Bourne Archive: Muspratt: Hide Preparation
http://boar.org.uk/aaiwxw3MusprattL6Preparation.htm Latest edit 27 Dec 2010
The Bourne Archive
Extracts Concerning Leather, 6: Preparation of the Hides.
The web pages linked from this introduction are from an article on tanning, under the heading ‘Leather’. The original article is presented here in several web pages, respectively dealing: 1, with leather; 2, with tanning materials; 3, Sources of tannin; 4, crushing mills; 5, varieties of skin; 6, hide preparation; 7, the common tanning process; 8, finishing processes; 9, fancy and speciality leathers.
Vol. 2. pp. 511-516
Preparation of the Hides.—The tanner, previous to submitting the hides, skins, of kips, to the true tanning process, is under the necessity of subjecting them to one or more preliminary operations, with the view of removing dirt, particles of flesh, and in most cases the hair and epidermis; and the manner in which these are effected is of great importance, as much of the success of the subsequent process, and, consequently, of the quality of the leather, depends upon it
Cleansing.—The hides which have been recently taken from the
carcass of the animal require but very little labor to purify them from any
filth which may remain attached to them, unless, indeed, as is said to be
sometimes the case, they are wantonly saturated with such matters in order that
they may weigh heavier. Sometimes, however, such green or fresh hides, as
they are called, when they cannot be immediately disposed of to the tanner, or
when the latter does not intend to submit them at once to the processes by
which they may be made into leather, are salted to prevent putrefaction 1.
When the hide has to remain for a period of a week, four or five pounds of
coarse salt are used to preserve it; if a month, double this quantity is deemed
requisite. Foreign hides—such as those from
that is, the removal from the water or steep—and scrapings are multiplied before they attain the softness and purity of clean fresh ones. In large establishments the mechanical labor required is executed by a kind of mill, similar to that for fulling cloth, wherein the hides are exposed to the continued action of two heavy stocks 4, which squeeze out the slimy matter and dirt that might induce a putrefactive fermentation. The immersion and handling even in this case require to be alternated with the action of the machine; and it is necessary to remark that where vats or tanks are used, the water should be frequently renewed, otherwise, if the contents happen to get stale or overcharged with the slimy extract from the hides, and the time of submersion be prolonged, there is danger of putrefaction, and, therefore, of injury to the skins. Where only water is employed for the above purpose, it is necessary to keep in view the quality of this liquid, irrespective of the change which it undergoes by saturation with the nitrogenous matters, dirt, et cetera, from the hides. When very calcareous, its cleansing qualities are less effective than when the water is soft; but, independently of this, it is believed that hard water affects the hides in another way, by which their softness and porosity are injured owning to the fatty and other matters which may be in an incipient state of decomposition, forming combinations with the earthy bases which fix themselves to the skin 5.
In many parts of
Depilation.—After the thorough softening and cleansing of the hides,
they are next submitted to a further immersion in baths or tanks with compounds
which effect the solution of the cellular matter in which the hair is rooted,
or are otherwise exposed to the action of agents which operate towards the same
end. A variety of methods are practised by tanners for the solution or
softening of the matter which binds the epidermis and hair to the true cutis,
or that portion of the skin that enters into the formation of leather, with the
view to their removal. The most important of these are the lime process, the sweating or fermentative process, the rhusma and gas lime process;
and latterly, in
By Liming 6.—Of these processes the liming
is by far the oldest, and is still very extensively adopted, but with greater
caution than in former times. In its action it is not so quick as certain other
agents, necessitates much future labor, and frequently is the cause of injury
to the leather; still, probably from its cheapness and the effect of custom, it
is more resorted to than any other. Generally speaking, it is now used in the
form of a semi-liquid or milk, formed by agitating a certain quantity of it
with a given body of water—about eight bushels to one hundred and fifty
gallons. The liming is carried on in a series of pits or tanks, sunk in the
floor of the liming house; these are built of bricks or masonry, and well
cemented, and have an outlet at the bottom for the purpose of drawing off the
fluid contents when necessary. Their dimensions vary according to the
requirements of the establishment, and the number of skins to be operated upon
at once. In some parts of
When the epidermis is sufficiently
softened it is detached, together with the hair on the beam. This is an arched wooden or stone bench—Fig. 347—upon which
the hide is laid, with the flesh side inwards, and the hair removed by the scraping knife,
shown in Fig. 348. After the separation of the hair and epidermis, the loose
