RAW MATERIALS AND THEIR PREPARATION FOR THE MANUFACTURE OF GLUE.
The raw materials used for the manufacture of glue consist of a variety
of animal offal. The principal substances employed are refuse from
tanyards, such as scraps of ox and other thick hides, the waste of the
workshops of leather dressers, morocco leather manufacturers, etc. The
tendons and intestines of many animals, rabbit and hare skins deprived
of their fur, cat and dog skins, scraps of parchment, waste of turners
and button makers, and offal from butcher shops and households, help to
swell the series of materials used for the manufacture of glue.
The materials are collected and sold either directly to the glue
boiler, or to dealers making a specialty of glue stock.
As a thorough knowledge of these waste products is of importance to
the manufacturer, this chapter will be devoted to their detailed
description, the success of the enterprise depending largely on
the selection of the raw materials and their careful sorting and
preparation. By bearing in mind the varied products—from the most
ordinary black glue to the colorless glassy gelatine for photographic
and culinary purposes—it will be understood that entirely different
raw materials have to be employed for the finer products than for the
ordinary qualities of glue.
According to their derivation the raw materials may be divided into
three groups, namely:
1. Skin-like raw materials: Skin, leather, tissues.
2. Bone raw materials.
3. Materials obtained from fishes: Air bladders, scales, etc.
1. ANIMAL SKIN.
[Illustration: FIG. 1.]
This consists of three layers, namely: 1. The thin upper-skin—the
epidermis—which consists only of cellular tissue, and is of no special
importance for the manufacture of glue. 2. The actual leather-skin, or
corium, which consists of fibres of connective tissue and forms the
actual object of the tanner as well as of the glue boiler. Underneath
the corium lies the under-skin, which consists only of cellular tissue
contaminated with particles of flesh and fat, which are detrimental
to the manufacture of glue. Fig. 1 represents a section of the animal
skin. _O_, is the epidermis, _L_, the corium, _U_, the under-skin. The
epidermis consists of two layers. The first, superficial one, _H_, is
known as the cuticle or lamellar layer, and the other deeper layer,
_S_, as the mucous or malpighian layer. The corium also consists of
two layers, the upper one _C_, and the lower one _C_{1}_, which is
the actual leather-skin. The under-skin, _U_, is an elastic tissue
containing many deposits of fat, _F_, and perspiratory glands, _D_,
which are connected with ducts, _D_{1}_, with the surface of the skin.
For the manufacturer of leather and glue, the corium is the only
material of value.
The tanner trims the skins before steeping them in the ooze. From sheep
and calf skins he removes the head portions, it being more advantageous
to use them for glue stock. He also cuts off the skin covering the
lower part of the thighs, and, to give the skin a neat finish, the
ragged edges of the belly part. Of bullock hides, the ears, tails and
foot pieces are utilized for glue stock, while the head parts are
tanned. Such tannery waste may yield 44 to 46 per cent. of glue. Scarf
skin of bullocks’ hides and waste in fleshing the hide, tendons and
hinder parts of cattle yield from 30 to 35 per cent.; horse sinews from
15 to 18 per cent.
Scraps of parchment and bullocks’ feet are highly valued as glue
stock, since they are in fit condition for boiling without further
preparation. They may yield up to 62 per cent. of their weight in glue.
Calf and sheep skins yield a superior glue; that from horse hides is
usually dark and poor in quality, although with careful working a
strong product can be obtained from the latter.
Of great value to the glue boiler are the so-called calves’ heads,
which, after liming and drying, form a special article of commerce.
Skins of hogs, hares and rabbits yield a light-colored glue of little
consistency. It is, therefore, best to use these last-named raw
materials for the preparation of jelly, such as is used in sizing, in
the manufacture of paper, etc.
The older the animals from which the skins have been derived, the more
solid the glue will be. In many cases, especially where a certain
quality of glue is to be produced, it may be recommended to separate
the different kinds of skin refuse into lots, provided there is enough
of each kind to boil it separately.
A considerable number of skins used for packing various articles, such
as indigo from South Africa, have been so much damaged in transit as to
render them useless for tanning, but they form good material for glue,
frequently yielding 50 to 55 per cent.
In reference to judging glue stock some valuable notes are given in
an article on glue, published by the American Provision Co., Chicago,
Ills.:
“Dry, uncured or salted stock, such as raw hide or South American, if
soaked for twelve hours in cold water, gains about 50 per cent. in
weight, and still remains tough, and the water sweet. The moisture,
dirt, and salt should not be over 10 per cent.
“Green salted stock, such as hide pieces, sinews, calf heads and pates,
should have no excess of salt, nor be foul, discolored or heated;
should be tough, with the hair not loose, and have a mild animal odor.
Moisture and salt not over 40 per cent.
“Dry limed stock soaked twelve hours develops a characteristic odor,
and should be firm, fibrous, and have no slimy pieces. The water should
not be dark. Lime, sand and dirt, not over 5 per cent.
