MANUFACTURE OF SKIN GLUE.
The thorough preparation of the raw materials will materially
facilitate all succeeding operations, which may be classified as
follows:
1. Boiling the glue.
2. Clarifying the glue-liquor.
3. Forming or moulding the glue.
4. Drying the glue.
However, before entering into the description of these operations, it
will be necessary to refer to an intermediate product, which has been
previously mentioned under the name of crude glue, and is prepared, for
instance, by tanners and manufacturers of parchment, but also forms in
some localities a special branch of industry.
This crude glue is actually not glue, but a glue-yielding substance in
such a state of preparation that it can be directly used for the first
operation, namely, boiling. It consists of waste of skins and leather
of all kinds, completely cleansed, dried and limed, and in the case
of leather treated with agents for the extraction of the substances
used in the tanning. As will be readily understood, the operations
required for the preparation of this stock are virtually the same as
those described in the previous chapter for raw materials and need not
further be here referred to.
The bulk of such stock is prepared by tawers and manufacturers of
parchment, though a considerable quantity of it is also derived from
waste in the manufacture of gloves. The product from the latter source
is also found in commerce under the French names _Colle franche_ or
_Brochette_. However, if such stock is used, it is best to again
immerse it in lime water, after which it should be thoroughly washed.
The manufacture of glue from hide and leather waste differs materially
in many respects from that of bone glue, it being the more simple
process, as no other preliminary operations than the preparation of the
glue-stock are required. The first operation is
1. COOKING OR BOILING.
For this operation any kind of boiler may be used, but the materials
should be supported on a perforated grid a little distance above the
bottom, so as to save them from risk of scorching. In the centre of
the grid stands a conical pipe 2 to 3¼ feet long, perforated like the
grid and communicating with the space between the grid and the bottom
of the boiler. The height of the boiler can be increased 1 to 1½ feet
by placing an annular piece upon the rim which is bent upwards for its
reception.
The size of the boiler depends on the quantity of raw material to be
worked at one time. It is best to choose boilers holding from 110 to
440 lbs. of glue-stock, and to place two, four or more of such boilers
in one hearth.
The manner of using such a boiler is very simple. Straw is placed upon
the false bottom in such a manner as to cover its entire surface, and
extend up the sides of the boiler at least as far as it is touched by
the flame. The object of the straw is to serve as a filter, and protect
the materials from injury by the flame. But for the production of
entirely pure gelatine or glue, straw cannot be used, as, by boiling,
it yields a yellow coloring matter, which passes into the glue. Barley
straw gives a less intense coloring matter than rye straw.
In case straw cannot be used, the material is placed in a large bag,
previously thoroughly boiled, and suspended in the boiler so as not to
touch the sides. By this means scorching is prevented even if the fire
touches the bottom as well as the sides of the boiler.
The boiler having been heaped with material so high as to overflow the
brim and fill the annular piece placed upon it, is filled with water
as far as touched by the fire. The fire may now be started. The hearth
in which the boiler is placed should, of course, be so constructed
that the gases are uniformly distributed and the water quickly brought
to the boiling-point. When the water commences to boil, bubbles of
steam ascend from the space beneath the grid and, passing through the
perforations of the conical pipe, penetrate the glue-stock. Thus the
first formation of glue takes place, and the stock begins to settle
down gradually as it goes into solution. The stock heaped up in the
annular piece also sinks down gradually, and being partly heated by the
hot vapors and thus prepared for solution, is finally submerged in the
boiling solution and becomes soon entirely dissolved.
Waste of hide and horn piths are completely dissolved in five to seven
hours. No more water should be used than is absolutely required for
cooking the entire quantity of stock, because too much water renders
the solution too thin and gives a jelly of little consistency and
difficult to dry. Concentrating the glue solution by continued boiling
is bad practice, as it is detrimental to the resulting product by
reason of the glutin undergoing a gradual transformation.
It is best to start with a slow fire to give the stock time to soften
and thus prepare it for solution. When somewhat softened, the mass is
brought to boiling and the latter kept up, gently and uniformly, until
solution is complete. Solution is promoted by careful stirring, but
care should be had not to disarrange the straw upon the grid and on the
sides of the boiler as this would interfere with proper filtration of
the glue solution.
The duration of cooking depends on the nature of the raw materials.
Scraps of skin from young animals, antlers, sheep trotters, etc.,
dissolve in three to four hours, while waste from ox and horse hides,
or bones from old animals, require six to eight hours.
The progress of the operation is readily ascertained by pouring a
small sample of the gelatinous fluid in half an eggshell, and setting
it aside for a few minutes to cool. If a clear and consistent jelly
be obtained, boiling has been carried on to a sufficient extent, and
the liquid is drawn off. Any undissolved glue-stock remaining upon the
straw filter can be boiled by itself, and the resulting gelatinous
liquor utilized in the next boiling.
It is evident that quick and uniform solution of the materials, which
enhances the quality of the glue, is promoted by comminuting the
glue-stock either by grinding, stamping, or mechanical means.
The succeeding clarification of the glue is much facilitated by
removing while boiling the scum, consisting of fat, coagulated albumen,
lime-soap, accidental admixtures, and other impurities. Before drawing
off the gelatinous liquor it is advisable to withdraw the fire and
allow the contents of the boiler to rest for fifteen minutes.