fragments of skin are pared off, as likewise the flesh and fatty matters adhering to the reverse side—such fleshings being
economized in the preparation of glue 7. The implement used for the latter purpose
is a long, straight knife, having a sharp edge. This is called the fleshing knife, and is represented in Fig. 349. After the
last operation, it is necessary to submit the hides to thorough rinsing in the
stream or vats, in order to remove all uncleanness, lime, et cetera, as much as possible. It often happens that the hides
present inequalities as to thickness and the like, and to reduce them to
average homogeneity, they are treated with the smoothing or polishing stone,
represented in Fig. 350. This tool is of sandstone,
interiorly moulded, so as to lie parallel upon the beam, fixed to a piece of
wood, and furnished with handles, with which the hide is rubbed, to bring the
part to an equal thickness as nearly as possible. This operation is not so much
practised by English as by continental tanners. By repeated scrapings in this
way with the knives and stone, alternated with steepings and washings, the
dirt, hair, fleshy parts, and lime are removed as effectually as possible,
previous to the true tanning operation. In the large English tanneries the
scraping and fleshing of the hides are executed by mechanical contrivances, and
so the manual labour is greatly diminished. Machines for this and other
purposes have been constructed by several, among others by
Bating 8.—With respect to the removal of lime from
the skin, it has been found that no amount of scraping and rinsing is capable
of making a perfect separation, a portion of the earth being retained in the
pores of the skin, combined with, perhaps, some of the decomposing tissue, and
with the grease and fat of the hide, as a lime soap. These combinations, if not
altered and removed, obstruct the union of the tannic acid with the gelatinous
fibre of the skin, and not only this, but they communicate a rigidity to the
leather which excludes it from such applications as require a soft, tough, and
pliable material. Several plans are employed to remedy this defect. The old
method is that which is known as bate,
or grainer, that is, a menstruum
prepared by macerating the excrements of pigeons, pen-fowls, and of dogs, in
water, and immersing the scraped and clean skins in it for a certain period,
when, it is stated, the lime is separated by a chemical decomposition that takes
place and renders it soluble. The proportions employed are from ten to twelve
gallons of fresh dung to one hundred, or less if they are large skins; and the
time of steeping stretches over ten days in ordinary weather, but less time is
required if the liquid be warmer. In this treatment, likewise, the repeated
handling and scraping on the beam are resorted to. Investigation has shown that
the above matters contain an ammoniacal chloride
that parts with its chlorine on coming into contact with the lime, and so gives
rise to a soluble combination of this base—chloride of calcium—that may be
readily abstracted by water. That this disgusting method should still continue
to be so extensively practised is disgraceful to modern science. It is stated,
Although this method of bating the hides is tolerably effective, yet it is well known to be attended with serious disadvantages, not the least of which is the putrefaction of the bate that is going on during the steeping, and which injures the hides by acting upon the tissue of the skin, reducing its weight in the first instance, and in the second, rendering it incapable of yielding the quality of leather it would doubtless produce, did the above change not occur. Taking the hint from the action of the alkaline chloride in this case, some tanners have attempted to prevent the putrefaction induced by the bating with excrements by avoiding its use altogether, using hydrochloric acid in a diluted state as a substitute. The innovation has not yet become very general in England, although many tanners, especially in Paris, are reported to have practised it successfully, the object of the expulsion of the lime being satisfactorily gained by it, and in addition to this, the swelling of the hides also. Sulphuric acid has likewise been proposed as a substitute of the bat, but the effect of this is not so powerful as that of the acid already mentioned, since it constitutes with the lime a very insoluble compound. The acid is, however, almost always resorted to for raising or swelling the skins, preparatory to submitting them to the tan-pits. MacBride in 1774 was the first who showed that it possessed this property. Turnbull introduced the use of saccharine liquids 9 for removing the last portions of lime from the hides, as well as for softening the hair and cuticle. Four or five pounds of coarse sugar or treacle dissolved in sixty to seventy gallons of water, constitute the bath. The sugar in this process forms a soluble saccharate of lime 10, which is removed by soaking and rinsings. A new process, in connection with others, for the purpose of facilitating tanning and improving the leather, was patented in 1841 by Warington. It consists in employing carbonate of ammonia instead of the usual bate for the graining of the skins; but although this salt converts the lime into a neutral compound, it does not remove it, and consequently, the flexibility and softness of the leather must suffer.
