MANUFACTURE OF GELATINE, AND PRODUCTS PREPARED FROM IT.
Gelatine, like glue, is produced from hides and skins, and bones. It is
distinguished by its purity, has a slight yellowish tint, and is very
hard and elastic. In cold water it softens, swells up, becomes opaque,
but does not dissolve. In hot water it dissolves completely, and on
cooling for several hours, an almost colorless, transparent and very
firm jelly results. This property of becoming jellied is in part lost
if the solution is for some time exposed to a temperature higher than
212° F.
The chemical constitution of gelatine is entirely changed by
concentrated sulphuric or nitric acid. Concentrated acetic acid, on the
other hand, renders softened gelatine transparent, and then dissolves
it; the solution does not become viscid, but preserves its adhesive
property. Dilute acids have no appreciable effect either on the
coagulating or the adhesive power.
Tannin is a valuable and delicate test of the presence of gelatine.
When added to a solution containing only 1/5000th part of gelatine,
nebulosity is immediately apparent. When more concentrated gelatinous
liquors are treated with tincture or infusion of nut gall, a dense,
white, caseous subsidence occurs which, on desiccation, becomes
brownish-yellow, agglutinates, and forms a hard, brittle mass easily
reduced to powder.
Gelatine is much used for culinary and medicinal purposes, and for
fining beer, wine and other liquids. Considered medicinally, it is
emollient and demulcent, and for this end is dissolved in water or
milk, and rendered palatable by the addition of acid and sugar. In
pharmacy, it is used for the formation of capsules intended to conceal
the nauseous odor and taste of medicinal preparations inclosed in them.
It is likewise employed for coating pills.
SKIN GELATINE.
But few changes have been made in the process of manufacturing skin
gelatine since the method introduced and patented, in 1839, by George
Nelson. This patent relates to the preparation of a transparent
gelatine from waste of calf skins, and of an inferior variety from
other skins freed from hair, wool, and fleshy and fatty matters. The
mode of procedure is the same in both cases, and is as follows: The
cuttings being washed are macerated in solution of caustic soda or
caustic potash at a temperature of 60° F., until they are partially
softened. Ten days is the average period required to effect this.
They are then placed in closed vessels and permitted to remain until
a thorough softening is effected. They are now washed in a revolving
cylinder, through which a current of water passes to free them from
adhering alkali; exposed in a well-closed chamber to the action of
sulphurous acid, and finally submitted to pressure to remove the
adhering water. The softened mass bleached by sulphurous acid is then
placed in a suitable vessel and subjected to the action of steam until
it is, as far as possible, dissolved. The liquor is then strained
and set aside at a temperature of 100° to 120° F. for the impurities
which may have remained to subside. The clarified solution is poured
upon slabs of slate or marble to the depth of about half an inch and
allowed to remain there till sufficiently solidified, when it is cut,
and washed to remove all traces of acid. It is subsequently redissolved
by means of a steam bath at a temperature of 95° F., finally again
solidified, and dried by exposure to dry air upon nets.
Messrs. J. and G. Cox, of Edinburgh, patented in 1844, a process by
which a perfectly pure substance, superior to that prepared from
isinglass is obtained. Shoulders and cheeks of ox-hides are preferred
by the patentees. They are thoroughly cleansed in water, after which
they are cut into pieces by a machine similar to that used for cutting
straw, and then subjected to the action of a paper-maker’s pulp-mill.
By this process the gelatinous fibre is well washed and cleansed, as
a stream of water flows through the mill during the whole operation,
carrying off all the impurities. The comminuted material is next
pressed between rolls, mixed with fresh water, sufficient to effect its
solution, and heated to a temperature varying from 150° to 212° F. The
resulting gelatine-solution is then allowed to cool to 150° F., and
mixed with fresh bullock’s blood—1 part of the latter to 700 parts of
solution. At a somewhat increased temperature the albumen of the blood
coagulates and rises in the form of foam to the surface, or subsides
in the shape of flakes, carrying with it the impurities, and thus
clarifying the liquor. The latter is allowed to stand for some time,
when it is poured upon stone slabs and allowed to solidify.
G. P. Swinborne’s improved patented process for the preparation of
gelatine from hides, skins and glue pieces, relates mainly to the
cleansing of the raw material. The latter is reduced by means of
suitable instruments to shavings or slices, and soaked in cold water,
which is drained off and replaced by fresh water three times a day,
until no odor or taste is perceptible. The shavings are then heated
with water, not above boiling, strained through filter cloths, and the
liquor is then run on to slate or other material to dry.
The modern process of preparing skin gelatine is, according to Thomas
Lambert, carried out as follows: The first treatment the cleansed skins
undergo is the “steeping” process with caustic soda or milk of lime.
In some factories a mixture of caustic (slaked) lime and soda ash is
used, in the proportion of 6 lbs. soda ash and 6 lbs. slaked lime to
every hundred-weight of skins treated, the chemical change being that
the whole of the carbonate of soda (soda ash) is converted into caustic
soda by its equivalent of caustic lime, the excess of the latter
remaining as such. The equation representing this is—
Na_{2}CO_{3} + CaH_{2}O_{2} = 2NaHO + CaCO_{3}
Soda ash. Caustic lime. Caustic soda. Carbonate of lime.
This steeping is conducted in large wooden vats, each having a length
of 12 feet, width 8 feet, and depth 3 feet, and fixed with a slight
gradient to the overflow, which is placed at the most convenient corner
of the vat and protected by perforated boards. The skins in the vat
are nearly covered with water, and then the solution of caustic soda,
or the slaked lime mixed in water to a cream, is sprayed equally over
the mass, the whole being intimately mixed with long stirring poles.
The water is renewed twice during a period of 12 days, the time allowed
for the skins to soak. They are now removed to a chamber, in which a
moderate increase of heat facilitates the saponification of the fat and
the dissolving of the fleshy matter. The chamber is a brick building,
with a cemented floor, on which the skins are spread to a uniform depth
of about six inches and is heated by steam pipes running round the
building. At a temperature of about 70° F. the skins are exposed two
or three days, with frequent turning. They are now transferred to the
washing machine and washed until the effluent is practically free of
soda or lime. The skins then undergo a bleaching process to whiten, and
thus destroy any injurious coloring matter. For this purpose the washed
skins are removed to a number of vats fitted with folding covers, and
treated to a solution of sulphurous acid of ½° Twaddell strength for 24
hours, each vat being well stirred at intervals to allow the “bleach”
to permeate equally the mass.