The residue remaining upon the straw filter consists of hair,
lime-soap, undissolved particles of hide and bones, lime, etc., and is
utilized, after repeated boiling, as fertilizer or for the manufacture
of gas.
The mode of glue boiling above described is the oldest and at present
is only in use in small establishments. Fig. 11 represents a convenient
apparatus for the purpose. It consists of three boilers upon as many
different levels. The lower boiler, _b_, serves for the settling and
clarification of the glue. It communicates with the second boiler, _a_,
which contains the material to be acted on, by means of a pipe provided
with a stopcock, and is sufficiently heated by a small fire to keep
the glue liquid without allowing it to reach ebullition. The upper
boiler, _c_, which is heated by the waste heat of the chimney, serves
as an economical reservoir for hot water. The end of the discharge-pipe
of the settling boiler is provided with a filter of woven wire. As
the sides and bottom of the second boiler are lined with straw, which
acts as a preliminary filter, the glue runs off quite clear from the
settling boiler.
When this mode of manufacture is adopted, two boilings can be made
per day, under favorable circumstances, so that, if the boiler has a
capacity of 220 lbs. of stock, which will yield from 110 to 132 lbs.
of dry glue, the daily fabrication will be about 220 lbs. of finished
product.
[Illustration: FIG. 11.]
In larger plants, the above described mode of extracting the glue-stock
with water has been superseded by the use of steam in a cylindrical
wrought-iron boiler, twice as high as wide, and capable of withstanding
a pressure of three atmospheres. The boiler is furnished with a
perforated false bottom underneath which terminates a steam pipe. It is
filled from above with previously softened glue-stock and the charging
hole hermetically closed. Steam is then gradually admitted and exerts
at once a dissolving influence upon the stock. A portion of the steam
condenses and forms with the dissolved glue-stock a concentrated jelly
which collects between the true and false bottoms.
For the escape of air a cock is provided which is closed as soon as
steam commences to escape from it.
[Illustration: FIG. 12.]
The advantages of this process are obvious. A larger quantity of
glue-stock can be extracted than in the boiler previously described,
and there is no danger of injury by scorching and consequent damage to
the color of the glue. More highly concentrated solutions are obtained
in a shorter time, and the spoiling of the glue solution by too long
continued cooking is prevented by drawing off the solution as quickly
as formed. The escaping hot vapors may be utilized for drying the glue,
softening the raw material, etc., the entire quantity of heat being
thus utilized. A further great advantage of this method is that there
is less annoyance from badly-smelling vapors than when boiling is done
over an open fire. A number of such boilers can be arranged in one
room and supplied from a common steam boiler.
Fig. 12 represents a boiler for extracting glue-stock with the use of
steam. It is provided with a lid, _D_, which is removed for charging
the boiler. The aperture, _E_, in front, serves for the removal of
the residue. Above the true bottom there is another false bottom,
perforated and movable, which can be covered with straw for preliminary
filtration. The steam reaches the glue-stock through a pipe which
passes through the actual and false bottoms, and is perforated above
the latter. The resulting jelly collects between the true and false
bottoms, where it is less exposed to the action of hot steam. The
escaping steam passes through the pipe, _F_, which is provided with a
stock-cock. The pressure in the boiler is indicated by the manometer,
_K_. After throwing the materials into the boiler they can be covered
with warm water, or, after the lid is closed, warm water is introduced
from a reservoir through a special pipe and distributed over the
material through a rose.
The boiler stands upon a frame sufficiently high to allow of
conveniently placing a vessel under the pipe _G_, through which the
jelly is discharged. The vessel, when full, is conveyed to the settling
vat, or the arrangement may be such that the jelly is directly run into
the settling vat.
In many large plants open jacketed pans heated by steam are still used
for treating the material. Fig. 13 shows an arrangement with two of
such pans; of course one, or a larger number may be used, according
to requirement. In the illustration the pan _I_ on the left is shown
in front view, and the pan _II_ on the right, in section. _K_{1}_ is
the actual pan enclosed by the jacket _K_. Steam circulates in the
space between pan and jacket, whereby the stock in the pan is heated.
_K_{1}_, in addition, is furnished with a steam coil _S_, which may,
however, be omitted.
The steam enters through the pipe _D_, the space between pan and
jacket, passes into the coil _S_, and escapes at _b_. The water formed
by the condensation of steam in the space between pan and jacket, as
well as that which runs off at _b_ from the coil _S_, is carried away
by the pipe _A_.
The pipe _L_ serves for conveying hot water to the pans, and the pipe
_F_ for the discharge of the finished glue liquor. The stirrer _R_, is
furnished with two paddles, and is set in motion by a transmission on
the ceiling of the room. It serves for keeping the stock in the pans
constantly agitated, solution being thus very much promoted.
[Illustration: FIG. 13.]
The mode of working with this apparatus is very simple. Water being
admitted into the pan through _L_, the glue-stock is introduced and the
mass brought to boiling by admitting steam. The finished glue-liquor
is from time to time drawn off through the pipe _F_ into the settling
vessel.
