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A
TEXT-BOOK OF TANNING.
[Illustration:
Pl. I.
_E. & F. N. Spon, London & New York._ "INK-PHOTO." SPRAGUE & CO. LONDON.
LIME PITS AND RINSING TANKS.]
A
TEXT-BOOK OF TANNING:
A TREATISE ON THE
CONVERSION OF SKINS INTO LEATHER,
BOTH PRACTICAL AND THEORETICAL.
BY
HENRY R. PROCTER, F.C.S.,
OF LOWLIGHTS TANNERY;
EXAMINER IN TANNING TO THE CITY AND GUILDS TECHNICAL INSTITUTE.
With 8 Plates and numerous Illustrations.
[Illustration]
E. & F. N. SPON, 125, STRAND, LONDON.
NEW YORK: 35, MURRAY STREET.
1885.
PREFACE.
[Illustration]
The aim of the following handbook is two-fold; to give, in a
compendious form, such a summary of our scientific knowledge as may
be useful to the practical tanner; and such a sketch of manufacturing
processes as may enable the chemist to apply his knowledge to their
improvement. Each may, therefore, find some superfluous matter, for
which his indulgence is asked. The book is an expansion of a short
article which appeared in Spons' 'Encyclopædia of the Industrial Arts,'
and to some extent still bears traces of its origin; and, having been
written under stress of limited leisure, and defective eyesight, is
very far from being so perfect as I should desire. For the sake of
completeness it has been necessary to describe many processes which are
outside the range of my own manufacturing experience, and in doing so
I have generally referred to the sources of my information. Chapters
III. and XXIV. are written by Mr. C. G. Warnford Lock, to whose kind
assistance I am much indebted. It may be well to state in conclusion,
that while the work is not intended for a cram-book for technical
students, it is hoped that it may be an assistance to teachers of the
subject.
HENRY R. PROCTER.
Tynemouth,
_August 1885_.
CONTENTS.
[Illustration]
PAGE
INTRODUCTORY NOTE 1
CHAPTER I.
Anatomical Structure of Hide 2
CHAPTER II.
Chemical Composition of Hide 17
CHAPTER III.
Commercial Tanning Materials 23
CHAPTER IV.
The Chemistry of Tannins 57
CHAPTER V.
Water as used in Tanning 83
CHAPTER VI.
Methods of Chemical Analysis for the Tannery 90
CHAPTER VII.
Sole-leather:--Preparing the Hides 132
CHAPTER VIII.
Sole-leather:--Unhairing Hides 139
CHAPTER IX.
Sole-leather:--Tanning Materials 157
CHAPTER X.
Sole-leather:--Treatment in the Tan-house 169
CHAPTER XI.
Sole-leather:--Treatment in the Shed 179
CHAPTER XII.
Dressing Leather 184
CHAPTER XIII.
Currying 193
CHAPTER XIV.
Enamelled, Patent, or Japanned Leather 203
CHAPTER XV.
Morocco Leather 206
CHAPTER XVI.
Russia Leather 208
CHAPTER XVII.
Chamois or Wash-leather 210
CHAPTER XVIII.
Crown Leather, or Preller's Leather 213
CHAPTER XIX.
Mineral-tanned Leather 218
CHAPTER XX.
Calf-Kid 223
CHAPTER XXI.
Glove-Kid 225
CHAPTER XXII.
Construction and Maintenance of Tanneries 231
CHAPTER XXIII.
Drying-sheds for Leather 243
CHAPTER XXIV.
Commerce, Statistics, and Bibliography 255
Index 275
A
TEXT-BOOK OF TANNING
ETC., ETC.
[Illustration]
INTRODUCTORY NOTE.
Leather manufacture may be broadly divided into two stages: "tanning,"
in which the raw hide is converted into the imputrescible and more or
less flexible material known as "leather"; and "currying," in which
this leather is further manipulated, and treated with fatty matters, to
soften and render it more waterproof, and to improve its appearance.