In some factories this process is carried out with dry sulphurous
acid, the gas being generated in a sulphur burner and, after washing,
passed into a chamber containing the skins. The vats are now drained,
then filled up with water, well stirred, and the water run off. This
is repeated until the effluent is practically free of any sulphurous
smell, and the work of dissolving the gelatine is proceeded with.
The digesting of the skins is carried on in stoutly built, circular
wooden vats, each having a size, generally of 4 feet 6 inches diameter
and 6 feet deep, and fitted with 2¼ inch copper steam coil. The vats
are provided with a double wooden bottom to divide the heat. They
are arranged on the first floor of the building, the liquors running
through a shallow filter of fine copper gauze to the clarifying vats
beneath. The skins are raised by an elevator and fed direct into the
vats by suitably-arranged troughs, then covered with water, and steam
is sent through the coils. The temperature is frequently taken by the
thermometer and should never exceed 177° F. During the digestion,
any unsaponified fat and dirt comes to the surface as a scum, and is
carefully skimmed off from time to time. Samples are also drawn of the
liquors, cooled, and the appearance and strength of the jelly noted.
After five to six hours’ heating, the first liquors are run off to the
clarifying vats, at a strength of about 17 per cent. gelatine.
The vats are refilled with water, and a second digestion made, the
liquor running to the clarifying vats at a strength of 12 per cent.
In the third digestion the temperature is raised a few degrees to
practically exhaust the gelatinous matter, and can either be clarified
to form an inferior gelatine, or concentrated for size. The exhausted
matter is taken to the manure-shed for mixing purposes. The clarifying
of the first and second liquors—and if used for gelatine the third
liquor—is done separately in the clarifying vats. The clearing agent
used is alum, to the extent of ½ per cent., or a little blood diluted
with water; both are mixed with a small portion of the hot liquors in
a bucket, and then well stirred in the vats. The liquors are raised
to 177° F. to coagulate the impurities, and then lowered to 149° F.
and allowed to stand for two hours. During this time the coagulated
matter rises to the surface, and is skimmed off. On leaving the vats,
the liquors are filtered through fine copper gauze into a receiver from
which is fed the vacuum pan.
Gelatinous liquors being peculiarly sensitive, especially as regards
color, to high temperatures, it is found advisable to evaporate them in
vacuo; and this method is adopted by all continental makers. The three
grades of the liquors, concentrated to the required strength, are run
on to squares of glass, 4´ × 4´, fixed in a wood framing, to a depth
of ½ inch for cake and ¼ inch for leaf gelatine, then placed perfectly
level on racks for jellying. In 24 hours the jelly is firm, and can
be easily cut to the desired size of cake or leaf wanted. A very fine
gelatine is produced by cutting the jelly into small pieces, washing
well with cold water, and remelting at a temperature of 176° F., and
poured again on the glasses for jellying.
The drying is carried on by exposing the cut cakes, on nets fixed to
a framework of wood, to a rapid current of dry air in the tunnels, as
described in the drying of glue.
BONE GELATINE.
The materials used for this purpose should be of the best description,
the most suitable bones being calves’ feet, waste of turners and button
makers, the bony cores of the horns of the ox and cow. Such bones do
not require comminution, but if large bones of oxen, horses, etc., are
to be used, it is recommended to break them as small as possible by
means of a wooden mallet, and to avoid the use of iron stampers, as the
bones become heated by the heavy blows and friction to which they are
subjected during the process, and acquire an empyreumatic odor which is
retained by the gelatine.
The next step in the process is the solution of the glue cartilage.
This was formerly effected by the use of steam and water. The crushed
bones were placed in a wire basket or cage and this inserted in a small
cast-iron cylinder and steam introduced. The apparatus is connected
with a steam boiler, and provided with an air-tight lid, and a pipe
and a rose connected with a water reservoir for pouring water over
the bones in order to promote the solution of the glue cartilage. But
this process is very slow, 20 hours being required without completely
exhausting the bones.
The resulting gelatinous liquor is drawn off every hour, the first run,
which contains the dirt and grease, being, of course, kept separate
from the rest.
[Illustration: FIG. 59.]
As will be readily understood this process consumes much fuel, and
leaves a residue which, though not completely exhausted, cannot be
further utilized for the preparation of gelatine. In fact the entire
process is obsolete, but as it is still in use in some localities,
a description is here given, for the sake of completeness, of the
apparatus and improved manner of manufacture employed in the factory of
D. J. Briers, which is well known for the beautiful product turned out.
Fig. 59 shows a longitudinal section of the entire apparatus.
Fig. 60 is the horizontal section of the boiler.
_a_, is the cylindrical boiler 6 meters (19.68 feet) long, and 2
meters (6.56 feet) in diameter. It is made of strong boiler plate
doubly riveted, and capable of resisting a pressure of six or seven
atmospheres.
_b_, is the manhole. It is closed by an oval lid secured by two iron
rods and two bolts, so that after placing the lid in position, the
boiler is hermetically closed by tightening the nuts upon the bolts.
_c_, is a cast-iron fork with two safety valves with levers graduated
from 1 to 100 atmospheric degrees.
[Illustration: FIG. 60.]
_d_, is a float upon the surface of the water, and provided with a
wheel graduated from Nos. 1 to 6. Its object is to indicate during the
operation how much water is lost and how much remains in the boiler.
Care must be had not to allow the indicator of the wheel to get below
No. 1. This figure indicates that the water has reached the highest
point in the boiler exposed to the fire, which is consequently the
lowest point which the water can be allowed to reach. On the other hand
the indicator must not move above No. 6, as the water when standing
too high in the boiler and too close to the pipes conducting the steam
into the various apparatuses might mix with the steam and spoil the
operation carried on in the drum, _e_, Fig. 60.