Depilation 11 by Acids.—Besides
these methods, by which the influence of the lime on the skin is counteracted,
others are employed which dispense with it in the first instance. Acid liquors
have long been known to operate upon the roots of the hair and epidermis, so as
to detach them from the skin. The Calmuck-Tartars employed a bath of sour-milk
for this purpose, and in
By Sweating.—The depilation is effected at the present day by many English and
continental tanners, more especially the latter, by what is known as the sweating process. Various means are
adopted, but the principle common to all is the softening or the epidermis, by
fermentation, or putrefaction. In
Steam and heated vapors have also been tried as agents for removing the hair from the skin. M. Dellut was the first to suggest the use of steam, and his method was carried into effect in several French tanneries. The hides are suspended in an airtight chamber of suitable dimensions, and which is furnished with a perforated, false, or second flooring. Steam is introduced beneath the perforated flooring, and ascends among the hides, through the small orifices, so as to keep the interior of the chamber at 68° to 80°,12 but not beyond this; for, in the event of the temperature becoming elevated, the gelatinous matter of the skins would more or less enter into solution with the condensed steam on them, and consequently, would afford a less weighty leather. When due attention has been paid to the operation, the hair and epidermis becomes sufficiently softened in twenty-four hours to be removed with the hairing knife. Compared with the putrefactive sweating process, this method is a decided improvement, as the liability to injury is very much diminished.
Bg 13 Cold Sweating.—In America, especially in New York, New Hampshire, and in many parts of the Northern division of Pennsylvania, a process is in general use, called the cold sweating, by which the hair can be removed, and a yield of leather estimated at seventy to eighty per cent. acquired; whilst, by other means employed, such as by steam and lime depilation, not more than thirty to forty per cent. of the dry hides are retained as leather. Besides this very material gain, and other advantage follows, namely, that the laborious cleansing and fulling required in the other modes, is unnecessary; and consequently, the hides retain their natural density, a condition which very much improves the quality of the leather. The operations by which this is effected, are thus stated in a foreign journal:—
First, a vault should be prepared for the reception of the hides; this, for convenience sake, should be twelve feet long, the same in depth, and ten feet wide. The walls may be of stone, brick, or a planked frame. There should be one alley or vestibule for entrance, not less than six feet long, having a door at each end, the outer being made double and filled in with tan, to prevent the communication of warm dry air from without. A ventiduct made of plank, ten or twelve inches square, should extend from the bottom of the vault to the distance of three or four rods, and should be fixed not less than three or four feet below the surface of the ground. This channel serves both as a drain for discharging the water of the vault, and for admitting cold damp air to supply the place of that which has become rarified, so that a current is thus kept up through the ventilator at the top of the vault. The ridge of the roof may be on a level with the ground, and on it, extending its whole length, are set up edgeways two planks two inches apart. The space between these is to be left open, but the remainder of the roof may be covered with earth to the depth of a yard. Such covering is intended to preserve a low temperature in the pit, so that the hides may unhair without tainting. Spring water should be conducted, either in pipes of logs, around the angles formed by the ceiling with the walls of the vault; and from these conduits the fluid should be allowed to flow in minute jets, so as to form a spray, or else to raise a mist of vapour, so as to saturate the atmosphere of the vault with watery vapour. This arrangement keeps the temperature of the vault much under that of spring water itself, which usually stands at 50°, owing to the heat rendered latent by the evaporation. Three bars furnished with iron hooks, at intervals of three inches, are laid longitudinally and equidistant near the ceiling, and upon them the hides are suspended. Before hanging up the hides, they are soaked as for breaking, and then suspended form the butts in a way that they remain fully open. In the course of a few days, when the hair begins to loosen on the upper parts, they are taken down, and the middle bar being raised, they are again suspended by the other end, and left so till the hair easily separated. The hides should not be broken till they are ready to unhair. In a good vault, where the heat is maintained between 44° and 56°, above which it should never be allowed to rise, and where there is a free circulation of damp air, the hides generally require from six to twelve days. If the temperature falls below 44°, the ventilator should be partially closed; but if it rises above 56°, cold damp air must be forced in, or a larger volume of spring water thrown into the pit.