It is generally preferred not to concentrate the glue-liquor in the
pans to such a degree as required to obtain a jelly, which after
cooling, can be immediately moulded, experience having shown that less
concentrated liquors can be more readily and better clarified, and
yield a lighter and more transparent glue.
Mr. Thomas Lambert gives the following process of cooking: The skins
are taken to the glue-boiler, which is an open vessel, 8 feet in
diameter at the top and 7 feet deep, and provided with a perforated
false bottom, through the center of which passes a two-inch pipe, one
end dipping below a layer of water at the bottom, the other projecting
about half the height of the boiler, this part being covered with a
perforated hood to spray the liquor through the mass. The skins are
placed on the false bottom and the added water at the bottom of the
boiler is brought to the boil by means of a steam pipe. The steam not
being able to escape quickly through the dense mass of glue-stock
above, exerts a pressure on the water, forces it through the pipe, to
be sprayed through the mass, and ultimately works its way to the bottom
of the boiler to be forced up again. This continual circulation of the
hot liquor rapidly dissolves the gelatinous matter, and when a strength
of 18 per cent. dry glue is reached, the first run is made to the
evaporating pan, the liquor passing through a filter of fine shavings,
to remove any suspended matter. Fresh water is added to the boiler, and
the boiling renewed. Three extractions are usually made, the last being
used for size.
In order to avoid annoyance to the neighborhood from foul odors,
Terne’s glue-boiler shown in Fig. 14 may be recommended. The lead-lined
iron boiler _A_, with manholes _B_ and _C_, on top and side, is
furnished with a false perforated bottom upon which the glue-stock is
placed. Underneath the false bottom lies the coil _E_ with valve-box
_e_. The boiler is filled through the upper manhole with glue-stock and
water admitted, steam being at the same time introduced in the coil
and in order to quickly heat the water, direct steam is also admitted
to the boiler through the pipe _F_ and cock _G_. When the water is
boiling the cocks _G_ and _F_ are closed, the coil furnishing now
sufficient heat. During boiling some steam is allowed to escape through
the partly-opened cock _L_, all badly-smelling gases being thereby
carried to the fire-box of a boiler where they are burned. When boiling
is finished, the glue liquor remains for a short time in the boiler to
allow the melted fat to separate on the surface, the cocks _K_{1}_ and
_K_{5}_ and serving for drawing off the fat. The insoluble residues of
the glue-stock remain upon the false bottom and are taken out through
the manhole _C_.
[Illustration: FIG. 14.]
2. CLARIFYING THE GLUE-LIQUOR.
The clearness of glue, _i. e._, its freedom from undissolved
substances, is by no means a criterion of its value as an agglutinant,
since pulverulent inorganic substances (white lead) are frequently
intentionally introduced into some varieties, for instance into
Russian glue, without injury to their adhesive power. But as a turbid
appearance may also be an indication of unsoundness and decomposition,
the manufacturer endeavors by all means to obtain a clear product.
A strict distinction should be made between clearness and color.
Very dark-colored glue may be very clear, and a very pale variety
the reverse, yet both possess excellent qualities. Both properties,
clearness and light color, cannot be obtained by the same process.
Clearness will be first referred to. If the glue-stock has been
properly prepared by rendering adhering particles of blood and fat
innoxious by liming and subsequent careful washing, the separation of
the few remaining impurities, which may have passed through the straw
filter, is readily effected by allowing the liquor to stand, care
being had to keep it liquid as long as possible to give the grease
time to rise and the flocculent and fibrous impurities to settle. This
is best effected in a wooden vat surrounded by a wooden or sheet-iron
jacket, the intermediate space between jacket and vat being filled
with a non-conductor of heat, or, if required, it may be heated by the
introduction of steam. The grease is skimmed off as it rises, and when
the solid particles have settled the liquor is drawn off through a pipe
placed a short distance above the bottom of the vat.
The size of the clarifying vat depends on the size of the boiler.
It is, however, best to have two vats for each boiler, in order to
keep the first liquor, which is always clearer and more concentrated,
separate from the last run. To be able to draw the upper layers of
purer liquor into cooling boxes by themselves, the vats are provided
with faucets at different heights.
To prevent putrefaction of the liquor which readily sets in during
settling at a higher temperature, the vats should be kept scrupulously
clean, and from time to time rinsed with clean, hot water. It is also
advisable to line them with sheet-iron.
Should the above-described mechanical separation not prove sufficient,
recourse must be had to other means. Alum and sulphate of alumina have
long been used for clarifying, 1 lb. of either of them, pulverized,
added to every 300 gallons of liquor, being as a rule sufficient.
Either of these chemicals removes the albuminous and extractive
constituents of the solution, and converts the dissolved free lime into
sulphate of lime, which settles readily, and prevents putrefaction of
the glue solution while drying under unfavorable circumstances. The
quantity of alum mentioned above does not impair the quality of the
glue.