_f_, Fig 60, is a manometer, which indicates the degrees of pressure
exerted by the steam in the interior of the boiler. It consists of
a wrought-iron pipe bent double, and is filled with mercury 1.22
meter (4.002 feet) high counted from its base. One end of the pipe
communicates with the boiler, while the other end is provided with a
small brass wheel. Upon the latter is a thread of twisted silk, to
the end of which is suspended an iron cylinder of somewhat smaller
circumference than the bore of the pipe, so that it can move up and
down in it without friction. This cylinder rests always upon the
mercury. To the other end of the thread is fastened an indicator of
somewhat less weight than the cylinder, which, by sliding in a groove
in a graduated board placed alongside the pipe, indicates the degrees
of steam pressure.
_g_ is a cast-iron pipe for heating the drying room, and
_h_ another cast-iron pipe for heating the store-room for the bones.
_i_ is the forcing pump for feeding the boiler with water.
_k_ is a sheet-iron reservoir placed close to the end of the boiler.
It is filled with water, which is heated by the heat lost in the
fire-place in consequence of the draught, and by allowing the smoke
to circulate under the reservoir before passing into the chimney. The
reservoir communicates with the forcing-pump by means of a pipe and
stopcock, so as to avoid feeding the boiler with cold water.
_l_ is the fire-place, consisting of the grate, door and cast-iron
frame.
The drum, _e_, is a spherical vessel of strong sheet-iron doubly
riveted. It is 3 meters (9.84 feet) in diameter, and capable of
resisting a pressure of six to seven atmospheres. It serves for
softening the bones with the assistance of steam passed into it from
the boiler, _a_. It is provided with a manhole similar to that of the
boiler.
_n_ is a wrought-iron shaft passing horizontally through the drum and
revolving in the brasses, _o_.
_g_, Fig. 61, is a gearing with a crank, by means of which the drum _e_
is revolved. The power of the gearing must be so calculated that one
man can turn the wheels when the drum is filled with water.
[Illustration: FIG. 61.]
_r_ is a false bottom perforated in its entire length with holes 12
millimeters (0.47 inch) in diameter, and is placed about 15 centimeters
(5.9 inches) above the true bottom of the drum. It consists of two
pieces, and is secured by two nuts, so that it can be easily removed
and replaced. Its object is to prevent the bones from clogging up the
pipe _s_, and the cocks _t_, _u_.
_a_, _a_, Fig. 61, are angular iron points inside of the drum _e_.
Their object is to facilitate the shifting of the bones when the drum
is revolving.
_x_, Fig. 59, is a cock near the manhole. It is opened about 2
millimeters (0.079 inch) during the operation in the drum. It serves
also for the escape of the steam from the drum when the operation is
finished.
The cocks _t_, _u_, placed in the lower part of the drum, serve for the
escape of steam condensed during the operation.
The steam pipe _p_, Fig. 59, conducts the steam from the vessel _a_
into the drum _e_.
_y_, Fig. 59, is a cock graduated into eight equal parts and placed
on the steam pipe _p_, to conduct the steam from the box _z_, into the
stuffing box _a´_, and from there into the pipe _s_, then under the
false bottom _r_, into the drum _e_.
The lid of the cast-iron box _z_, is provided with a safety valve
loaded with a weight corresponding to the pressure of one atmosphere.
The wooden vessel or box _d_, the ground-plan of which is shown in Fig.
62, serves for boiling the comminuted bones in order to extract the
jelly. This box consists of the following parts:—
[Illustration: FIG. 62.]
_n_, are cast-iron steam pipes occupying the entire surface of the box,
being placed at equal distances from each other, and connected on their
ends by semicircular pieces. The steam, which is allowed to circulate
in the pipes in order to boil the liquid, enters through one of the
ends which rises up vertically and is connected with the cock _h´_,
Fig. 59. The other end is secured to the inner side of the box, which
is perforated for the admittance of the cock _o´_. Upon the steam pipes
lies a wooden framework with linen nailed upon it, the object of which
is to prevent the comminuted bone substance from falling under the
pipes. The frame must, of course, fit closely into the box.
The cock _h´_ graduated in eight equal parts serves to admit steam into
the pipes _n´_, and is opened either entirely or half, or one-quarter,
or one-eighth, according to the stronger or gentler ebullition to be
produced.
To prevent the steam from becoming stagnant in the steam pipes _n´_,
a small jet is allowed constantly to escape through the cock _o´_. The
latter serves also to run off the condensed steam when it no longer
possesses the heat required to keep up ebullition.
_p´_, Fig. 62, is a cock in the bottom of the box _d´_ for drawing off
the gelatinous solution from the residue. The box _e´_, Fig. 62, a
ground-plan of which is shown in Fig. 63, serves for evaporating the
gelatinous solution, which is effected by circulating steam through
several tubular pieces of cast-iron which form the bottom of the box
and are connected in a similar manner as the pipes in the box _d´_.
The cock _i_ is graduated and similar to _h´_.
[Illustration: FIG. 63.]
The cock _r´_, Fig. 63, is similar to _o´_, Figs. 59 and 62.
_s´_, Fig. 63, is a cock for drawing off the evaporated gelatinous
solution.
The wooden box _f´_, Fig. 59, a ground-plan of which is shown in Fig.
64, serves for the reception and settling of the evaporated gelatinous
solution. Its bottom is constructed in a manner similar to that of the
box _e´_.
[Illustration: FIG. 64.]
The cock _n´_, which is placed 14 millimeters (0.55 inch) above the
bottom of the box, serves for running the gelatinous solution into the
wooden cooling-boxes.
The bones as received in the factory are sorted by throwing out the
spongy material, etc. They are then steeped in lime-water for a few
days to free them from adhering particles of flesh, after which they
are dried and stored away for future use.
The boiler _a_, is filled two-thirds with water, and heated until the
manometer indicates a pressure of 30°. In the meanwhile the drum _e_,
is filled seven-eighths with perfectly dry bones, and steam is then
admitted from the boiler _a_, through the graduated cock _y_. The fact
that the bones in the drum are exposed to the proper temperature of
250° F. is recognized by the thermometer _b´_, placed between the cock
and the drum.
