Glue, gelatine, animal charcoal, phosphorous, cements, pastes and mucilages

by F. Dawidowsky

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.