“Green limed stock should be smooth and soft, any remaining hair being
easily detachable, while the liquor should be fairly clean, sweet, and
not too alkaline.
“A large quantity of waste bones accumulates in the preparation of
tinned provisions. If these have not been overheated and are in good
condition, a considerable amount of glue can be obtained from them, the
bones of the head, ribs, and feet giving a better yield than those of
the thighs and legs.
“Horn piths should not contain over 12 per cent. moisture, and should
not have been overheated in drying; they should have been cleansed from
skin and hair, which are of little value to the glue-maker.
“The age of the animals yielding glue stock has an important influence
on the product. While from younger animals the product, as a rule, is
of lighter color, more abundant and more easily obtained, it contains
more chondrin, so that from solutions of equal strengths, those from
mature animals will be found to be of greater consistence and the glue
more solid.
“Abroad, dry hides are often, for weighing, soaked in chlorbarium, a
solution of barium chloride, and then in a bath of dilute sulphuric
acid, 1½ per cent., which readily soaks in, combines with the barium
to form the white insoluble powder of barium sulphate, leaving weak
hydrochloric acid in the fibre, to be afterwards neutralized in
liming, the chloride of calcium dissolving out. This treatment affects
considerably the subsequent making of glue, as, beside the effects of
the acids, the sulphate of barium will render the liquors cloudy and
difficult to clarify. Of course if colored glues are to be made this
will be no detriment.”
To prevent putrefaction, which is always accompanied by decomposition
of glue-yielding substance and consequent loss, the scraps must be
carefully preserved, especially in summer.
The tanner prepares the waste by liming, _i. e._, steeping it during
fifteen to twenty days in milk of lime which is frequently renewed.
By the action of the lime, adhering particles of blood and flesh are
dissolved and the fatty matter is saponified. After this treatment the
glue-stock is dried.
In case this work is not done carefully in the tanyard, as is only
too frequently the case, the stock is of but little value to the
glue-boiler.
By allowing the refuse to lie too long in a heap, as is sometimes done,
putrid fermentation sets in, the injurious effects of which cannot be
remedied by subsequent liming, or the lime bath has not yet been strong
enough, or has not acted sufficiently long upon the scraps to destroy
the adhering particles of blood and flesh. The lime bath, on the other
hand, may have been too strong, so as to attack the glue-yielding
substance. Frequently it is also the case that the scraps having been
dried under unfavorable circumstances, mould has commenced to form,
and finally they may be spoiled in winter by allowing them to freeze.
Frozen glue leather yields glue of very little consistency.
It will be seen from the foregoing that great precaution and care are
required when buying glue leather. The manufacturer should especially
see that it is dry and tough, free from mould and all organic and
inorganic substances, and not too strongly limed.
The glue-boiler should, in all cases, be prepared to undertake the
preparation of the glue stock himself. The following arrangements are
required for the purpose:
Let us assume that the glue factory is located on a stream of water. In
the immediate neighborhood of the stream a sufficient number of pits
to prepare all the glue stock used, each about 6½ feet deep and 6½ to
10 feet in diameter, and lined with cement, are so arranged that their
bottoms are about 3 to 3½ feet above the level of the water. They are
supplied with water by means of a pipe line connecting one with the
other. Each pit is provided with a discharge pipe to draw off the dirty
water.
As the glue-stock, before undergoing other operations, has to be
freed from the lime by washing with water, the simplest plan is to
place the limed stock in nets or wicker baskets suspended in running
water by means of a traveling crane or other contrivance erected on
the bank of the stream. This elementary method, however, is open to
several disadvantages, as it fouls a large volume of water and may
lead to legal interference, and its very simplicity is apt to lead to
neglect of precautions, such as brushing away solid particles of lime
or softened animal matters. Further, the great quantities of water
carry off small pieces of glue-stock and fat, if catch-basins are not
provided sufficiently large to allow fat, glue-stock, hair and lime to
separate from the water.
The object is better accomplished and in a shorter time by the use of a
washing drum. This consists of a perforated iron cylinder about 6 feet
in diameter and 4 feet in length, and open on both ends. Around the
inside of the cylinder are fixed a number of wooden shelves 6 inches
broad, which, as the cylinder revolves, carry the glue-stock partly
round, ultimately falling to the bottom again, the movement dashing
it about under a spray of water. In the cylinder is also arranged an
iron plate supported by stays from the outside. While the washing
is in operation the plate is turned perpendicular; on completion it
is brought to a horizontal position, forming a table, on which the
glue-stock falls, and the latter is then removed to a hand-press to
squeeze out the water. The washed stock is then removed to the drying
ground, which should be in a sunny and airy location, and provided with
an inclined floor of planks or cement so arranged as to allow of the
admittance of air from beneath.
As it is well known that small quantities of liquid, frequently renewed
and thoroughly drained off each time, effect the most complete and
economical washing, and in the shortest time, S. Rideal recommends the
use of pits or vats with proper arrangements for stirring, draining and
inspection. The lime scum from the pits can be used in the manufacture
of fertilizers.