To prevent the stagnation of the steam in the drum, _a_ small jet of
it is allowed constantly during the operation to escape through the
cock _x_. The cock must not be opened wider than is necessary to keep
the temperature at 250° F. By opening it wider, this degree would be
exceeded, and the gelatine-yielding substance would, in consequence,
undergo alteration. A quarter of an hour after admitting the steam into
the drum, the cock _t_, is opened, and again closed after allowing a
small portion of the condensed steam to escape to the cock _u_, and,
through this, into a box. This operation is repeated every quarter of
an hour.
To change the position of the bones, the drum is revolved twice every
half hour, by means of the gearing _q_, of course closing the cock _x_
during the operation.
By carefully following the above rules, the bones will be thoroughly
reduced in four hours. If, for instance, steam has been introduced into
the drum at 5 o’clock a. m. the operation will be finished at 9 a. m.
The cock _y_, is then closed, and the steam allowed to escape through
the cock _x_. After the escape of the steam, the drum is emptied, by
removing the lid and turning it upside down. It is then refilled with
entirely dry bones, and the operation continued in a like manner, day
and night, if necessary.
After the bones have been taken from the drum, they are spread out
under a shed, and, when dry, ground in a suitable mill. The resulting
flour, which contains the jelly-yielding substance, is brought into
the vessel _d´_, which, in the meanwhile, has been furnished with
sufficient water to cover the flour 65 centimeters (25.59 inches) deep.
The mixture is boiled for three-quarters of an hour, being constantly
stirred to prevent the flour from forming a heavy and dense mass
which would hinder the quick extraction of the jelly. Ebullition is
then interrupted by closing the cock _h_, and the fat floating on the
surface skimmed off. After allowing the gelatinous solution to settle,
it is drawn off by means of a faucet placed above the level of the
flour. Thirty bucketfuls of the gelatinous solution are then at once
poured into a vat and mixed with the condensed steam drawn off by means
of the cocks _t u_, during the reduction of the bones in the drum.
After allowing the mixture to cool to 160° to 155° F., 20 kilogr. (44
lbs.) of pulverized alum are added at once and as quickly as possible.
When the gelatinous solution has become transparent, it is drawn off
into the box _e´_, and a few bucketfuls of hot water are poured upon
the sediment in the vat in order to extract the remaining jelly, which
is effected by thorough stirring and allowing to settle until the water
is entirely clear.
After disposing of the 30 bucketfuls in the manner mentioned,
the remainder of the gelatinous solution is evaporated. This is
accomplished in the box _e´_, which is filled 8 centimeters (3.15
inches) deep with gelatinous solution, and steam is then admitted
into the tubular bottom pieces. To promote evaporation and keep the
fluid constantly in motion, the cock _i´_ is only opened far enough
to keep up gentle ebullition. During evaporation the solution should
be frequently stirred with an implement resembling a rake. The nearer
the required degree of concentration is approached, the greater care
must be exercised to prevent the solution from boiling too strongly.
The proper degree of consistency is obtained when half a saucer full
of the solution placed in a shady place in the air acquires in a short
time such a consistency that when touched with the finger no impression
remains. The cock _i´_ is then closed, and the jelly is drawn off into
the box _d´_, which contains the 30 bucketfuls of clarified jelly,
care being had to mix the two solutions as quickly as possible. After
evaporating all the gelatinous solution and mixing it in the box _d´_,
the whole is heated to 158° F., by admitting steam through the cock
_k´_, care being had not to forget closing it as soon as the above
temperature has been reached. The solution is then thoroughly stirred
and permitted to settle for three hours to allow of the precipitation
of the lime salts decomposed by the alum. The fluid, which is now
perfectly transparent and of a beautiful dark-yellow color, is then
drawn off into wooden cooling boxes 2 to 2.5 meters (6.56 to 8.2 feet)
long, 20 centimeters (7.87 inches) wide, and 16 centimeters (6.30
inches) deep. The following day the gelatine is cut into leaves 25
centimeters (9.84 inches) long and 12 centimeters (4.72 inches) wide,
which are dried upon nets. When quite dry, the drying process is
finished by bringing the leaves into the drying-room which is heated by
the pipe _g_. (Fig. 59.)
The bone flour remaining in the box _d´_ still contains much jelly,
which is extracted by pressure. This is accomplished immediately after
running off the gelatinous solution into the evaporating vessel. The
liquid which has drained through the cloth frame previously mentioned,
is drawn off by opening the cock _p´_, Fig. 62, while the residue
in the box _d´_ is placed in coarse bags and the jelly extracted by
subjecting the bags to strong pressure under an iron screw-press.
Before mixing the extracted fluid with the solution in the evaporating
vessel, it is recommended to allow it to settle, as it is always more
or less turbid. The residue remaining in the bags is an excellent
manure.
The modern process of preparing bone gelatine, which is now almost in
general use is as follows: Clean bones are selected for the purpose,
and the operation commences with the extraction of the fat by means
of benzine, though some manufacturers prefer carbon disulphide as a
solvent, because it is claimed that by reason of its low boiling-point
it does not injure the gelatine in the same manner as benzine, and
besides it leaves no smell in the degreased bones. It is advisable
to bleach the degreased bones, spread out in thin layers and kept
constantly moist, by exposure to the action of air and light. The
bleached bones are conveyed to large vats for the purpose of extracting
their mineral constituents by digestion with hydrochloric acid. If
the gelatine is to be used as an article of food, or for medicinal
purposes, only the purest hydrochloric acid obtainable should be used,
while the ordinary article suffices for that for technical purposes.
The vats are filled three parts full with bones and the latter covered
with a solution of hydrochloric acid of 10 per cent. strength.
Digestion is carried on until the bones become soft, flexible and
semi-transparent. The acid water is now drained off, and a supply of
fresh water added, and then discharged. This is repeated until the last
water is entirely free of acid, which is known by adding a few drops of
silver nitrate, the absence of any white precipitate indicating that
the water is entirely free of acid.
The bones are now bleached in the manner as described under
skin-gelatine, preferably with solution of sulphurous acid, the
process with gaseous sulphurous acid being difficult to carry out and
considerable time is required for the gas to completely permeate the
cartilaginous mass. The bleached bones are then conveyed to the boiling
vats and the resulting liquors treated in the same manner as described
under skin-gelatine.
