METHODS OF BLEACHING GLUE.
Many experiments have been made to bleach glue, _i. e._, to obtain
masses as colorless as possible, or at least slightly colored, the
resulting product being more valuable than the dark-colored one.
a. _Bleaching in the Air._
The principal requisite for obtaining a beautiful bleached glue is that
the unbleached product is clear, _i. e._, transparent, even if of a
dark color, this being the best criterion of well-made glue.
Glue may be bleached whilst being prepared, or the finished cakes may
be subjected to the bleaching process.
For the purpose of obtaining pale-colored glue from skin or cartilage
it is advisable to expose the materials in thin layers to the direct
action of the sun. Moist oxygen when acted upon by the sun is converted
into ozone, which exerts an extraordinary bleaching effect upon organic
substances.
b. _Bleaching with Chlorine._
The powerful bleaching effect of a solution of chlorine in water upon
organic matter is well known; the water is decomposed, and bleaching is
effected by the oxygen which is liberated. Hence skin and cartilage may
also be bleached by placing them in a vessel filled with weak solution
of chlorine in water and leaving them in contact with it until the
fluid shows no longer an odor of chlorine. When bleaching is finished
the materials are suspended in a certain quantity of hydrochloric acid,
which has finally to be removed by repeated treatment with water.
c. _Bleaching with Animal Charcoal._
Animal charcoal is distinguished by its great power of absorbing
coloring as well as odoriferous matter, and may also be used for
discoloring glue-liquors. This may be done by allowing the thin liquor,
as it comes from the glue-boiler, to run through a filter filled with
animal charcoal, or with the use of charcoal dust.
In the latter case the glue-liquor is collected in a clarifying vessel
and a quantity of charcoal dust amounting to about 3 to 4 per cent. of
the weight of the glue in the liquor stirred in. The finely divided
charcoal sinks slowly down, carrying with it the solid particles
suspended in the liquor, and collects on the bottom of the clarifying
vessel in the form of a black slime.
In order to discolor the glue-liquor as much as possible, when working
on a large scale, it is recommended to use a number of cylinders filled
with animal charcoal. These cylinders are connected one with the other
in such a manner that the glue-liquor runs into the first cylinder from
the top, passes out from the lower end of this cylinder through a pipe
into the second cylinder, traverses this from bottom to top, passes
into the third cylinder from the top, and so on. The animal charcoal
in the first cylinder loses its discoloring power first. The cylinder
is then disengaged, freshly charged and placed last in the series of
filters, the process being the same with the second cylinder, and so
on, so that after a certain time all the cylinders have alternately
occupied the first and last places in the series of filters.
By the use of animal charcoal it is even possible to render very dark
and badly-smelling glue, entirely colorless and free from odor. The
darker the glue is, the longer it must, of course, be subjected to the
action of the animal charcoal.
d. _Bleaching with Sulphurous Acid._
Bleaching of the glue-liquor itself by means of sulphurous acid is
best effected in the clarifying vat. For this purpose, the latter is
fitted with a lead pipe reaching to the bottom and terminating in a
perforated coil. Through this pipe gaseous sulphurous acid, generated
in a suitable sulphur-burner is forced through the liquor by means of a
force-pump.
The sulphurous acid is dissolved in the glue-liquor, the latter being
thereby bleached. When the liquor has acquired a much paler color and
a strong odor of sulphurous acid is perceptible in the air over the
vat, the introduction of gaseous sulphurous acid is interrupted and the
liquor allowed quietly to clarify, the acid dissolved in it exerting
during this time a further bleaching effect. By this means ordinary
brown joiners’ glue of good quality may be converted into a pale-yellow
product similar to the variety known as gilder’s glue.
[Illustration: FIG. 57.]
For bleaching finished glue, solution of sulphurous acid in water may
be used, the apparatus shown in Figs. 57 and 58 being suitable for the
purpose.
The apparatus for the production of the acid solution consists of the
sulphur-burner _O_, the wash-vessel for the gas _W_, and the vessel _T_
for dissolving the gas in water.
The sulphur-burner _O_ is a small brick vault of sufficient size to
hold a vessel _S_ having a capacity of a few quarts. In front the
sulphur-burner is provided with a well-fitting door _J_, which is
furnished with a small aperture for the introduction of an iron pipe
into the burner. The lead-pipe _R_ leads from _O_ to the bottom of the
wash-vessel _W_, and from the lid of the latter rises a pipe _R_{1}_,
and runs along the bottom of the trough filled with water. This trough
is provided with a wooden lid and the cock _H_, and by means of the
latter the fluid can be discharged into the vessel _G_.
[Illustration: FIG. 58.]
The trough _T_ is filled with water and _W_ is also filled
three-quarters full. In the burner _O_ is placed a dish _S_ filled with
sulphur, and the latter ignited. The door is then closed and air blown
in through _A_ by means of a bellows, the joints of the door _J_ being
at the same time luted with clay.
In contact with air, the sulphur burns to sulphur dioxide. The latter
is freed in the wash-vessel from sulphur vapor which has been carried
along, and passes from _R_{1}_ through the numerous perforations into
the water in _T_ where it is dissolved to sulphurous acid.
Saturation with sulphurous acid of the fluid in the trough is complete
when the suffocating odor of the acid is perceptible in the proximity
of _T_. The fluid is then discharged, replaced by water, which is again
saturated with sulphuric acid, and so on.
The cakes of glue to be bleached are placed in a trough (Fig. 58), in
which are arranged several frames, _B_, covered with linen. The cakes
of glue are placed upon these frames and the trough is filled with
sulphurous acid so that it stands a few inches deep over the uppermost
frame. The cakes of glue swell up rapidly in the solution of sulphurous
acid, and yielding up their salts become bleached. After twelve hours
the fluid is discharged through the cock, _H_, and if glue of a
particularly fine appearance is to be produced the cakes are treated
twice more with solution of sulphurous acid.
When bleaching is finished the trough is filled with clean water, in
which the glue is allowed to remain for a few hours, when the frames
are lifted out and the cakes dried.
By this method glue may be bleached to such an extent as to render it
fit as a substitute for gelatine for many purposes, for instance, for
the imitation of thin plates of ivory.
Dr. Bruno Terne’s apparatus (Fig. 40), previously described, may also
be used for the generation of sulphurous acid for bleaching purposes.