The glue-stock washer shown in Figs. 2 to 5, is the invention of W.
A. Hoeveler (American patent), and it relates to the construction of
apparatus for washing glue-stock.
In apparatus for this purpose the stock is very commonly damaged by
being broken up too much, and considerable loss results, besides,
from the fact that the small particles are allowed to escape with the
wash-water. By the present construction and arrangement these defects
are remedied and other advantages derived.
Fig. 2 is a transverse vertical section on line x-x of Fig. 3 of this
apparatus;
[Illustration: FIG. 2.]
Fig. 3 is a longitudinal vertical section of the same;
[Illustration: FIG. 3.]
Fig. 4 is an enlarged plan illustrating the screen and hinged covers,
one being opened and one closed; and
[Illustration: FIG. 4.]
Fig. 5 is a detail of the hub, stems and part of one paddle.
[Illustration: FIG. 5.]
The apparatus is constructed in the form of a rectangular trough-like
structure, with its sides and ends, _A_, substantially water-tight by
means of the double walls, _a a_. The upper portion of the interior is
occupied by the swinging wash-box, _B_, semicircular in shape, with
flat sides and rounded bottom throughout, the bottom being perforated.
Upon a transverse shaft, _c_, journaled at the axis of box, _B_, is set
a paddle-wheel composed of a suitable hub, _d_, and adjustable paddles,
each composed of the radial stem, _e_, and the blade, _f_, or spoon.
The spoons, _f_ are set on the stems, _e_, so as to be capable of being
reversed or turned half-way round, more or less. One side of the spoon,
_f_, is rounded off, so that while passing through the stock the latter
will not cling to or remain upon it. The other side of the spoon is
flat, but slightly skewed or bevelled, so that when turned to face
with the direction of motion of the wheel it not only gathers up the
stock and holds it till out of the box, _B_, but upon further elevation
causes it to roll or slide along the paddle to a predetermined point,
where it falls off gently into a discharge-spout, _g_, which carries it
off for further treatment practically undamaged. During the operation
the box, _B_, and the body, _A_, are kept supplied by a stream of clean
or chemically-prepared water, and the wheel, _e f_, revolves slowly
in the box, the edges of the paddles sweeping around, while the box,
_B_, or its bottom, is kept oscillating, thus preventing an injurious
clogging of the perforations in the box-bottom. After the stock is
placed in the box, _B_, and the latter filled with sufficient water,
the wheel, _e f_, is caused to slowly revolve (by motive or hand
power), with the rounded sides of the spoons, _f_, presented forward.
This operation thoroughly agitates and cleanses the stock, while the
rounded form of the spoons prevents the breaking up of the natural
condition of the stock. The inventor gathers the finer particles as
follows, after they have escaped through the perforated bottom of
the wash-box, _B_. At the lower part of the trough, _A_, elevated on
crossbars or blocks, _h_, he places two parallel strips, _i_, and
between these, which are grooved to form ways, _k_, inside, is set a
long screen, _l_, placed on rollers, _m_, and movable thereby on the
rails or ways, _k_. To give movement to the screen, _l_, the inventor
attaches to its end a rod, _n_, which projects outwardly through the
walls, _a a_, by means of the packing-box, _p_, and cap or door, _q_,
which, when opened, allows the withdrawal of the screen, _l_, and
its burden. The shaking of the screen is accomplished by a suitable
motor applied to rod, _n_, and is kept going during the operation as
required. To the strips, _i_, which are placed at a little distance
from the side walls, _a_ (to leave a passage for the water and refuse
to go through), are hinged the two doors, _r_, which shut down upon
the rod, _s_, as a support, in which case nothing can fall upon the
screen, or which open up and rest against the sides, _a a_, in which
case the screen is exposed and the side passages closed by the doors,
_r_. During the initial or rough-washing stage the doors, _r_, are
kept closed, and the dirty water and refuse pass freely down the side
passages and out at a suitable opening at the bottom. After this stage
it becomes desirable to catch the particles which get detached from the
stock in box, _B_, and come through the perforations therein. Then open
up the doors, _r_, thus closing the side passages and compelling all
the water and small stock to go to the screen, _l_, which catches the
remaining stock. When sufficiently accumulated the screen may be drawn
out and the stock thereon removed. When the main body of stock in box,
_B_, has been cleansed, the paddles or spoons, _f_, are reversed, so as
to present their flat, skewed faces to the stock, and in revolving the
paddles now gently lift the stock and discharge it into the spout or
hopper, _g_. The washing and removal of the stock when washed are thus
accomplished without further manipulation than to reverse the paddles,
which obviously could be done by a reversing-gear on the motor, thereby
reversing the direction of movement of the paddle-wheel.
Instead of the whole box _B_ being oscillated back and forth, its
bottom may be set on slides or rollers and oscillated, while the sides
remain stationary.
