supplied by an American firm, in which glucose is mixed with a small
amount of nitrogenous matter and phosphates, together with a lactic
ferment, and which only requires dissolving in warm water some little
time before use. Its results are good for some purposes, but rather
resemble those of a drench than a bate. In a similar way, puer may be
added to bran-drench liquors, and induces in them a fermentation which
brings the skins down much lower than the ordinary drench. It is
probable that a weak glucose solution, with traces of mineral
constituents similar to Cohn’s solution (see L.I.L.B., p. 269) and “set”
with sour milk, or fermenting drench-liquor, might in some cases be used
with advantage for drenching, with a saving of cost. A writer in ‘Hide
and Leather’ describes a bate in which two parts by weight of glucose
are dissolved in about 25 parts of water, and fermented, for about three
days, till a foam gathers on the top, with about one part of old bran
drench-liquor, or 0·1 part of pressed yeast, and then made up with water
to 1000 parts. The goods are bated 24-36 hours at a temperature of about
35° C, and the bate is strengthened for a second pack with about
one-fifth of the original glucose, a new bate being made at the end of a
week, and set with one part per thousand of the old one. A short bating
of say 10 hours produced very nice harness-leather, but the general
tendency was to make the goods looser and more spongy than a dung-bate.
It is obviously not a matter of indifference whether old drench, or
yeast, is used to start the fermentation, since in the latter case only
alcohol could be produced directly by the ferment introduced, though
this might be fermented later, by other accidental organisms, into
acetic acid. These mixed bates, containing glucose, are however probably
wrong in principle, since the true puering and bating bacteria will not
thrive in presence of acids, and require nitrogenous nutriment.
As regards the relative effect of dog- and hen- or pigeon-dung bates,
the chief of the published experiments are those made by W. J. Salomon
at the Vienna Versuchsanstalt für Lederindustrie,[106] in which he
determined the relative solvent power of equal quantities as being, for
dog-dung 2¹⁄₂, for pigeon-dung 2, and for hen-dung 1. It is obvious that
these figures, though interesting, must be taken with some reserve, as
the composition even of pure dungs is by no means constant, depending on
the feeding of the animals, and adulteration is common. The writer has
heard stories of a certain dealer who used to fabricate his product from
clay by the aid of a popgun, though he does not vouch for the statement!
It is generally held that the action of bird-dung is more penetrating,
but less softening and loosening than that of dog-dung, which is thus
generally used for descriptions of leather where great softness and
stretch are required. It is to be remembered in this connection that
bird-dung bates are generally used cold, and hence are much slower in
their action, which allows them time to penetrate thicker hides more
uniformly. Few analyses of the dungs used in leather manufacture have
been published, and these mostly with a view to manurial value.
Schulze[107] gives the result of forty analyses of pigeon-dung as
follows:--
Min. Max. Mean.
per cent. per cent. per cent.
Water 3·80 40·00 21·00
Nitrogen 1·47 5·04 2·53
Phosphoric acid 1·00 2·77 1·79
Potash 0·71 2·57 1·46
One sample contained 43·3 per cent. of sand!
[106] Tech. Quart., 1892, v. p. 81.
[107] Der Landwirt, 1895, li. p. 301.
Wood[108] quotes the following:--
_Hen-Dung._
Per cent.
Water 60·88
Organic matter[109] 19·22
Phosphates 4·47
Calcium carbonate and sulphate 7·85
Alkaline salts 1·09
Silica and sand 6·69
_Dog-Dung._
Water 31·0
Ca 43·0
Na, K, Mg 0·8
PO₄ 3·4
CO₂ 7·5
Organic matter 14·2
Traces Fe, Cl, Si, loss 0·1
[108] Journ. Soc. Chem. Ind., 1894, p 220.
[109] Containing nitrogen equal to 0·74 per cent. of ammonia.
This was apparently a sample from a dog fed on bones; that from the
kennels, which is more commonly used in leather manufacture, contains
much less lime; a sample analysed by Wood gave 4·7 per cent. mineral
matter, 9·7 per cent. organic, and 85·6 per cent. of water, part of
which was no doubt added.
_Analysis._--Little or no attention has been paid to the analysis either
of dungs for bating purposes, or of the bating liquors, and although the
total cost of manure bates is a high one, it is evident that such low-
priced and irregular articles will not pay for elaborate analysis.
Probably in some cases it would be worth while to make a determination
of moisture and organic and mineral constituents by drying and ignition.
Where a further investigation is desired, the determination of the
soluble matter by filtering and evaporating a portion of the solution to
dryness, and that of the nitrogen by Kjeldahl’s method (see p. 70),
would be advisable, and of course in the future, when the subject is
better understood, a bacteriological examination may be useful. If it is
desired to estimate the solution of hide-substance in the use of bate or
drench liquors, the determination of the nitrogen in a measured quantity
by Kjeldahl’s method will afford the best basis of calculation,
allowance being made for the nitrogen present in the original bate
liquor. Hide-substance contains about 17·8 per cent. of nitrogen. In
many cases, simple weighing of the solid residue, left on evaporating
the liquor to dryness and drying for several hours at 100° C., with
subsequent ignition to determine lime and other mineral matters, will
suffice.
The quantity of hen- or pigeon-dung used in bating hides is very
variable, but may be stated at from 12 to 60 litres per 1000 kilos of
raw hide, in at least 2000 litres of water. The bate is generally used
cold, the hides remaining in it 4-8 days, with frequent handling; but
some tanners, especially in the United States, prefer bating in a paddle
or drum at a temperature of about 35° C., in which case the time must be
diminished to a few hours. The dung is best infused with warm water in a
separate vessel,[110] and allowed to ferment for at least a week without
use, when it will be found to swarm with micrococcus-chains. Only the
clear liquor should be run into the bate-pit, the sediment and dirt
being thrown away, or used as manure. In this way the danger of stains
and flaking is much reduced. Bates may be mended with fresh portions of
dung-infusion for several successive packs of hides, but should not be
used too long, as they gain in solvent power by the dissolved
hide-substance and the increased fermentation, and the method is not
without risk.
[110] This seems to have been first suggested by T. Palmer, Eng. Pat.
13,636, 1886.
After bating, the hides are usually “worked” (“scudded,” “fine-haired”)
on the beam, to remove dirt and grease, but in America a wash in the
wash-wheel is often considered sufficient. Goods are occasionally
“stocked” (p. 116) from the bates, but this is not to be recommended, as
it is likely to drive out much of the partially dissolved hide-substance
and produce undue looseness and loss of weight.
It is difficult to give any definite marks of sufficient bating other
than the soft and fallen feel of the hides, which is easily recognised
by a practised hand. One of the earliest signs of commencing overbating
is the occurrence of bluish patches, or a bluish tinge somewhat similar
to an iron-stain, which, if slight, generally disappears in a few days
after the hides are taken into the liquors. Hen- and pigeon-dung is
probably best kept air-dried, though, if very wet, or for convenience
for immediate use, it may be kept in paste like dog-dung.