Glove-kid, and certain other leathers, however, are not tanned at all,
but "tawed," or prepared with a mixture in which alum and salt are
the most active ingredients; chamois, "shammy," or "wash" leather, is
produced by fulling with oil alone, and many leathers can scarcely be
said to be curried, although more or less oil is used in the final
processes of "finishing" or "dressing." The first subject to be treated
of in this work will be the operation of tanning, properly so called,
taking for example the tannage of sole- and belting-leather. This
demands thorough explanation, in both its practical and theoretical
aspects, not only because it is one of the most important branches of
the trade, but because the principles involved are those which equally
underlie all other tanning methods. The next to be dealt with will
be the modifications of the process which are necessary in tanning
the more flexible leathers used for boot-uppers, hose-pipes, and
saddlery purposes; then the currying of these leathers; and finally,
the manufacture of moroccos, Russian, and japanned leathers, calf- and
glove-kid, &c.
CHAPTER I.
ANATOMICAL STRUCTURE OF HIDE.
Before speaking of actual processes of manufacture, it is necessary to
devote some attention to the structure and chemical constitution of
hide or skin, which forms the raw material. Although a great variety
of skins are employed in tanning, they are all constituted on the same
general type, and an anatomical description of the hide of the ox will
apply almost equally to those of the calf, sheep, and goat; but from
differences in thickness and closeness of texture, their practical uses
differ widely. Fig. 1 shows a section of ox-hide, cut parallel with the
hair, magnified about 50 diam.: _a_, epithelial layer or _epidermis_,
consisting of horny layer above, and _rete malpighi_ below; _b_, _pars
papillaris_, and _c_, _pars reticularis_ of _corium_, _derma_, or true
skin; _d_, hairs; _e_, sebaceous or fat-glands; _f_, sudoriferous or
sweat-glands; _g_, opening of ducts of sweat-glands; _h_, _erectores
pili_ muscles, for erecting the hair.
The fresh hide consists of 2 layers: an outer, the epidermis; and an
inner, the true skin. The epidermis is very thin as compared with the
true skin which it covers, and is entirely removed preparatory to
tanning; it nevertheless possesses important functions. It is shown in
Fig. 1 at _a_, and more highly magnified in Fig. 2. Its inner mucous
layer _b_, the _rete malpighi_, which rests upon the true skin _c_, is
soft, and composed of living nucleated cells, which are elongated in
the deeper layers, and gradually become flattened as they approach the
surface, where they dry up, and form the horny layer _a_. This last
is being constantly worn away, and thrown off as dead scales of skin;
and as constantly renewed from below, by the continued multiplication
of the cells. It is from this epithelial layer that the hair, as well
as the sweat- and fat-glands, are developed. It will be seen in Fig. 1
that each hair is surrounded by a sheath, which is continuous with the
epidermis. In embryonic development, a small knob of cells forms on the
under side of the epidermis, and this enlarges, and sinks deeper into
the true skin, while the root of the young hair is formed within it;
this is shown in Fig. 3, _a b_. Smaller projections also form on the
stalk of the knob, and in due time produce the sebaceous glands. The
process of development of the sudoriferous glands is very similar to
that of the hairs. There is a great analogy between this process and
that of the ordinary renewal of hair in the adult animal. At _d_^1
Fig. 1, is seen an old and worn-out hair. It is shrunken and elongated,
and is almost ready to fall out. It will be noticed that its sheath or
follicle projects somewhat below the hair to the right. This is the
first production of a young hair, and is quite analogous to the knob of
epithelium which has been described as forming the starting-point of a
hair in embryo. At _d_^2, the same process is seen further advanced,
the young hair being already formed, and growing up into the old
sheath. At _d_^3, it is complete, the old hair having fallen out, and
the young one having taken its place.
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
[Illustration: Fig. 3.]