As bone-gelatine solidifies with a larger content of water than the
product from skins, evaporation of the liquors may be early interrupted
and the resulting jelly cut into thin leaves which are finally dried.
COLORED GELATINE.
Cakes or leaves not entirely colorless may be utilized for colored
gelatine which is employed for various purposes. Coloring is effected
by simply dissolving and distributing uniformly a suitable quantity
of coloring matter in the clarified gelatine liquor previous to
solidifying; of course only water-soluble coloring matter should be
used.
Colored gelatines are frequently used by confectioners and in the
household in the preparation of jellies, and the use of poisonous
colors should under all conditions be avoided. Sufficient attention is
not paid to this, because many of the aniline colors which are now so
much used for the purpose are at least open to suspicion, while others,
especially picric acid, which yields a beautiful yellow color, are
decidedly poisonous.
The following coloring matters are perfectly harmless and yield good
results in coloring gelatine:
_Yellow_: Caramel or sugar color. A still more beautiful yellow is
obtained by the use of an aqueous extract of saffron.
_Red_: Extract of cochineal.
_Blue_: Indigo-carmine solution.
_Green_: Mixture of indigo-carmine and caramel.
_Violet_: Mixture of extract of cochineal and indigo-carmine.
While gelatine colored with the above-mentioned coloring matter does
not present such a beautiful appearance as the product colored with
aniline colors, it is entirely harmless and suitable for culinary
purposes.
Leaves of gelatine colored with aniline colors exhibit beautiful colors
and are used for many technical purposes. The following colors may be
employed:
_Yellow_: Picric acid soluble in hot water.
_Red_: Fuchsine or eosine.
_Blue_: Water-soluble blue.
_Green_: Iodine green.
_Violet_: Methyl violet.
_Gelatine for Fining Purposes._
For fining beer, wine, etc., gelatine in leaves or in powder is brought
into commerce. For the preparation of leaves, gelatine particularly
well dried is carefully melted over a water-bath and then ladled into
sheet-metal moulds, and allowed slowly to solidify.
The product brought into commerce under the name of _Gelatine Lainée_
which commands a high price is in many cases nothing but thoroughly
purified bone-glue of a dark honey-yellow to brown color.
Fining powder for wine and beer is made by grinding off-color gelatine
cakes and freeing the resulting powder from coarser pieces by sifting.
The powder is white.
_Liquid fining gelatine._ This preparation consists of
gelatine-solution suitably prepared, and represents a colorless, or at
the utmost slightly opalescent, product just sufficiently concentrated
to remain liquid.
Skin-liquors can only be used for the preparation of liquid gelatine,
as bone-liquors already gelatinize when they contain scarcely more than
1 per cent. of glue. The skin liquors are concentrated so far as to
remain liquid at a temperature of from 60° to 68° F.
A product answering all demands is obtained as follows: A good quality
of gelatine of a pale color is dissolved in a sufficient quantity of
water, the solution, in case it shows a slight odor, filtered through
animal charcoal, and then brought into bottles. To prevent putrefaction
the liquid gelatine is sterilized as follows:
The filled bottles being placed in a boiler filled to a proper depth
with water, the latter is slowly brought to boiling which is kept up
for 15 to 20 minutes. The bottles are then closed with corks previously
boiled in the water.
_Preparation of Gelatine from Ordinary Glue._
For this purpose ordinary pale-colored glue is allowed to swell up
for two days in cold strong vinegar. The vinegar is then drained off
and the glue which is now almost colorless, is placed upon a sieve and
allowed to float in a vessel filled with water for 10 to 12 hours. It
is then placed upon a linen cloth, stretched in a room heated to at
the utmost 68° F., allowed to drain and dry to such an extent that
on heating it to between 158° and 167° F., a thick clear liquid is
obtained. This liquid is carefully poured upon plates of glass or
marble, so as to avoid the formation of air bubbles and when solidified
the leaves are removed and completely dried in the air. The product
thus obtained being entirely tasteless and almost colorless may be used
in the preparation of jellies, as well as for foils for pictures, etc.
_Preparation of Gelatine for Photographic Printing and for Photographic
Purposes in General._
Gelatine suitable for this purpose should be colorless and free from
all salts, since the latter would exert a disturbing effect upon the
chemical processes which take place during the photographic operation.
For the removal of the salts the gelatine is treated as follows:
Colorless gelatine of the best quality is broken into small pieces and
covered with 10 to 12 times the quantity of water, the latter being
changed every 15 to 20 minutes until the last water shows no trace
of lime, which is known by adding to the water solution of oxalate;
the absence of turbidity indicates that the water is free from lime.
The white of an egg is now mixed with five drops of ammonia and twice
the volume of distilled water and shaken to froth in a bottle. This
quantity suffices for 6 to 8 ozs. of gelatine. The washed gelatine is
next melted in a dish and mixed with the white of egg. One part of
glacial acetic acid mixed with 250 parts of water is then added drop
by drop, stirring constantly until sensitive litmus paper shows an
acid reaction. The liquid is now rapidly brought to the boiling-point,
whilst stirring constantly, and then filtered in a warm place to avoid
gelatinization. The gelatine now contains only the salts of the white
of egg, some ammonium acetate and free acetic acid. For their removal
the solidified gelatine is cut into pieces, which are placed in water.
_Gelatine Capsules for Medicinal Purposes._
Considerable use is made of gelatine in medicine. To disguise the
disagreeable taste of some medicines they are either mixed with
gelatine solution or inclosed in gelatine capsules. To prepare the
latter, dissolve 8 parts of gelatine, 2 of sugar, and 1 of gum-arabic
in 8 of water in a water-bath, and dip the pear-shaped ends of iron
rods into the lukewarm solution. To facilitate the detaching of the
gelatine film from the rods, grease the pear-shaped ends with oil. The
capsules are dried by placing them in holes of a corresponding size
in boards. When dry they are filled with the respective medicine, and
closed with a drop of the same solution.
_Court Plaster._
Gelatine or isinglass 155 grains, alcohol 13½ fluid drachms, glycerine
15½ grains. Water, tincture of benzoin, each a sufficient quantity.