Albumen is sometimes used for the better qualities of glue, and
generally for gelatine, but a cheaper substitute is fresh blood, which
contains albumen and fibrin. Dry albumen is dissolved in cold water,
or white of egg is used direct, if procurable. Before adding either of
these substances, the liquor is cooled to 130° F., and the clarifier
well stirred in; then the temperature is raised to about 200° F., when
coagulation occurs, and the precipitate entangles the impurities and
falls to the bottom, requiring, however, from twelve to twenty-four
hours to clear. It is said that glues clarified with albumen have a
characteristic soapy smell and show a tendency to foam.
The precipitation of the lime might be better effected by oxalic acid,
and the organic substances removed as scum by adding to the boiling
mass some astringent matter, such as a decoction of oak bark or hops;
but the purification has, in either case, to be done at the expense of
glutin.
A glue liquor, which does not clarify by these means, is not sound, and
is derived either from spoiled raw materials, or such as have not been
thoroughly prepared, or has been injured in boiling.
A far more difficult matter than the removal of mechanical admixtures
is to free the liquor from the coloring substances from which
it derives its color, and to discolor it without injury to the
characteristic qualities of the glue.
The use of animal charcoal for such large quantities of somewhat
thickly-fluid solutions, which are liable to spoil at the high
temperature at which they would have to be filtered, is very
difficult, and the result not favorable, except the solutions could be
successfully deprived of their tendency to putrefy. The use of carbolic
acid is also in this case the only means of removing the great tendency
of the liquor to putrefy, and hence, if the liquor is to be discolored
by treatment with animal charcoal, it can only be done without danger
to the glue, by mixing it with carbolic acid.
The object is more easily effected by bleaching the raw materials
previous to boiling them to glue.
This is accomplished by placing the glue-stock, thoroughly limed and
while still moist, in a bath of chloride of lime, not too strong, as
otherwise the solution of the materials becomes difficult. A bath of
the proper concentration is made by dissolving about 9 ozs. of chloride
of lime in sufficient water to cover 110 lbs. of glue-stock. After one
hour add sufficient hydrochloric acid to obtain an acid reaction, which
is recognized by litmus-paper dipped in the bath turning red.
Although the glue-stock is not bleached entirely through by this
process, the thin portions and outsides of the thick material acquire
a lighter color, and the first run of glue solution will have a light
color and can then be treated further without much difficulty.
Sulphurous acid has been successfully used for the production of
colorless glue without the necessity of boiling.
Waste of hides and skins is the only available material for this
process. Place the waste in water until putrefaction sets in. When this
is the case wash the material in a bag or wicker basket in running
water. Then pour 2½ parts of sulphurous acid over 12 parts of wet
material, mix the whole thoroughly and let it stand in a hermetically
closed vessel for 24 hours. Now draw off the acid, and after washing
the material thoroughly repeat the operation. When the vessel
containing the mixture of material and sulphurous acid is opened for
the second time the foul odor should be entirely superseded by that of
sulphurous acid, this being a sure indication of the correct execution
of the process. Wash the material, and, after squeezing, throw it
into a vat large enough not to be filled by it more than two-thirds
full. After filling the vat with water allow the mass to digest at a
temperature of 109.4° F. for 24 hours. The result will be a gelatinous
solution, which is drawn off and converted into glue. The undissolved
residue is transformed into gelatinous solution by pouring water over
it and allowing it to stand at a somewhat higher temperature.
For carrying out this process and that of bleaching with chloride
of lime it is best to use a vat provided with a stirring apparatus,
somewhat like a hollander used by pape-rmakers, as being most suitable
for washing, disintegrating and mixing the material.
Glue-liquor may also be successfully bleached with sulphurous acid, and
in speaking later on of the manufacture of bone glue, a very practical
apparatus for this purpose will be described.
Glue-liquor bleached by sulphurous acid clarifies very readily and
is protected from spoiling. The resulting glue remains, however,
quite acid, and cannot be used for all purposes, especially not in
combination with colors, chemicals, etc., upon which the acid has a
destructive effect.
3. FORMING OR MOULDING THE GLUE.
After clarifying, the liquor is run into moulds of deal wood or sheet
iron, lightly joined and of a rectangular form, slightly converging
towards the bottom so as to allow the more ready detachment of their
contents. They are about 3.25 feet long, 10 inches wide at the top, and
7¾ inches at the bottom, and 5 inches deep. When very regular cakes of
glue are desired, cross grooves of the required shape are cut in the
bottoms. After being well cleansed and ranged upon a level the boxes
are filled to the brim through large funnels with strainer cloths
affixed to their barrels. It is best to place them upon perfectly clean
stone flagging slightly inclined towards a reservoir for the reception
of such portions of their contents as may run over. The apartment in
which the work is performed should be clean and airy, a dry cellar
being the best for the purpose. In place of a large number of boxes, a
shallow vessel lined with sheet-iron and capable of holding the entire
quantity of liquor is sometimes used, from which the solid jelly is cut
out in cubic masses, which are further divided.
This arrangement can only be recommended for establishments where
but one variety of glue is produced, and the different layers in the
clarifying vat are not separated according to their clearness. Before
running the liquor into the boxes the latter should be moistened with
water, or, if made of wood, coated with oil, stearine, or paraffin to
prevent the liquor from penetrating the wood and the solidifying glue
from adhering to the sides.