In the drawings the box _B_ is shown as hung upon the shaft _c_ as a
centre; but as the provision of means of reciprocating or oscillating
the box or its bottom is within the skill of any machinist, it is not
necessary to describe any specific form. As the box with its contents
will be very heavy, the inventor prefers a special motor for it, which
may also be geared up to reciprocate the screen _l_.
Instead of the door _q_, as located in Fig. 2, it can be located as at
_q’_, same figure.
The entire plant must of course be arranged according to sanitary
regulations, especially as regards river pollution, etc.
The sheds for sorting and storing the glue-stock should, if possible,
be in close proximity to the pits and washing drums, and be dry and
airy. In arranging his plant, the glue manufacturer must, in short,
exercise his ingenuity with a view to carrying on the business with
as little loss of material, and as much saving of time and labor as
possible.
The work in a factory arranged in the above manner, is carried on as
follows:
The raw materials brought by the dealer are weighed, and if in green
state, the customary percentage—generally 50 per cent.—taken off. To
facilitate future operations, and to enable the manufacturer to produce
different varieties of glue, the dry materials are sorted and stored in
different compartments of the store-shed.
Green waste, _i. e._, such as has not been limed must be taken in hand
at once, as otherwise it would taint the air, be attacked by rats and
other animals, and suffer injurious alterations by decomposition. The
manner of operation is as follows:
_Liming._ Prepare “milk of lime” by filling the pits, which are to
serve for the reception of the skin waste, with the required quantity
of water and dissolve in it 2 per cent. of calcium hydrate obtained by
slaking a good quality of quick lime. Stir thoroughly, and in order
that the water may become thoroughly saturated with the lime, let the
liquor stand for 8 or 10 days before placing the waste in it. The
liquor should stand about 9 inches deep over the waste in the pits.
The length of time the waste has to remain in the milk of lime varies
according to the material; calf skins requiring 15 to 20 days, sheep
skins 20 to 30 days, and heavy ox hides 30 to 40 days. The milk of lime
should be renewed once or twice a week, and thoroughly stirred.
For the purpose of liming, the quality of the lime used is of the
utmost importance, the milk of lime being frequently quite valueless
by reason of having become carbonated or a bad quality of quick lime
having been originally employed. It should be borne in mind that only
the hydrate of lime which is present in solution in lime water is of
use, whereas in milk of lime so much carbonate and other impurities
may be present that the liquid, though thick, may be quite useless.
The value of a lime should always be tested by determining the amount
of real calcium hydroxide, Ca(OH)_{2}, contained in it. The operation
according to S. Rideal, is conducted as follows: Water free from
carbonic acid is first prepared by boiling distilled water for half an
hour in a strong, round-bottomed Bohemian or Jena flask. While steam
is still issuing, the flask is removed for an instant, closed by a
well-fitting greased cork or a rubber stopper, and allowed to cool.
When the temperature has somewhat fallen, the cooling may be cautiously
accelerated by dipping into a pail of warm water, then transferring to
the cold stream from a tap. The water may be preserved in the flask
or, preferably, a number of bottles with vase-lined stoppers should be
filled quite full and retained for use.
From the sample of lime, well mixed, a small portion (about 0.25
gramme) should be accurately and rapidly weighed, placed in a
wide-mouthed, stoppered bottle holding about 300 cubic centimeters, 250
Cc. of the boiled water added, and then allowed to settle. The whole of
the calcium hydrate will now have dissolved. Fifty cubic centimeters
of the clear liquid should now be withdrawn by a pipette, transferred
to a flask, colored with an indicator—either phenol-phthalein,
methyl-orange, or litmus may be used—and its alkalinity determined by
running in decinormal hydrochloric or sulphuric acid from a burette
till the change of color occurs. Each cubic centimeter of the acid
corresponds to 0.0028 gramme of calcium oxide, or 0.0037 gramme of the
hydrate, Ca(OH)_{2}. The amount by calculation will give the percentage
of real lime present in the sample. It is well to notice that any soda
or potash present will equally neutralize the acid, and be returned
as lime, but as these are of almost equal efficiency their presence
in _small quantity_ has no disadvantage. For special work it will be
necessary to have a full analysis. As a rule the product made from
limestone, or “stone lime,” is the best article in commerce, and is
much more free from stones and clay than “gray lime” or “shell lime.”
The best stone lime contains sometimes only ½ per cent. of impurities,
and seldom more than 5 per cent., while inferior kinds of gray lime
may contain as much as 50 per cent., and would be of little use in
glue-making.
After removal from the lime pit, the material is placed in willow
baskets or nets, and immersed in the stream to remove the greater
portion of the lime, which is generally effected in a few days. It
is still more effectively accomplished by placing the waste, after
soaking in the willow baskets, in the wash drums. After taking it from
the baskets or wash drums it is spread in the drying yard to drain and
dry, the desiccation being accelerated by turning it over with a fork
several times a day. While drying, the quick-lime is converted into
carbonate, the latter exerting no disturbing effect in the manufacture
of glue. When sufficiently dried, the material is ready for boiling,
and the crude glue thus obtained can be stored for any length of time,
until wanted for further manipulation.