Dissolve the gelatine in a sufficient quantity of hot water to make the
solution weigh 4½ ozs. Spread one-half of this in successive layers by
means of a brush upon taffeta stretched upon a frame so as to present
a level surface, waiting after each application until the layer is
dry. For the first two applications the gelatine solution should be
warmed merely to above its congealing point, so that when spread out
it may rapidly solidify and at the same time adhere to, but not pass
through the fabric. Mix the second half of the gelatine solution with
the alcohol and glycerine, and apply it in the same manner. Then
reverse the taffeta, coat it on the back with the tincture of benzoin
and allow it to become perfectly dry. The tincture of benzoin applied
to the reverse side leaves a thin layer of resin which in a measure
renders the plaster water-proof; it is, however, advisable to repeat
this application once or twice. The addition of glycerine to the last
portion of the gelatine solution prevents the plaster from breaking,
and preserves its flexibility for a long time. When the plaster is dry
it is cut in pieces of suitable length and preserved in well-closed
vessels.
_Gelatine Foils._
Large quantities of gelatine foils, which are leaves of gelatine about
as thick as a sheet of paper, are produced in England and France where
their manufacture forms a special branch of industry. They are either
simply colored or printed with neat designs in gold or silver.
The fabrication is quite simple. Cover pure gelatine with water, and
after swelling up, pour off the water and dissolve the jelly over a
water-bath. After allowing the solution to cool somewhat, add the
coloring matter previously dissolved in water.
In place of pure gelatine, a solution of ordinary bone-glue may be
used. In order to clarify it add O.14 oz. of oxalic acid dissolved in
water to every 5.5 lbs. of glue. To make the foils more flexible add
also one-half pint of spirit of wine and O.28 oz. of rock candy or a
small quantity of glycerine.
Aniline colors soluble in water are best adapted for coloring the
foils; for _red_, fuchsine, eosine or ponceau, for _blue_, _blue de
Parme_, for _green_, aldehyde green, for _yellow_, picric acid, and for
the various shades, mixtures of the above colors.
A durable blue is also produced by indigo solution, yellow, by
decoction of saffron, green, by mixing blue and yellow, red, by a
solution of carmine in spirit of sal ammoniac, and violet, by mixing
blue and carmine.
The gelatine solutions are poured upon ground-glass plates, previously
polished with elutriated rouge, and rubbed with Spanish chalk. The
foils are so smooth upon the glass side that when dry they can be
detached without much difficulty. If both sides are required to be
smooth, the foils are dried between two glass plates. In many respects
their manufacture resembles that of “Gelatine Veneers.”
Gelatine foils are used for printing sacred images, visiting cards,
labels, in the manufacture of fancy articles, artificial flowers, etc.
For the manufacture of artificial flowers very soft and flexible sheets
are made by adding ½ part of glycerine to 1 part of gelatine and mixing
intimately in dissolving the gelatine.
Such gelatine sheets brushed over in addition with Peruvian balsam can
also be advantageously used for tying up wounds instead of gutta-percha
cloth which tears easily and rots soon. They form an air-tight bandage
which clings closely to the parts of the body, and beside the glycerine
contained in them exerts a beneficial cooling effect and acts as an
antiseptic.
_Gelatine Veneers._
Franchi, as far back as 1814, prepared artificial ivory by mixing
gelatine solution with earthy substances. This idea has been again
taken up in modern times for the manufacture of veneers imitating
not only ivory, but also avanturin, lapis lazuli, malachite,
mother-of-pearl, and tortoise shell. These imitations are much liked by
manufacturers of fancy articles, workers in leather, cabinet-makers,
etc. They are prepared as follows:—
The process may be divided into five principal operations: 1.
Preparation of the glass and marble plates; 2. Preparation of the
glue solutions; 3. Pouring the colored solutions upon the plates; 4.
Transferring the layer of glue to the layer of gelatine; and 5. Drying
the veneers and detaching them from the plates.
1. _Preparation of the plates._ Both marble and glass plates are used
for imitations of marble, but glass plates only for imitations of
mother-of-pearl. The glass plates must be ground, but need not exceed
O.11 to O.15 in. in thickness, and only require careful washing and
drying for imitations of mother-of-pearl. For imitations of marble
they should be rubbed with an oiled linen rag. Other glass plates,
after being washed and polished with elutriated rouge and water, are
wiped with a soft rag to remove any particle of the polishing powder.
The polished surface is then gently rubbed with a rag dipped in pure
Spanish chalk, and the excess of chalk carefully dusted off.
2. _Preparation of the glue solutions._ For one dozen plates, each 10¾
square feet, soak 2 lbs. of good, colorless glue in water for 24 hours,
pour off the water and melt the glue in a water-bath and stir in 3½
ozs. of glycerine. For imitating marbles of two colors, compound 20 to
24 fluidounces of this glue solution with the quantities of thoroughly
ground mineral colors given below; the rest of the glue solution is
mixed with 6.34 ozs. of zinc white ground very fine. For imitating
marble of three colors mix 14 fluidounces of the glue solution with
one of the coloring matters and 14 fluidounces with the other coloring
matter, and the remainder with zinc white. For imitating marble with
four colors, take 10 fluidounces of the glue solution to each of the
three coloring matters, and mix the rest with 4½ ozs. of zinc white.
The proportions by weight of the mixtures for 10 different varieties of
imitations of marble and enamel are as follows:—
_a._ Mix 20 fluidounces of the glue solution with 1¾ ozs. of rouge and
2½ ozs. of zinc white, and the rest of the glue solution with 6⅓ ozs.
of zinc white.
_b._ Mix 20 fluidounces of the glue solution with 1¾ ozs. of rouge, and
the rest of the glue solution with 5¼ ozs. of zinc white.
_c._ Mix 14 fluidounces of the glue solution with 1¼ ozs. of zinc white
and 1 oz. of rouge, 14 fluidounces of the glue solution with 1 oz. of
yellow ochre, and the rest with 5¼ ozs. of zinc white.
_d._ Mix 14 fluidounces of the glue solution with 1 oz. of rouge, 14
fluidounces of the glue solution with ¾ oz. of sepia, and the rest with
5¼ ozs. of zinc white.
_e._ Compound 20⅓ fluidounces of the glue solution with 1 oz. of quite
concentrated and filtered solution of aniline black, and the rest with
6⅓ ozs. of zinc white.