After the solidification of the glue, which generally takes place in
twelve to eighteen hours, the boxes are inverted upon a table with a
smooth top of wood or stone previously wetted, so as to prevent the
adherence of the gelatinous cake to its surface. To detach it from the
sides of the boxes the moistened blade of a large knife is generally
used.
Cutting the cubes of glue into commercial cakes or sheets is readily
accomplished by observing the following instructions:—
The shape of the cakes depends principally on custom. The consumer is
used to a certain variety of glue, and if it is not offered to him in
the customary shape, he might refuse it and take his custom elsewhere.
The quality of the glue is the next point to be considered. If very
dark, it is advisable to cut the glue into thin cakes, and if turbid,
into thick ones, in order to make this defect the less apparent.
Thicker cakes can also be cut if the conditions for drying them are
favorable, and thinner ones if the reverse is the case.
The mass is first divided by a steel or brass wire stretched over a
frame, like a bow saw, into horizontal layers. The size of these layers
is regulated by guides which are placed at distances corresponding with
the desired thickness of the cake of glue. Instead of one wire, as many
as the cakes of glue to be cut, can be stretched over the frame, which
is best made of iron and provided with conical pins by means of which
the wires can be tightened, in the same manner as piano strings, when
they have become slack by use.
The width and thickness of the cakes of glue are regulated by the
distance of the wires from each other, and the length by the width of
the box. The cakes thus formed are dexterously lifted from the block
with the moist blade of a large knife and placed upon nets.
Instead of using wooden or sheet-iron cooling-boxes, it is recommended
to pour a layer of liquor of the desired thickness of the glue cakes
upon large polished stone slabs, and when congealed, cut it into
sheets, which are placed upon the nets to dry. The advantages of this
method are obvious. The liquor cools more quickly by being exposed in a
thin layer upon a large surface, which reduces the danger of spoiling,
and a strong evaporation of water and consequent concentration take
place. Besides, the cakes show the smooth surface of the polished
stone, and become in a short time so hard, that when placed upon the
nets, the twine will make no impression upon them.
Liquors which in gelatinizing do not become very solid, are not run
into forming boxes, but upon glass or zinc plates, and thus spread out
in a thin layer, acquire sufficient solidity to be removed cake by cake
after being cut. The plates upon which the glue is run are placed in
frames and laid upon a table furnished with a rim about 1 inch deep.
To accelerate gelatinization of the liquor, the table is flooded with
water before placing the plates upon it.
Where cooling-boxes are used, the jelly when completely congealed is
placed upon a table with a stone plate, by inverting the boxes, and
then cut into cakes. Figs. 15 and 16 represent the tools for cutting
the jelly into cakes. The block of glue is laid upon the surface _A_,
Fig. 15, and the frame, _B_, is gently drawn along in the grooves, _a_.
In the upright portion of the frame are fixed wires at such a distance
from each other as required for the thickness of the cakes to be cut.
[Illustration: FIG. 15.]
[Illustration: FIG. 16.]
When the block of glue has been cut in this direction, it is divided
by cuts perpendicular to the former, into cakes of a size in which the
finished product is to be brought into commerce. The apparatus shown in
Fig. 16 serves for this purpose. The vertical bars, _a_, furnished with
the wire, _b b_, serve as guides. The sheets thus formed are lifted
from the block with the moist blade of a large knife, and laid upon
nets.
The machine shown in Figs. 17 and 18 is the invention of Mr. J.
Schneible, and it is for slicing and spreading glue-jelly preparatory
to drying, and it consists in the combination of a reciprocating cutter
with the jelly-box and a traveling belt-carrying frame for receiving
the slices as cut by the knife.
Fig. 17 is a partly sectional side view of the machine, and Fig. 18 is
a cross-section of the same.
[Illustration: FIG. 17.]
_A A_ are side bars of the supporting frame, fitted at the ends with
cross-shafts, _a´_, carrying pulleys, _a a_, around which are endless
belts, _b b_. _c c_ are slide-ways upon the bars, _A_, and _d d_ are
slides carrying a cross-plate, _e_, and also a plate, _f_, to which
plate _e_ is attached a knife or cutter, _g_, the cutting edge of which
is at the edge of the plate, _f_, and about the same thickness as the
slices to be cut. The cross-shaft, _h_, is fitted in boxes on bars,
_A_, and near one end thereof it is provided with cranks at its ends,
which connect by rods, _i_, to the slides, _d_.
[Illustration: FIG. 18.]
From the opposite ends of the slides, rods, _k_, pass to loose arms,
_l_, on the shaft at the opposite end of the machine, and the arms,
_l_, carry pawls, _l´_, that engage ratchet-wheels, _m_, fixed on
the shaft, so that the shaft, _h_, being revolved, the slides, with
plates, _e f_, are reciprocated, and at the backward movement of the
cutter the pawls engage the ratchet-wheels, and belts, _b_, are moved a
distance equal to the movement of the knife.
The jelly-box, _n_, is fixed to side bars, _A_, by brackets at its
ends, as shown in Fig. 18, and is placed above the cutter and the
plate, _e_, so that when the plate, _f_, is drawn out from beneath the
box the plate, _e_, takes its place for holding up the block of jelly.