The hair itself is covered with a layer of overlapping scales, like
the slates on a roof, but of irregular form. These give it a serrated
outline at the sides, strongly developed in wool. Within these scales,
which are sometimes called the "hair cuticle," is a fibrous substance,
which forms the body of the hair; and sometimes, but not always, there
is also a central and cellular pith, which is mostly transparent,
though under the microscope it frequently appears black and opaque,
from the optical effect of imprisoned air. On boiling or long soaking
in water, alcohol, or turpentine, these air-spaces become saturated
with the liquid, and then appear transparent.
The fibrous part of the hair is made up of long spindle-shaped cells,
and contains the pigment which gives the hair its colour. The hair
of the deer differs from that of most other animals in being almost
wholly formed of polygonal cells, which, in white hairs, are usually
filled with air. At its base, the hair swells into a bulb, which is
hollow, and rests on a sort of projecting knob of the _corium_, called
the hair-papilla. This has blood-vessels and nerves, and supplies
nourishment to the hair. The hair-bulb is composed of round, soft
cells, which multiply rapidly; as they grow, they press upward through
the hair-sheath, become elongated and hardened, and form the hair. In
dark hairs, both the cells of the hair itself and those of its follicle
or sheath are strongly pigmented, but the hair much the more so, and
hence the bulb has usually a distinct dark form. The dark-haired
portions of a hide from which the hair has been removed by liming
still remain coloured, from the pigmented cells of the hair-sheaths,
which can only be got out completely by bating and scudding. The
cells outside the bulb, shown at _f_, in Fig. 4, pass upwards as they
grow, and form a distinct coating around the hair, which is called
the "inner root-sheath." This again consists of 2 separate layers,
of which the inner is "Huxley's," the outer, "Henle's." They arise
from the same cells in the base of the hair; but in the inner layer,
these remain polygonal and nucleated, while in the outer, they become
spindle-shaped and without nuclei. The inner root-sheath does not
extend to the surface of the skin, but dies away below the sebaceous
glands. This figure represents an ox-hair root, mag. 200 diam.: _a_,
fibrous substance of hair; _b_, hair cuticle; _c_, inner root-sheath;
_d_, outer root-sheath; _e_, dermic coat of hair-sheath; _f_, origin of
inner sheath; _g_, bulb; _h_, papilla.
[Illustration: Fig. 4.]
Outside the inner root-sheath is a layer of nucleated cells, continuous
with those of the epidermis, and of the same character. This is the
"outer root-sheath," and is shown at _d_, Fig. 4. This, together with
the whole of the epidermis, is covered next the _corium_ with an
exceedingly fine membrane, called the "hyaline" or glassy layer. It
is possible that this forms the very thin buff-coloured "grain" of
tanned leather, which evidently is of different structure from the rest
of the _corium_, since, if it gets scraped off before tanning, the
exposed portion of the _pars papillaris_ remains nearly white, instead
of colouring. The whole of the hair-sheath is enclosed in a coating of
elastic and connective-tissue fibres, which are supplied with nerves
and blood-vessels, and form part of the _corium_. Near the opening of
the hair-sheaths to the surface of the skin, the ducts of the sebaceous
or fat-glands (_e_, Fig. 1), pass into them, and secrete a sort of
oil to lubricate the hair. The glands themselves are formed of large
nucleated cells, arranged somewhat like a bunch of grapes; one is shown
highly magnified in Fig. 5: _a_, sebaceous gland; _b_, hair-stem; _c_,
part of _erector pili_ muscle. The upper and more central cells are
most highly charged with fat, which is shown by the darker shading.
[Illustration: Fig. 5.]