_f._ Mix 10 ozs. of the glue solution with 0.8 oz. of rouge, 10
fluidounces of the glue solution with 0.8 oz. of yellow ochre, 10
fluidounces of the glue solution with 0.8 oz. of sepia, and the rest
with 4¼ ozs. of zinc white.
_g._ Mix 20.3 fluidounces of the glue solution with 1.41 ozs. of
lampblack. For gray add sufficient zinc white to produce the desired
shade. The rest of the glue solution is mixed with 6⅓ ozs. of zinc
white.
_h._ Mix 10 fluidounces of the glue solution with 0.8 oz. of umber, 10
fluidounces of the glue solution with 0.8 oz. of bole, 10 fluidounces
of the glue solution with 0.8 oz. of ochre, and the rest with 4½ ozs.
of zinc white.
_i._ _For enamels_ mix 20.3 fluidounces of the glue solution with 1 oz.
of ultramarine, and the rest with 6⅓ ozs. of zinc white.
_k._ Mix 20.3 fluidounces of the glue solution with 1.41 ozs. of chrome
green, and the rest with 6⅓ ozs. of zinc white.
For imitating mother-of-pearl veneers, 0.42 oz. of silver bronze, which
need not be genuine, is ground with a little glue solution or water and
intimately mixed with the above solution of glue. The bronze powder
must not be in a dry state when stirred into the glue, as lumps would
be formed and the veneers become spotted. In place of bronze, essence
of fish scales, which is of course far more costly, can be used.[2] The
glue solution thus prepared is then compounded with different aniline
colors according to the tint desired.
[2] This preparation is also known by the name _Essence d’Orient_. The
material for its production is furnished by a small white fish very
common in the rivers of continental Europe. It accompanies the scales
of this fish, and is detached when the scales are triturated for a
considerable time and thrown into a vessel of water. To collect the
essence the water is poured off upon a fine hair sieve, which retains
the scales and allows the water and the product sought to pass through.
The latter sinks to the bottom, and is obtained pure by decanting the
water. A little ammonia is added to prevent decomposition.
_a._ For yellowish veneers no coloring matter is required, or the
desired shade is obtained by an addition of some solution of picric
acid.
_b._ For colorless veneers or those of slightly reddish tints a smaller
or greater number of drops of a concentrated solution of fuchsine are
added in order to counteract the yellowish color of the glue solution.
For these imitations of mother-of-pearl veneers, concentrated solution
of gelatine compound with 15 per cent. of glycerine can be employed,
especially when essence of fish scales is used.
_c._ For _blue_, the glue solution is compounded with _bleu de Lyons_,
care being had not to use too much, as otherwise the imitation becomes
indistinct. The proper degree of coloring is tested by allowing a few
drops of the colored glue solution to fall upon a glass plate.
_d._ For _red_, solution of fuchsine or carmine is used, the latter
being obtained by dissolving commercial carmine powder in alcohol.
_e._ _Orange colors_ are produced by an addition of solution of
_chrysaniline_ generally sold under the name of Victoria orange, and
_violet_ by adding aniline violet. For these, as well as for the
solution colored with fuchsine, the plates must _not_ be rubbed with
oil, as even the smallest trace of the latter discolors these colors in
drying, or at least the veneers will show spots without color.
3. _Pouring the colored solutions of glue upon the plates._ For
imitations of marble and enamel, the glass plates, after rubbing with
oil, are placed, rubbed surface up, in a perfectly level position.
The proper portion of the white ground-mass, after becoming somewhat
thickish, is then poured upon the plates, and the gaps left free in
pouring filled in and smoothed with a knife-shaped tool of horn or
bone. Upon this white ground the respective colored glue solutions
are then poured in a zigzag form, and in conformity with the desired
design, drawn through the ground-mass with a glass rod. If several
differently colored glue solutions are to be applied, as given, for
instance, under 2_f_, they should be poured in quick succession, so
that the succeeding color runs into the preceding, and a white strip
or spot remains between each color. The whole is then intermingled
by the glass rod, according to the design. If the latter is to have
sharply defined lines and spots, the respective colored solution of
glue is used somewhat thicker, but if, on the other hand, the design is
to be somewhat blended, the glue solutions are used somewhat warmer,
and consequently more thinly fluid. After solidification of the glue
solutions the plates are placed in a cool room for two or three hours.
Imitations of malachite are prepared in a similar manner. Four glue
solutions of different shades of green from the darkest to the lightest
tint are prepared and poured upon a slightly greenish ground, so as
to imitate the characteristic curves and veins of malachite, which
are then further traced with a comb with teeth standing at unequal
distances from each other.
The glass plates set aside to be used for imitation of mother-of-pearl
are now taken in hand. The solutions of glue are kept warm over a water
bath and thoroughly stirred every time before pouring them upon the
plates. The formation of a film on the surface of the glue solution
must be strictly avoided.
For pouring out the solutions it is best to use a porcelain vessel
provided with a spout and handle, and having a capacity of about 6¾
fluidounces. The portion of glue solution required for each plate
(1¾ flu. oz.) is then measured into one of the porcelain vessels,
and, after standing a short time, poured upon the plate and uniformly
distributed. The production of a mother-of-pearl design requires some
skill and practice. A comb with teeth set ½ inch apart is used. It is
held in a somewhat oblique position, the teeth are gently pressed upon
the glass plate, and, with frequent turnings of the comb at a right
angle, cycloidal motions executed. The operation is carried on from the
front to the back edge of the glass plate, and when the glue begins to
thicken on the edges, continued at the softer places until the desired
design is produced, care being had not to touch places which have
already acquired a certain degree of solidity, as this would mar the
pattern. After treating all the plates in this manner, they are set
aside in a cool room for two or three hours.
4. _Transferring the layer of glue to a layer of gelatine._ For each
dozen of veneers soak 2½ ozs. of gelatine, and then melt them in a
water-bath, and after adding glycerine equal to 10 per cent. of the dry
gelatine, let the mixture settle.
The glass plates treated with rouge and Spanish chalk are now placed in
a perfectly level position, and after pouring 5½ fl. ozs. of gelatine
solution upon each of them, the gaps left in pouring are filled in
and smoothed with the glass rod. The front edge of a plate covered
with a colored layer of glue is now, glue side down, placed upon the
front edge of a gelatine plate, while the back edge of the former is
gradually lowered until the glue plate lies firmly upon the gelatine
plate.