In operation the block of jelly is placed in box _n_, resting on plate
_e_. A frame provided with netting—such as is used for drying glue—is
placed on belts, _b_, beneath the box, and the shaft, _h_, being
rotated by power, the cutter moves forward and cuts a slice from the
jelly. The plate, _f_, at the same time moving away, the slice passes
upon the frame, and the return movement taking place, plate _f_ is
carried beneath the jelly-block, and the belts being at the same time
moved, the frame is carried forward in position for receiving the next
slice apart from the first one. In this manner, as slice after slice
is cut, they are spread on the frame, and the frames, when filled,
are carried to the end of the machine for removal. The plate, _f_, is
adjustable, so as to vary the thickness of the slices cut.
The box may be divided into cells of any size desired, so that each
movement of the knife will cut a slice from the bottom of each cell,
and the box extending the full width of the drying frames, all the
slices cut at once will be properly spread.
In order to keep the plates, _e f_, moist, so as to prevent the
glue-jelly from sticking thereto, there are fitted at the sides of the
jelly-box, _n_, open-bottomed boxes, _o_, containing fibrous material
soaked with water, which, resting on plates, _e f_, keeps their
surfaces moist.
The machine saves the troublesome and expensive work of spreading the
jelly by hand, as has been heretofore practiced.
The knife is to be attached to plate, _e_, in any suitable manner, and
the surface of plate, _f_, may be corrugated, so as to slide on the
jelly more readily.
[Illustration: FIG. 19.]
The cutting apparatus patented by M. Devoulx, of Marseilles, is much
used in France. The machine stands upon a board or table, upon which
are fastened two uprights, far enough apart to allow of the passage of
a truck carrying the glue, which is cut into cakes by blades or wires
stretched between the uprights.
[Illustration: FIG. 20.]
Fig. 19 shows the perspective elevation of the machine with its truck.
The upper part is filled up for the reception of the glue to be cut up
into cakes. The sides are omitted in this figure in order to admit of
a better explanation of the separate parts.
Fig. 20 gives the same view, except that the truck, the upper part of
which is closed, is between the uprights, and contains the glue to be
cut.
Fig. 21 represents the moment the wires have passed through the glue
and cut it into cakes. In all the figures, _a_ is the wooden frame upon
which the machine rests, _b_ the table-plate fastened to the frame,
_c_ and _d_ are the uprights, between which the cutting wires are
stretched, and _f_ the truck carrying the glue.
[Illustration: FIG. 21.]
[Illustration: FIG. 22.]
Figs. 22 and 23 show the truck by itself, _g_ representing the bottom,
and _h_ the back, which is provided with slight grooves, into which
the wires catch to assure the entire cutting through of the block of
jelly; _i_ is the upper part of the truck, which opens by means of a
hinge, and when closed is fastened with the pin, _k_. This upper part
of the truck is fastened to the back part of the truck by means of a
screw, which allows it to be set higher or lower, according to the size
of the block of jelly to be cut; _m_ is the bar of a rack fastened to
the truck, and serves for moving the latter. The driving gear, _n_, the
shaft of which carries a crank, _o_, catches into the rack.
[Illustration: FIG. 23.]
Two boards, one on each side of the truck, serve to keep the block of
jelly in position, and guide the truck.
With this machine 120,000 to 130,000 cakes can be cut in five or six
hours.
3. DRYING THE CAKES OF GLUE.
Drying the cakes is without doubt the most precarious part of the
manufacture. The jelly contains a large quantity of water which, to
prevent decomposition of the jelly before it is converted into glue,
must be evaporated as quickly as possible. In favorable weather, drying
may be accomplished either in the open air or in covered sheds.
Drying in the open air is connected with many inconveniences, for if
the sun strikes the cakes of jelly when they still contain a large
quantity of water, they may become soft so as to run through the
meshes of the net, or they may dry so quickly as to prevent them from
contracting to their proper size without numerous cracks and fissures.
If frost supervenes, numerous cracks may be formed in the cakes from
the congelation of their water, or a shower of rain may cause much work
and damage. In consideration of all these inconveniences, it is best to
conduct the operation in a drying-room.
To insure a constant circulation of air, which is absolutely necessary
for the expulsion of the aqueous vapor caused by the evaporation of
such a large quantity of water, the drying-room should be at least 10
feet high, even if intended for summer use only, and the windows be
provided with Venetian blinds so as to shut out the sun, if necessary,
without disturbing the circulation of air.
To dry the cakes in heated rooms in winter is a more difficult matter,
as provision has to be made for the removal of the aqueous vapor, and
a current of warm dry air has to be kept up at the same time. But such
a room is an absolute necessity for the manufacturer on a large scale,
who, in order to carry on his business without interruption throughout
the entire year, must be independent of the changes of wind and weather.
[Illustration: FIG. 24.]