As already remarked, the sudoriferous or sweat-glands are also derived
from the epidermis layer. They are shown at _f_, Fig. 1, and on a
larger scale (200 diam.) in Fig. 6: _a_, windings laid open in making
section; they consist, in the ox and sheep, of a large wide tube,
sometimes slightly twisted. In this, they differ considerably from
those of man, which form a spherical knot of extremely convoluted
tube. The walls of these glands are formed of longitudinal fibres of
connective tissue of the _corium_, lined with a single layer of large
nucleated cells, which secrete the perspiration. The ducts, which are
exceedingly narrow, and with walls of nucleated cells like those of
the outer hair-sheaths, sometimes open directly through the epidermis,
as shown at _g_, Fig. 1, but more frequently into the orifice of a
hair-sheath, just at the surface of the skin. Each hair is provided
with a slanting muscle (_h_, Fig. 1), called the _arrector_ or _erector
pili_, which is contracted by cold or fear, and causes the hair to
"bristle," or stand on end; by forcing up the attached skin, it
produces the effect known as "goose-skin." The muscle, which is of the
unstriped or involuntary kind, passes from near the hair-bulb to the
epidermis, and just under the sebaceous glands, which it compresses.
[Illustration: Fig. 6.]
The _corium_ or true skin is principally composed of interlacing
bundles of white fibres, of the kind known as "connective tissue";
these are composed of fibrils of extreme fineness, cemented together
by a substance of different composition from the fibres themselves.
This may be demonstrated by steeping a small piece of hide for some
days in a stoppered bottle in lime-, or baryta-water, in which the
inter-fibrillar substance is soluble, and then teasing a small fragment
of the fibre with needles on a glass microscope-slide, and examining
with a power of at least 200-300 diam. In the middle portion of the
skin, these bundles of fibre are closely interwoven; but next the
body, they gradually become looser and more open, forming the _pars
reticularis_ (or netted part); and the innermost layer is a mere
network of loose membrane, generally loaded with masses of fat-cells,
and hence called adipose tissue.
It is this adipose tissue which is removed in the "fleshing" process.
On the other hand, the outermost layer, just beneath the epidermis,
is exceedingly close and compact, the fibre-bundles that run into it
being separated into their elementary fibrils, which are so interlaced
that they can scarcely be recognised. This is the _pars papillaris_,
and forms the lighter-coloured layer, called (together with its very
fine outer coating) the "grain" of leather. It is in this part that the
fat-glands are embedded, while the hair-roots and sweat-glands pass
through it into the looser tissue beneath.
Besides the connective-tissue fibres, the skin contains a small
proportion of fine yellow fibres, called "elastic" fibres. If a thin
section of hide be soaked for a few minutes in strong acetic acid, and
then examined under the microscope, the white connective-tissue fibres
become swollen and transparent, and the yellow fibres may then be seen,
as they are scarcely affected by the acid. The hair-bulbs and sweat-
and fat-glands are also rendered distinctly visible.
The nerves of the skin are very numerous, each hair being supplied
with fibres passing into both the papilla and sheath. They also
pass into the skin papillæ. They cannot readily be seen, without
special preparation, and, so far as is known, exercise no influence
on the tanning process. "Breaking the nerve" is a technical term,
which signifies a thorough stretching and softening of the skin, but
has nothing to do with nerves properly so called. The blood- and
lymph-vessels are, from the present point of view, somewhat more
important. They may often be seen in sections, and are lined with
nucleated cells, similar to those of the glands. These are surrounded
by coatings of unstriped muscular fibre, running both around and
lengthways, and also by connective-tissue fibres. In the arteries, the
muscular coating is much stronger than in the veins.
It may be thought that the space devoted to a discussion of the
anatomical structure of the skin is disproportionately large;
but there can be no doubt that, in order to make improvements,
nothing is of more importance than a clear conception, even to the
smallest details, of the materials and causes to be dealt with. The
illustrations are from actual specimens, and enable the various parts
of the hide to be identified under the microscope.
As this instrument is a most useful means of investigation in the
tanning industry, and one likely to be of increasing importance, it
will be well, before proceeding further, to say a few words, both on
the selection of a suitable instrument, and on its manipulation in
general.
To do useful work, it is not necessary to possess a very elaborate
or expensive instrument, but it is essential that the microscope be
well made and good of its kind. As high powers are often required in
the examination, both of hide sections and of ferments, which are the
principal objects of investigation in a tannery, it is of the first
importance that the fine adjustment should be perfectly steady, without
vibration or backlash. This, in the writer's experience, is never the
case with cheap microscopes, in which the fine adjustment is made by
a screw at the side of the tube moving the nose by means of a lever.