It may here be remarked that the gelatine solution must only be allowed
to cool off sufficiently to prevent the melting of the glue plate on
touching it. If it is cooler the veneers will have blisters. It must
further be looked to that, before placing the first plate upon the
gelatine plate, no gelatine escapes, and that any excess only runs off
after the back edge of the glue plate touches that of the gelatine
plate.
The plates are now allowed to rest quietly until the gelatine is
congealed, when they are removed to a cool place where they remain five
or six hours.
Imitations of mother-of-pearl are treated in the same manner with the
exception that the gelatine solution is colored with the same coloring
matter as the glue solution. For colorless or yellowish veneers the
gelatine solution is not colored.
After six hours the first glass plate is detached from the layer of
glue by loosening the latter around the edges with a knife blade, and
the plate gradually lifted off commencing at one corner. With some care
and skill, this operation is readily accomplished without detaching the
gelatine mass.
5. _Drying and detaching the veneers._ The veneers with the gelatine
layer still adhering to the glass plate are now dried. This is done
in a heated room in which the veneers are arranged upon frames so
that they stand almost perpendicular. The hot air for heating the
room enters near the ceiling while the moist air is drawn away near
the floor. The temperature of the lower zone where the fresh plates
are placed should not exceed 68° F. The plates are moved up higher
every day until, on the third or fourth day, they have become entirely
dry. Before removing the veneers from the drying-room they should be
tested in regard to their dryness. They are sufficiently dry, when, on
pressing the finger nail upon the glue, no impression is made.
After removal from the drying-room the plates are allowed to cool off
for at least two hours before detaching the veneers. The operation
begins by detaching the gelatine layer on the edges with a very thin
knife blade. The operator then takes hold of one corner of the veneer
and draws it gradually and carefully from the glass plate. After
trimming the edges the veneers are ready for use.
If the veneers are required to resist the action of water, mix with the
solution of gelatine compounded with glycerine ⅓ fl. oz. of a solution
of 5 parts of chrome-alum in 100 of water to every plate, and immerse
the veneers for a short time after they have been detached from the
first plate, in a similar solution of chrome-alum.
Veneers prepared by these methods can be used for various purposes in
architecture and in the manufacture of furniture. Tennessee and other
marbles have been so closely imitated, that when used for table plates,
etc., the fact of their being imitations could only be detected by the
closest scrutiny. The veneers are also much used for fancy and inlaid
work, for coating columns, etc. To prevent their blistering and coming
off, it is recommended to add one-quarter of its weight of glycerine to
the glue with which they are to be attached to the articles.
_Formo-Gelatine._
This product is used as a dressing in surgery, and, according to
Samuel Rideal, is obtained as follows: Gelatine in aqueous solution is
precipitated by formaldehyde H.COH, or CH_{2}O, as a substance which on
drying is a white powder, neutral, inodorous, and insoluble in water
and dilute chemical agents. In commerce formaldehyde is met with in
aqueous solution as “formalin” containing 40 per cent. of formaldehyde.
If 1 Cc. of formalin be added to 200 Cc. of a 5 per cent. gelatine
solution, the latter is changed into a gelatinous mass, not melted by
heat nor reduced by water. If a smaller quantity of the formalin is
added (1 in 1000; the jelly is said to be meltable, but with a higher
tenacity) when dried it becomes insoluble in warm water. Formalin in
less proportion, though it does not prevent the dried gelatine from
dissolving in warm water, is said to improve the “body” of the jelly
and its keeping qualities, and also the tenacity of the glue. The
results of investigation show that with up to 1 per cent. of formalin
the solidity of the jelly increases; above that it declines. Up to
0.02 per cent. (1 in 5,000) it redissolves in water after drying.
Even with this small proportion the firmness of the jelly is sensibly
increased. The English patent, 4,696, 1894, claims the addition of
formalin during the manufacture of size and glue in such proportion
that the product shall liquefy in warm water.
In examining commercial sheet gelatines for printers’ and photographic
use, they were often found to contain small quantities of formalin.
It seems to improve the quality, a very small percentage increasing
the tenacity, flexibility and keeping qualities, while not affecting
the transparency or rendering the material acid. When applied to
articles that may be used as food, a strength of 1 in 50,000 in the
final prepared product is not injurious to health, but should not be
exceeded. (Rideal and Foulerton, _Public Health_, May, 1899, p. 568.)
Zimmermann applies dilute formaldehyde mixed with glycerine, vaseline,
oil or yolks of eggs, with or without flour, to the surface of
photographic films which are claimed to be thereby rendered more
pliable and not so hard as when the formalin is applied by itself.
It will be seen that formaldehyde in more than traces renders gelatine
insoluble; the product, moreover, is almost unaffected by water, is
more or less hard and elastic and, owing to the antiseptic properties
of the formaldehyde is nearly imperishable.
_Use of Gelatine in Bacteriology._
Gelatine fit for this purpose must be clear and bright, fairly neutral
and of high gelatinizing power. For bacteriological purposes a 10 to 20
per cent. solution of gelatine in sheets or strips is made with meat
broth, and this nutrient jelly, which is obtained by clarification with
albumen perfectly clear and bright, forms a most useful medium for the
cultivation of micro-organisms.
_Artificial Silk from Gelatine._
For the production of textile threads, Millar utilizes the property of
gelatine solution mixed with potassium dichromate, becoming insoluble
on exposure to light. For this purpose a clear solution of gelatine is
mixed with solution of potassium dichromate in the proportion of 100
parts of gelatine to 2 or 2½ parts of potassium dichromate. The fluid
should be of such a consistency as to allow of its being drawn out into
fine threads which on exposure to light become insoluble. Silk woven
from such threads is equal in appearance to the genuine article but of
course is not so strong. It is affected by moisture, becoming limp, but
regains its normal strength when dry.[3]
[3] For further information on this interesting subject, the reader is
referred to “Cellulose and Cellulose Products.” By Dr. Joseph Bersch.
Henry Carey Baird & Co., Philadelphia, 1904.