The size of the drying-room should be proportional to the daily
production. Constructions are fitted up with the requisite frames for
the reception of the glue cakes, and are heated by steam pipes arranged
along the walls. In the floor in the immediate neighborhood of the
steam pipes are openings, which can be opened and shut at pleasure,
for the admission of fresh dry air. The latter on entering the room
is heated, and after passing over the frames and absorbing water from
the glue cakes, escapes through openings in the ceiling to a space
above it from which it is withdrawn by means of ventilators in the
roof. A constant change of air must be kept up. The quick drying of
the glue is of the utmost importance, as otherwise the jelly putrefies
either entirely or partially, and the glue acquires a turbid and mean
appearance. Too much heat causes the cakes to bend and crack. The
cakes are laid upon widemeshed nets of twine stretched in frames 6½
to 8 feet long and 3¼ feet wide. Fig. 24 represents the form of nets
commonly used. The nets are placed upon frames, such as shown in Fig.
25, arranged around the drying-room in the neighborhood of the steam
pipes and air flues. As the cakes have to be occasionally turned upside
down upon the nets, the latter must be placed at convenient distances,
one above the other in the frames.
[Illustration: FIG. 25.]
The use of twine netting has been found to be attended with many
disadvantages, the principal ones of which are given by S. Rideal as
follows:
1. “Being freely handled in the making, the netting is almost always
impregnated with dangerous organisms which penetrate the moist glue
cakes, and cause moulding or putrefaction. When this occurs, it is
usually attributed to a state of the atmosphere, but if the cakes are
examined, the alteration will generally be found to originate along
the lines made by the netting. The fault could be cured by sterilizing
the net for an hour at 212° to 248° F. in a hot oven, but besides the
expense, the fibre is thereby weakened. Moreover, the spores of a few
bacteria, such as _Bacillus subtilis_, which is widely distributed and
has the power of liquefying gelatine, will bear a heat of 248° F. for
over an hour, and still be capable of growing.
2. “However smooth the fibre, the glue will stick in places, leaving
small remains, which being hygroscopic, become ’sour,’ and set up the
objectionable bacterial changes in the subsequent batches.
3. “Owing to sagging, rotting, scouring, or wearing into holes, the
life of cotton or hemp netting is so short that the constant renewal is
a considerable item. A whole batch is frequently spoilt by the fault
of a net. In some works, heaps of old netting are found, which become
very putrid in the rain and sun, and give rise to mysterious bacterial
inroads in the factory. In others they are regularly burnt under the
boilers.
4. “The considerable overlap or selvedge required for securing the
edges of the net involves a waste of the area, and also some difficulty
in refixing.”
For this reason metallic netting has been largely adopted. The best
material has proved to be a heavily galvanized iron-wire netting having
no less than 15 to 25 per cent. of its weight of zinc. It can be
strengthened by longitudinal and transverse wires or ribs. It must be
examined by the microscope to see that it is perfectly free from holes
or cracks, and should last at least two years in constant use.
The temperature of the drying-room requires careful regulation, and
should never be allowed to rise above 68° to 77° F., as otherwise the
glue would soften and run through the meshes of the net, or adhere so
firmly to the twine as to require the nets to be put in hot water for
its separation. Dryness of air is of far greater importance in the
drying process than a high temperature. To promote this dryness of air
and prevent the aqueous vapor from condensing, evaporating, and again
condensing upon the cold walls of the room, they are wainscoted. Thus
protected by a bad conductor, they acquire a higher temperature, and
the aqueous vapor, instead of being precipitated upon them, is carried
off by the air-currents.
As the cakes placed in the immediate neighborhood of the steam pipes
and near the floor where the dry air enters, dry quickest, the nets
containing them are shifted after some time to a higher part of the
drying-room and their former places filled with cakes still wet. When
the cakes are dry, they are finally desiccated in a room at a higher
temperature, which serves to harden and improve them.
In modern times drying-rooms have been almost entirely abandoned and
in this country long drying galleries are used, sometimes 250 feet
in length and 6 to 8 feet square, with traveling platforms on rails
carrying the sheets of glue on stout galvanized netting. Wood is found
to be a better material for the galleries than stone or brick.
Figs. 26 to 28 show an apparatus for drying glue which is the invention
of W. A. Hoeveller.
Fig. 26 is a plan section, and Fig. 27 a side elevation in section, of
this improved drying-alley. Fig. 28 is an end view in section.
The form and arrangement are as follows:—
_A B_ represent the two parts of the alley, separated by the partition
_C_, which is shorter than the alley, so as to leave a communicating
space at both ends.
At the front of section _A_, is located a blower, _D_, actuated by a
steam-engine or other motor, _E_, also located within the walls of the
alley. The whole current from blower _D_, is directed through section
_A_ of the alley, whence it turns into section _B_, and comes back
through it, to be again drawn into and forced out of the blower into
section _A_. By this means the contained air of the alley is set in
continuous motion through the two sections successively, and as the
structure is made as air-tight as practicable in such cases, the air
remains unchanged until the doors _F_, or either of them, are opened to
discharge the vitiated air and let in the fresh.
In sections _A_ and _B_, is placed the railway _a a_, to admit of the
convenient movement of the contents in process of drying, which are
generally set on cars or buggies.
[Illustration: FIG. 26.]
[Illustration: FIG. 27.]
[Illustration: FIG. 28.]