In summer it is scarcely possible to cleanse the raw material as
rapidly as it is brought to the factory, and to work it immediately
without putrefaction setting in, and for this reason it would
frequently be risky to purchase larger quantities of it, even if
offered at very favorable terms. During the colder season of the year,
drying of the cleansed raw material is such a slow operation, that
in order to prevent putrefaction, recourse would have to be had to
artificial heat.
These drawbacks can, however, be overcome by the use of carbolic
acid, which possesses in a high degree the property of preventing
putrefaction. It is quite cheap, and as but a comparatively small
quantity of it is required, the additional cost need scarcely be taken
into consideration, since the value of glue-stock annually destroyed by
putrefaction is considerably greater than the expense for carbolic acid.
The raw material is thoroughly cleansed, and while in a moist state
is gradually brought into a brick cistern or large vat, carbolic acid
solution being poured over each layer, so that, when the cistern or vat
is filled, it stands about an inch or two deep over the material. The
latter may be left in this state until wanted.
The carbolic acid solution is prepared by dissolving 2 lbs. of carbolic
acid in 1000 quarts of water; the fluid thus obtained possessing a
slight odor of smoke. The washed glue-stock treated as above described
with carbolic acid remains absolutely unchanged, and when wanted needs
only be taken from the cistern and worked like fresh material.
In plants having no running water at their disposal and depending
entirely on well water, and where the waste water has to be discharged
into rivers or creeks, water containing carbolic acid should be used
for all the washing operations, a fluid containing 1 to 2 parts of
carbolic acid in 10,000 parts of water being sufficient for this
purpose. Such an addition of carbolic acid prevents the wash-water from
becoming foul.
Carbolic acid has the tendency of hardening the glue-stock and
imparting its odor to the glue, and among other antiseptics,
formaldehyde and boric acid have been recommended for the purpose of
preventing putrefaction for a reasonable time. Formaldehyde in weak
solution (1 part in 10,000 to 100,000 parts water) has been found
beneficial. In this small quantity it does not harden the stock nor
affect the subsequent boiling, as it is dissipated by the heat. Boric
acid and its preparations, notwithstanding their low antiseptic power,
are much in favor. A fluid containing 1 part boric acid in 200 parts
water will have to be used.
The principal varieties of hides and leather for glue-stock may be
classed as follows:
1. Bullock leather from old animals, highly limed, mixed with rump
pieces, also with horse leather, the latter being thin, of a dark color
and soft, and is of less value than bullock leather, because it yields
a dark glue. Fat leather is bullock’s leather from fat, stall-fed
cattle, and before use has to be freed from fat (by means of benzine).
2. Pieces of hide from the lower parts of the limbs of cattle, not
limed and with the hair; they form excellent glue-stock, yielding a
very adhesive glue.
3. Worn-out hinges from weavers’ looms, consisting of strongest
untanned bullock’s hide. When treated with lime they yield a very
strong glue, but are worked with difficulty.
4. Whip leather. This is waste in the manufacture of whips, and
is derived from thick tawed bullock hide. It yields an excellent,
light-colored glue.
5. Calf leather. Broad, thin, translucent strips, slightly limed,
yields glue of a very light color.
6. Calves’ heads. The skin of calves’ heads, limed, without hair. They
constitute the best material for gelatine, and form a special article
of commerce.
7. Calves’ feet. The skin from the last but one leg-joint which is cut
off from dry, unlimed, haired skins. It is the best material next to
calves’ heads.
8. Knapsack leather. Old knapsacks of calf skin and waste in the
manufacture of new ones, tawed with the hair on with alum and common
salt. When suitably washed this yields good glue-stock. The alum and
common salt have to be completely removed by washing. The hair is no
detriment to the process of boiling, it serving as a filter for the
glue running off. To this class belong also all kinds of fur waste,
especially remnants of old fur coats (sheep skin coats), from which the
wool is removed and the skin used as glue-stock. All these materials
having been treated with alum and common salt have to be freed from
them by suitable manipulation.
9. Hare and rabbit skins freed from their fur. They yield a
light-colored glue of little consistency.
10. Cut rabbit skins. In depriving these skins of their fur, they are
cut by a machine into fine threads of even size. In France they are
worked into size for gilders’ use which is highly valued.
11. Sheep and lamb leather (goat leather) limed, thin and very light,
yields but a small quantity of glue of little consistency. To this
class belongs the waste in the manufacture of kid gloves. Waste of
morocco and other varieties of similar leather, pressed into bales and
secured with wire, comes into commerce under the name of Levant leather.
12. Waste obtained in paring kid leather and in the manufacture of
gloves. It constitutes a flocculent powder and yields very thin glue
liquor with slight adhesive power. Before boiling, the substances used
in tanning must be completely removed by washing.