A much more satisfactory arrangement is that in which the whole body
of the microscope is raised or lowered by a screw in a pillar at the
back of the stand on which it slides. A rack for the coarse adjustment
is useful, but not essential. If a sliding tube only is provided, it
must be tight enough not to slip, but must move easily up and down
with a sort of screwing movement. A mechanical stage is not at all
necessary, and for most purposes one of black glass is better as well
as cheaper. The diaphragm for regulating the light should be as near
level with the surface of the stage as possible, and when examined with
a low power should appear in the centre of the field. For research work
on the minuter ferments, an achromatic condenser and the finest oil-
or water-immersion lenses are necessary, but directions for this are
beyond the scope of the present work. It may, however, be mentioned
that Prof. Flügge,[A] a first-class authority on the subject,
especially recommends Abbé's illuminating apparatus as made by Zeiss.
[Footnote A: "Fermente und Mikroparasiten," Leipzig, 1883.]
A frequent defect in cheap English microscopes is that the mirror for
substage illumination does not bring the rays of a lamp to a focus
exactly on the slide, but frequently some inches above it. This may be
to a great extent overcome by the use of a bulls'-eye condenser between
the lamp and the microscope. Another defect is that sometimes the
centre of the mirror is not in a line with that of the microscope body.
The objectives (or lenses at the lower end of the microscope) are the
most important part of the instrument, and however good it may be in
all other respects, if these are defective the whole is useless. The
most useful lenses for our purpose, if only two are to be selected, are
a 1-in., magnifying about 50 diam., and a 1/4-in., magnifying about 200
to 400, according to the eye-piece; a 1/8-in. giving, say, twice this
magnification will be needed to see the smaller bacteria distinctly,
but it is possible just to see even the small putrefaction bacteria
with a really fine 1/4-in. In any case, the highest power should be as
perfect and of as large an angle as attainable. A good 1/4-in. should
resolve _Pleurosigma angulatum_ with direct light, and should show the
movement of the granules of protoplasm in the round corpuscles which
are present in saliva. In using the latter test, it must be remembered
that the motion only lasts a very short time on a cold slide.
About 5_l._ is the very least for which a microscope can be obtained
which is suitable for tanners' use; where it can be afforded, a better
one is advisable.
Without disparaging other makers, it may be mentioned that the writer
has generally used both the eye-pieces and objectives of Dr. Hartnack
of Potsdam; and that they are moderate in price, at least for the dry
combinations, and perfectly satisfactory for all technical purposes.
Numbers 2, 5, and 8 objectives with No. 3 eye-piece, are sufficient
for all ordinary work. If only 2 objectives are to be obtained, Nos.
3 and 7 would be perhaps the best selection. It is always better
to use objectives on the stand, and with the eye-pieces for which
they are intended, but in case Hartnack's objectives are used on an
English stand (which is easily done by means of an adaptor ring), it
is important to remember that they are constructed to work with a
shorter tube than that customary on English microscopes, and that they
will not perform well if its length is much more than 6 in.; these
objectives are not provided with a movable adjustment for thickness
of cover-glasses, which for technical purposes is not required, and
in inexperienced hands is apt to prove troublesome. Extra-thin covers
must therefore always be used. Where this adjustment is provided, the
object must be accurately focused, and then, maintaining this focus
with the fine focusing-screw, the collar must be cautiously turned till
the best definition is obtained. Practically it will be best to make
this adjustment accurately once for all, and to take care to use covers
selected of a uniform thickness.