Morfit states, that the hides carried carefully through this process are, when received by the tanner from the beamsman’s hands, free from extraneous matter, and retain all their gelatine, albumen, and fibrin in an unimpaired state; and that it is not a fermentative change that produces the unhairing, because there is no ammonia generated. The effect is owing to the softening action of the absorbed vapour upon the epidermis, in conjunction with a swelling or distention of the roots of the hair, and tissue of the skin immediately underlaying them. Should these advantages be found, upon trial, attendant upon this simple and quite feasible method, it would undoubtedly be for the interest of tanners to adopt it, as not only does it require less labor, but the requirements in the shape of pits and water are still less than for the systems practised on this side of the Atlantic, leaving the consideration of the great gain in the amount of leather out of the question; and which, of course is its most powerful recommendation.
By Rhusma.—Warington recommends a solution of carbonate of soda for unhairing the hides, but the effect is not so rapid as when the alkali is in the caustic state. The same chemist, in attempting to depilate hides by the use of rhusma, the long-known mixture of caustic lime and orpiment, or tersulphide 14 of arsenic, discovered that the active agent was sulphide of calcium, formed by the decomposition of the arsenical compound in the presence of that base, and that the arsenic was without effect in the operation. He therefore tried the action of sulphide of calcium per se, and found that it acted so promptly, that only from twenty-four to thirty-six hours were required to soften the epidermis, and loosen the hair.
the removal of the hair and epidermis, and the bating, in case of hides
intended as upper 15 or pliable
leather, it is necessary, to submit the pelt,
as the depurated 16 unhaired skin
is commonly called, to a further operation, in order to distend its cellular
organs, with a view of enabling it the more readily to combine with the tannin.
In England, sulphuric acid, in the ratio of one part of acid to a thousand of
water, is the agent usually employed for this purpose; but in some cases
immersion for a limited period in the spent tan-liquor, wherein a certain
quantity of gallic and other acids have been developed, is substituted, and
even this treatment is abandoned by may, a weak tanning liquor being preferred.
This is especially the case in
The finishing of the raising is effected occasionally with strong tan-juice, which has been mixed by agitation with about four pounds of sulphuric acid. Eight or ten hides being submitted to this bath, they are taken out the first day twice, and permitted to soak of drain for two hours, the second day once, the same time being allowed for draining. In the morning of the third day they are transferred to another, composed of the densest tan-liquor, mixed with the same quantity of sulphuric acid as the foregoing. Whilst here, the hides are handled once a day, being allowed to drain at each handling one hour. About three days suffice to give them the necessary finish.
The red bate is regarded as being by far the best means in use, notwithstanding it is much slower in its effects than some of the others adopted in practice. This may arise from the fact, that the combination of the gelatinous tissue of the skin with the tannin is slowly going on from the immersion in the first vat of the series, at the same time that the lime is being removed, and, consequently, that the finished leather is more compact, and tougher, than if the swelling were made with acids in a shorter time.
Next Page: the Tanning Process.
2. ^ It is tempting to see this as a form of the
French tremper, to soak but it will
mean, in origin at least, walking on a vat full of soaking skins, in the manner
of a mediaeval fuller, on cloth. The personal name,
4. ^ A stock is a tree trunk or post. Again, the process is similar to that of fulling. The stocks would be repeatedly raised by power from such as a water wheel then allowed to fall, so strongly agitating the contents of the vat.
7. ^ In Bourne, T.W.Mays had two businesses. One, alongside the former navigation basin, included fellmongery, the preparation of sheep skins. The other, by the South Fen Slype, was a knackery, producing among other things, glue.
13. ^ This looks like a compositor’s error. It is not a mistake which is readily made in typing but an italic g would readily find its way into the type case with the y’s. It is a reminder of the enormity of the task of composing a work of well over 2,000 pages, letter by letter.
Link to the Berkeley Laboratory for a modern view of the interaction between a protein and a salt.