13. Surrons. These are untanned, unlimed skins of various wild animals
(antelopes, gazelles) which have been used for packing leaf tobacco and
various drugs. They form good glue stock.
2. BONES AND CARTILAGES.
In addition to hides, bones are a material highly valued by the glue
boiler. Chemically speaking, the framework supporting the fleshy
tissues of the animal order, and which we call bones, is a combination
of phosphates of lime and magnesia, carbonate of lime, and alkaline
salts, united with fatty and cartilaginous matter. To the latter we
look for our yield of glue; to the fatty matter for the fat, and to the
phosphates for the basis of fertilizers.
Bone cartilage is composed of carbon, hydrogen, oxygen and nitrogen,
the percentage composition being practically constant, whether the
cartilage be from an old or a young animal. The bones of the young
are, however, much richer in cartilage than those of the old. This is
reversed in the case of the inorganic or mineral matter, the old having
the greatest yield of phosphates.
Then again, the fatty matters are more in evidence in full-grown
animals than in youth or age; also in the thigh and leg bones the
yield is higher than in the heads, ribs or shoulder blades, the latter
averaging 12 to 13 per cent., whilst the former runs 18 to 19 per
cent.[1]
[1] Bone Products and Manures. By Thomas Lambert. London, 1901.
Bones being less subject to putrefaction than skin-stock, they are
not brought into commerce in a prepared state. They are mainly bought
by contract from various dealers within easy access to the works. The
rates are generally fixed for a certain period, and cover all classes
of common bones, whether fresh butchers’ or a mixture with partly
boiled bones. Bones differ considerably in their value. A fresh bone
will yield the highest percentage of fat and glue. On the other hand,
partly boiled bones may contain only 6 per cent. fat with 30 per cent.
water. In buying bones the manufacturer should exercise great care, as
the dealer sometimes finds ways and means of including hoofs, horns,
iron, beefy matter, and even pieces of brick. Naturally they form
weight, but, excepting the horns, have no value.
To separate the different classes of bone coming into the works, and
arrange them according to the amounts they would produce of fat and
glue, is no doubt a desirable object, but in practice it is seldom
carried out. However, if the manufacturer wishes to undertake this
tedious work, it is recommended to make the following distinctions:
1. Bones of young animals, sheep, calves, dogs, cats, etc., being
readily disintegrated, are thrown into one pile, and also the light
bones of oxen, such as skull bones, shoulder bones, the vertebra of the
tail, etc.
2. A second pile is made of the foot bones of goats, sheep and cattle,
provided they can be had, as is the case in the United States and
England, in sufficiently large quantities.
3. Scraps and shavings from bucks’-horn from turners and button-makers.
4. Thick bones of oxen, horses, etc., which must remain longer in the
lime-bath, together with waste of hard bones from turners.
5. Where large quantities of bones are handled it is advisable to sort
out the bones of the upper thigh, as they can be more advantageously
used for the manufacture of piano-keys, handles for tooth-brushes, etc.
Hoofs, which are frequently found, should be thrown out, as they yield
no glue and can be utilized for other purposes.
The further manipulation of the bones for the manufacture of glue
requires first of all their crushing or grinding in a stamper or mill.
By this crushing or grinding of the bones two objects are attained,
namely, they are more readily deprived of their fat and present more
points of attack to the corrosive agents to be used later on. The
crushed bones are put in a large boiler, and for a few hours subjected
to the action of steam. Leg bones, as well as horns, should not be
boiled, as they contain no fat, and would lose too much glue-yielding
substance. After boiling, the bones are placed in a lime vat for 8 to
14 days. The water used for boiling the first portion of bones may be
used for a second one.
The extracted fat amounting to 4 or 5 per cent. of the quantity of
bones used, is taken off the surface of the cold liquor and the latter
may be utilized as a fertilizer, or fed to cattle.
For crushing the bones, a stamping mill is generally used, it yielding,
when properly constructed, material for the manufacture of glue, as
well as granulated bones which form an excellent product for the
preparation of animal charcoal.
Since animal charcoal in pieces of quite even size is now in
general demand, it is recommended to manipulate the bones in the
above-described manner, to sell the granules to the manufacturer of
animal charcoal, and use for boiling glue only the completely-crushed
portions and the porous bones which are not at all suitable for the
manufacture of animal charcoal.
[Illustration: FIG. 6.]
Fig. 6 shows a stamping mill very suitable for the crushing of bones,
the illustration showing the mill open on the left side and closed
on the right. It is furnished with 16 stamps, _D_, each stamp being
provided with a cast-iron shoe. The stamps are lifted by means of a cam
shaft in such a manner, that the height of fall of the outermost pairs
of stamps is least and that of those in the centre greatest. Between
the inner stamps is a sieve _H_ with meshes of sufficient size to allow
the largest pieces, which can be produced by granulation, to fall
through.
Underneath the sieve is an Archimedean screw _K_ for carrying off the
pieces of bone passing through the sieve.