High-power objectives of wide angle (which condition is essential to
good defining power) necessarily work extremely close to the object,
and it is always best to use the thinnest cover-glasses which can be
got. Even then, with such glasses as Hartnack's No. 8, unless the
sections are very thin, it will be impossible to examine their lower
parts; and one of the greatest difficulties of microscopic research
is to obtain them thin enough. It will be obvious, from what has
been said, that the greatest care is needed to avoid screwing the
objective down on the cover, and so breaking one or both of them. One
way to avoid this is to screw down as close as possible to begin with,
and then focus upwards. Another plan, when the object on the slide
is small, is to keep continuously moving the slide gently with the
fingers, while looking into the tube. It is then easy to notice when
the dust and small particles on the slide come into focus, and if the
point should happen to be overstepped the contact will generally be
felt before serious damage is done.
Illumination is one of the most important points in practical
microscopy. With powers of not less than 1/2-in. focus, objects may
generally be examined by light thrown upon them from above by a
bulls'-eye condenser, or by good daylight. In this case they need not
be transparent; and the plan is often convenient for a mere surface
examination. In examining bodies illuminated in this way, prominences
often appear as hollows and _vice versâ_, by a sort of optical
illusion, which, once established, is very difficult to overcome. By
remembering the direction of the light, and that this appears reversed
in the microscope, it is easy to decide the truth.
For all finer work and higher powers, and most generally with the low
powers also, it is necessary to render the object transparent, and to
examine it by light transmitted from the mirror below the stage.
Good daylight is least trying to the eyes. Where artificial light must
be used, that of a small paraffin lamp is best; and a blue chimney,
or blue glass interposed between the stage and mirror, or lamp and
microscope, spares the sight, and makes it easier to distinguish
colours. The light should be sufficient, but not too dazzling. Work
should never be prolonged after the least strain is felt, nor should
the microscope be used for some little time after a meal. It is well to
accustom oneself to keep both the eyes open while observing.
If it be required to see how far the cellular structures of the hide,
such as hair-sheaths and fat-glands, are affected or destroyed in any
stage of liming or bating, the following ready method may be employed.
If a strip of hide be cut 2/3 through from the grain side, as shown
at _a_ in Fig. 7, and the flap be turned down, and held between the
finger and thumb, the fibrous tissue will be put on the stretch, and
will then allow a moderately thin shaving (including the grain and
parts immediately below it) to be cut by a sharp razor. The hide
should be held in the positions shown, and a steady drawing cut be
made from flesh to grain, the razor being steadied on the tip of
the forefinger, and its hollow surface flooded with water. If the
thin section be now placed on a glass slide, moistened with a drop of
water, and examined on the microscope under a strong light from above,
with a 1-in. objective, the fat-glands will be seen as yellow masses,
embedded in the white fibrous tissue. If a drop of a mixture of equal
vols. of strong acetic acid, glycerin, and water be used to moisten the
section, the fibrous tissue will become quite transparent, and whatever
remains of the cellular tissue will be easily visible, and may even be
studied under tolerably high powers if covered with a thin glass, and
lighted by the mirror from below. (The cover-glass must be carefully
cleaned by rubbing with a linen handkerchief, and placed in position
with a pair of tweezers, one side being supported by a needle, which is
gradually withdrawn, so as to avoid air-bubbles.) Care must be taken
that this mixture does not touch the brass-work of the microscope; even
the vapour is apt to tarnish, so that the preparation must not remain
longer than necessary on the microscope. The same method is applicable
for ascertaining the completeness of the tannage of leather, and to
decide whether the hide fibre is really tanned, or only dyed. Actually
tanned leather is unaffected by the acetic acid, but raw or only
stained hide swells and becomes transparent.
[Illustration: Fig. 7.]