In section _A_, in front of blower _D_, is placed a steam or other
heating device, _G_, which may be of any form or design adapted to
allow the air from blower _D_ to pass through it and to heat such air
while passing therethrough. The inventor prefers the radiating coil
for such purpose, the steam entering at _b_, and emerging at _c_. At
the other end of the alley, which by the double construction is in
section _B_, just back of the blower and heating-coil, there is placed
a condensing-coil, _H_, of a construction similar to coil _G_, and
having inlet _d_, and outlet _e_. Through this condenser there is kept
flowing a refrigerating liquid or brine, which renders the condenser
very cold. The continuous current of air from the blower passes over
the contents of the cars or trays in the alley and takes up moisture in
its passage. After such passage the air is charged with moisture and
comes in contact with the coils of the condenser _H_, upon which the
charge of moisture is condensed, and the air emerges dry again, enters
the blower, and is again made the vehicle by which the moisture of the
glue or other contents is transported to and deposited on the condenser.
In drying glue by this method do not use the steam-coil at the first
stage of drying a charge, as the drying should not be effected too
rapidly; but as soon as the product begins to stiffen properly, admit
the steam to the coil _G_, and thereafter the operation is continuous,
as above described.
By doubling up the alley into two sections, as shown, the inventor is
enabled to erect the alley in a more contracted space. In a length of
ninety feet he obtains the benefit of a single alley one hundred and
eighty feet long. Section _B_ may, if desired, be located on top of
section _A_. Doors may be located wherever desired, to facilitate the
movement of the trays or cars and the placing of them in and their
removal from the alley.
[Illustration: FIG. 29.]
By the above apparatus the drying can be perfected in a very much
shorter time than can be done by the old alleys, and operations can be
conducted in hot weather without hindrance from the condition of the
atmosphere.
In cases where the atmosphere is dry enough to dispense with the
heater and the condenser, the inventor can throw the doors _F_ wide
open, extend the partition _C_ out to that end of the alley, and then
preserve a continuous forced draft of sufficiently dry air in both
alleys for the purpose. As there are many days during the year fine
enough to give reasonably dry air, operations can be conducted with
the blower alone in this way, and thus economize the steam and the
refrigerating-brine.
[Illustration: FIG. 30.]
Figs. 29 and 30 show a longitudinal section, with upper and ground
floor plans of a modern drying-house, as given by Thomas Lambert. In
the ground floor the whole of the liquors are jellied in coolers,
and then cut into cakes by the two cutting machines in the centre.
Here a hoist, _E_, Fig. 30, is placed which carries the cut cakes on
“glasses” to the floor above. This forms the drying-floor, and is
partitioned off in three divisions, running nearly the length of the
building. The two outer divisions form the tunnels proper, and at the
ends are fixed two powerful revolving fans, driven at high speed and
drawing the air through the tunnels at a high rate. At the opposite
ends of the fans are fixed a series of 6-inch pipes, heated up by
waste steam, and the air passing between is warmed up to any desired
temperature, of necessity below 78° F. In the center passage a number
of girls are employed in transferring the cut cakes to the nets, which
are built upon a carriage running on a small railway. The carriage
with the complement of filled nets is run to the end of the division,
transferred to the lower railway, _C_, by which it is carried either
to the right or left hand tunnels as desired; the glue when dried on
the nets is run to the other end and by another lower railway brought
to the middle division, and ultimately by the hoist raised to a large
store-room, seen in section, where the glue is sorted and bagged. At
the end of the store-room is placed a grinding machine, and all the
off-color and twisted cakes are ground to a powder and sold as powdered
glue. The manufacturer by varying the size of the cake, its thickness,
and its color, may make any number of grades from the same boiling.
Fleck proposes to accelerate the drying of glue by utilizing the
water-absorbing power of some salts, such as Epsom and Glauber’s salts,
ammonium sulphate, crystallized acid sodium sulphate, etc., for the
purpose of withdrawing water from the glue cakes. For the practical
application of this principal a shallow, water-tight, wooden box is
required. The bottom of the box is sprinkled with a layer of the
water-absorbing salt about half an inch deep, and covered with a moist
linen cloth. Upon this is placed the jelly cut into sheets and also
covered with a moist cloth, a layer of salt being finally scattered
over it. After standing for a few hours, the box is slightly inclined
and the salt solution allowed to drain off through a hole in the
bottom, the dropping ceasing in from 12 to 18 hours. If now the upper
cloth is taken off with its layer of salt, the glue beneath it will be
found so far deprived of its moisture that when placed in the sun or
exposed to other heat, it will become completely dry in a short time
without either melting or spoiling, and in winter may be laid upon
drying-floors with the same result. The salt-solution formed may be
evaporated to crystallization and the salt thus obtained be again used.
After the treatment with the salt, the jelly contains 70 to 75 per
cent. of anhydrous glue, while the content in jelly not thus treated
varies between 7 and 28 per cent. according to the concentration of the
liquor from which it has been derived. It is claimed that the adhesive
power of the glue is not injured by this treatment.
Commercial glue must not only be thoroughly dry, but should also
present a good appearance, showing especially lustre. However, after
drying, the glue is dull, spotted, dusty and sometimes even mouldy. To
give a good lustrous appearance the dry cakes are dipped in warm water
and replaced upon the nets to dry.