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
The base of the stamping mill consists of iron plates so arranged
as to form steps, the plates lying towards the center of the mill
constituting the lowest steps. Every two stamps standing alongside one
another rest upon such a step. When the mill is set in motion, the
bones reaching the stamping trough from the right and left fall upon
the steps, and are crushed by the descending stamps.
As a rule, the bones to be stamped are not brought directly into the
stamping trough, but are first passed through a crushing mill and
the coarser pieces thus obtained are subjected to the action of the
stamping mill.
Figs. 7 and 8 show a well-constructed bone crusher. It consists
essentially of two cast-iron rollers _A_ and _B_, furnished with
case-hardened cutters. The bones are introduced through the hopper _B_,
and the rolls set in motion by means of cog-wheels _a_ and _b_. The
bearings of the roll _B_ run in a carriage which can be shifted by the
lever-construction _f i_. The object of this contrivance is to allow
of the roll _B_ giving way in case a harder material than bones, for
instance, a stone, passes between the rolls.
[Illustration: FIG. 9.]
The Crosskill bone mill, Fig. 9, as described by S. Rideal, is intended
to be driven by a strap from the fly wheel of a common portable
engine. It consists of a pair of strong rollers made of wrought-iron
with case-hardened cutters, and a revolving or oscillating riddle for
separation of the ground bones as they fall from the cutters; the whole
carried by a substantial cast-iron frame. The mill will grind from 6 to
16 hundred-weight per hour with a three to eight horse-power engine.
For sorting the crushed bones into pieces of equal size, a sieve, Fig.
10, is used consisting of a drum constructed of narrow boards covered
with wire-netting of different degrees of fineness. The upper portion
_A_ of the drum consists of narrow-meshed net and through this falls
the fine meal which is conveyed by the Archimedean screw _F_ over the
frame _F G H_ into vessels serving for its reception.
The lower section, _B_, of the drum is furnished with netting, the
meshes of which become gradually wider towards the lower end, and,
hence, the smallest particles of bone fall through the funnel, _D_, the
medium-sized ones through _E_, and the largest ones through _F_. Pieces
which cannot pass through _F_, leave the drum at _G_.
In factories manufacturing glue as well as animal charcoal, the larger
pieces are steamed by themselves to obtain their fat, and then charred,
while the small pieces and the meal are utilized for glue.
[Illustration: FIG. 10.]
The lime-bath used for bones should be of the same strength as that
for skin-stock. After removal from the lime vat and washing, the bones
are put in a tank of stone or wood (brick pits should not be used)
containing cold hydrochloric acid of 70° Bé. or 1.05 specific gravity
(= 10.6 per cent. HCl) for thick bones, or half that strength for thin
bones, and are thus left to digest for 8 to 14 days, being frequently
stirred and fresh acid added. By the action of the acid the calcium
phosphate is dissolved and the bones become cartilaginous, flexible
and transparent. The phosphates can be precipitated by ammonia, or
the whole evaporated with charcoal or silica, and distilled to make
phosphorus.
When sufficiently softened, the stock is washed in wicker baskets or a
washing drum to remove adhering acid. They are then placed for one day
in the lime liquor, again washed, and then either dried or stored away
for future use, or boiled at once to glue, while in a moist state.
Leg bones, horns, and other soft bones which contain scarcely any fatty
matter are not steamed for the reason previously stated, but in all
other respects are treated like steamed bones.
It is of the greatest importance that the bones should be thoroughly
freed from acid, since even the smallest quantity remaining behind
exerts an injurious effect upon the finished glue. It is therefore
recommended to test the water draining off, or the bones themselves,
with litmus. If the tincture turns red, it is a sure indication of the
presence of free acid, and the washing must be continued until the blue
color of the tincture remains constant.
Gerland’s suggestion, to use dilute sulphurous acid in place of
hydrochloric acid for dissolving the phosphates of the bones, and
to evaporate the sulphurous acid by heating, whereby the phosphates
are precipitated in an insoluble state, has now been quite generally
introduced.
For the preparation of gelatine from bones, Jullion and Pirie’s
process may be recommended. It requires a somewhat expensive plant,
but saves hydrochloric acid and time. The process consists essentially
in dissolving the phosphates of the bones in vacuum. A box of wood,
or better of granite, which can be closed air-tight, is required
for this purpose. The box is filled with bones, and acid of the
previously-mentioned strength poured over them. The box is then closed
and the air pumped out by water or steam power. The smallest cracks
and pores of the bones are thus freed from air, and the latter is
replaced by hydrochloric acid, which in this manner acts rapidly and is
completely exhausted. The remaining crude glue is then further worked
in the usual way.
Bones honey-combed by putrefaction, exposure to the weather, or burial
in the ground are of little or no value to the glue-boiler, as nearly
all the glue-yielding substance has been destroyed; they should
therefore be thrown out in buying stock. The ammonia which is formed
when putrefaction sets in, colors the glue dark.
