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.
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.