To prepare the very thin sections necessary for detailed study of the
hide, more complicated methods are required. Small slips of hide, not
exceeding 1/4 in. wide, and cut exactly across the lie of the hair,
are placed first in weak alcohol (equal parts methylated spirit and
water), and, after a few hours, are removed into strong methylated
spirit. It is then kept for some days in absolute alcohol, which must
be repeatedly changed, until the hide is hard enough to give fine
shavings, and may be cut either when held as above described, between
cork or pith, or when embedded in paraffin wax. This is accomplished
by placing the piece of hide in a little paper-box and covering it
with melted paraffin (candle), which is just beginning to stiffen. The
piece of hide may be fixed in position with a needle, which must of
course be withdrawn before cutting. When hard, the paraffin is shaved
away till the object is exposed, when it may be cut. The razor must be
wet with alcohol, and the section be made exactly in the plane of the
hair-roots, which may be seen with a hand-lens. (The use of a microtome
for hide-sections is rarely successful, as it is almost impossible to
fix the fragment of hide so that it is cut exactly with the hairs.)
The slices may now be stained by placing them in a watch-glass with
water and a few drops of the logwood or picrocarmine staining-mixtures
sold by opticians, and afterwards either examined in glycerin, or,
after soaking some hours in absolute alcohol, may be transferred to
clove-oil, and afterwards to a slide, and covered with a drop of
dammar varnish or Canada balsam dissolved in chloroform. The sections
moistened with glycerin may also be mounted in Farrant's solution
or glycerin jelly, under a cover-glass for permanent preservation.
If picrocarmine be used, the connective-tissue fibres (gelatinous
fibres) and the nuclei of the cells will be coloured red, and the cells
themselves of both epidermis and glands, together with the muscles and
elastic fibres, will be yellow.
Franz Kathreiner, who has made very elaborate researches on skin, and
the changes which take place in it during the processes of tanning,
employs a mixture of osmic and chromic acids for hardening, and at the
same time staining the tissue. This mixture was first used by a German
histologist with whose name I am not acquainted, in a research on the
internal organs of hearing, and was applied by Kathreiner in 1879 to
the investigation of skin, and communicated by him to the writer in
the autumn of that year. His method is briefly as follows. The pieces
of hide to be examined must, if salted, be well washed, or if dry, be
thoroughly softened. For the study of hide in its unaltered and natural
condition, it is essential that it be quite fresh, and taken from the
animal as soon as possible after death. In any case the _Panniculus
adiposus_ or fatty layer is, as far as possible, removed with scissors,
the hair cut short, and the skin cut up into little pieces of 3-4
millimetres wide by 10-12 millimetres long (about 1/8 in. by 1/2 in.);
the hair must lie exactly across these pieces.
They are then placed for 4-8 days, according to the thickness of the
hide, in about 12 times their volume of a solution consisting of
0·2 parts osmic acid.[B]
0·5 " chromic acid.
200·0 " water.
[Footnote B: Solution of osmic acid is best preserved in sealed tubes
in the dark. If obtained in solution it is rarely of full strength,
for which allowance may have to be made. Care must be taken to avoid
inhaling its fumes, which are very irritating to the eyes and to the
respiratory organs, producing severe catarrh.]
This solution must be kept from dust and light, in a glass stoppered
bottle, and in a cool place. On removing the hide-pieces from this
solution, they are placed in about 12 times their volume of absolute
alcohol for 4-8 days, during which time the spirit must be at least 3
times renewed. The sections are cut with a razor flooded with absolute
alcohol, so that the thin shavings float without friction upon it. The
hide-pieces may be held either between soft cork, or, as is generally
preferable, simply between the forefinger and thumb as shown in Fig.
7. The cut must be made exactly parallel with the direction of the
hair roots, and from the grain towards the flesh; and the sections
cannot possibly be too thin. After lying for 1/2-1 hour in absolute
alcohol, the sections are soaked till quite clear in clove oil (which
must be pale and of the purest kind), and may then be mounted in dammar
varnish, or solution of Canada balsam.
In these sections, fat and the oily contents of the fat glands are
stained black, and the limits of the cells both of these glands and of
other elements of the hide (_rete malpighi_, hair-bulb, &c.) are made
very distinct, so as to be capable of the most delicate investigation
under the highest powers; but the beginner will learn most easily to
recognise the different tissues by studying at first some sections
stained with picrocarmine as before described. The method is admirably
adapted for the study of hide as affected by the limes and bates.