3. LEATHER WASTE.
Leather tanned with a substance insoluble in water is not directly
suitable for manufacturing glue, but can be made so by a special
process, which, though somewhat tedious, nevertheless pays for the
trouble.
In using such stock the manufacturer should make a distinction
between old and new leather. The principal materials of this kind,
large quantities of which contribute their quota to the glue-boiler’s
stock, are old shoes, straps, harness, etc., and further, waste from
shoemakers, trunk-makers, and in fact from the shops of all workers in
leather except those using alumed leather.
Before boiling the leather waste to glue, the removal of all traces
of tannin becomes absolutely necessary, since the retention of the
smallest quantity prevents the animal tissue from dissolving in water.
The various methods proposed for the preparation of the leather waste
differ either in the chemical solvent used, or in the mechanical
manipulation of the waste.
The principal point in all methods is to comminute the waste as
uniformly as possible to facilitate the complete removal of the tannin.
Various machines, some very complicated, have been proposed for the
comminution of the waste, but a rag engine or “hollander” such as is
used by paper-makers deserves preference for the purpose, as it not
only comminutes, washes and prepares the waste in a suitable manner for
the manufacture of glue, but the leather pulp when mixed with rags or
woody fibre gives a substitute for leather which is very tough and of
good appearance, and can be worked into many articles.
After the preparation in the hollander and careful washing the waste is
treated, according to Stenhouse, under a pressure of two atmospheres in
a boiler with water to which is added 15 per cent. of the quantity of
waste to be treated at one time of slaked lime.
By another method the extraction of the tannin is effected by boiling
the leather pulp with caustic soda of 1.025 specific gravity for from
six to twelve hours. After drawing off the water and pressing out, the
pulp is again boiled with caustic soda of the same concentration. The
next process is to carefully wash out the soda, which is best effected
in the hollander.
By neutralizing the soda lye in the fluid drawn off after the first
boiling, it can be re-used for tanning or purposes for which tannin is
required.
According to another method, the _modus operandi_ is as follows:
Dissolve 1½ lbs. of oxalic acid in 3 gallons of water, pour the boiling
solution over 110 lbs. of waste, and keep the mixture in a water-bath
at a temperature of 176° to 212° F. This effects the solution of the
pulp. Then dilute the solution by adding gradually 4 gallons of water
until a uniform mass is formed. Now add 5 lbs. of lime slaked to a
thin paste, and mix the whole thoroughly. The mass becomes friable and
pulverulent. It is passed through a wire sieve and then exposed to the
air. In three to four weeks the tannin is entirely destroyed, which
is recognized by the mass assuming a lighter color. The lime is then
removed by washing with water and hydrochloric acid. If the tannin
has not been entirely destroyed by exposure to the air, add 1 lb. of
liquid ammonia and a like quantity of pyrolusite to every 110 lbs. of
leather substance when boiling it to glue. The oxygen yielded up by
the pyrolusite, which, in the presence of ammonia, exerts no injurious
effect upon the glue, destroys the last traces of tannin. Frequent
stirring with a shovel while the material is exposed to the air and
moderate heating, facilitates the destruction of the tannin.
4. RAW MATERIALS FOR FISH GLUE.
The air-bladders or sounds of various fishes contain much glue-yielding
substance and on account of its purity, the product known as isinglass
obtained from them is preferably used for culinary and medicinal
purposes. The high price of the raw material excludes it from being
used by the glue-boiler, but as he manufactures substitutes for
isinglass, and should therefore have a thorough knowledge of the
article with which he has to compete, its manufacture will be included
in this treatise. Since, however, the work of the manufacturer is
finished with the preparation of the raw material, _i. e._, of the
air-bladders into crude glue, isinglass and its substitutes will be
referred to later on.
There is a material difference between isinglass and glue manufactured
from entire fishes. The raw material is, of course, limited to certain
localities. The principal point to be observed in the manufacture of
fish-glue is the removal of the skin, which is effected by means of
dilute sulphuric acid.
After removal of the last traces of acid, the fatty matter of the
fishes is saponified by a treatment with milk of lime frequently
renewed. After washing out the lime, the pulpy mass is placed in a
solution of sodium hyposulphite, alum, and common salt, where it
remains for a few days. The liquor is then drawn off and replaced by
a mixture of solution of alum, dilute sulphuric acid and nitric acid.
After macerating in this mixture for a few days, the mass is thoroughly
washed and boiled to glue, and the resulting product clarified with
sulphurous acid or alum solution. As will be seen, the entire process
is tedious, requires many chemicals, and besides the yield of glue,
which has no specially good qualities, is small. It is used as a
substitute for isinglass for clarifying liquids. The best proof that
the business is of but little importance is found in the fact that
no fish-glue has been exhibited at any of the late international
exhibitions.
The scales of large fishes, such as carp, give more favorable results.
They are treated with hydrochloric acid in a similar manner to bones.
The scales do not dissolve entirely, a horny insoluble mass, giving
no glue, remaining behind after the solution of the glue-yielding
substance.