Instead of using hydrochloric acid for the solution of mineral matter, sulphurous acid is sometimes employed, and has the advantages that its bleaching effect is thereby obtained throughout the process, and that it is recoverable for subsequent use. The Bergmann process, most generally favoured, is described very concisely by Rideal thus: "A sulphurous acid solution is made to circulate over the bones in a series of closed tanks, the solution being continually enriched with sulphurous acid from a cylinder of the liquefied gas. The resulting liquor, containing an acid calcium phosphate and calcium bisulphite, is heated by steam in a leaden digestor, when the excess of sulphurous acid is liberated and passes back to the tanks, while neutral calcium phosphate and sulphite are precipitated. The latter is decomposed by an equivalent of hydrochloric acid, setting free the remaining sulphurous acid, which is returned to the tanks, leaving calcium chloride in solution, and neutral calcium phosphate in suspension." Not more than 5 per cent. of sulphurous acid is said to be lost in this process, and the gelatine is more thoroughly bleached. It is subsequently well washed before extraction.

Recovery and Purification of Grease.—The degreasing operation, which is applied usually to bones (p. [224]) and to skin glue scutch, was once brought about by steaming only, but is now accomplished with the assistance of fat solvents, though in the latter case steaming together with mechanical centrifugal force has proved sufficiently successful. On the Continent carbon disulphide was once largely used as solvent, and in this country benzene has been employed, but their low volatility and high inflammability, as well as their expense, make both these substances somewhat unsuitable, and it is now usual to make use of petroleum oils, whether Scotch, American or Russian. A fraction which boils about the same temperature as water is usually employed, and all of it must be volatile under 280° F. Before the actual grease extraction the bones should be sorted over and unsuitable substances (horns, gravel, iron, etc.) removed. They are also usually put through a mill and roughly crushed or broken. The actual grease extraction plant consists of large copper vessels which will each take 5 tons of bones. These extractors are arranged in sets so that the degreasing is proceeding in some whilst the others are being emptied and recharged. The doors for charging and emptying must be securely fastened. When the extractor is charged the solvent is run in and heated by a steam coil which eventually causes it to distil. After some hours the remainder, which has dissolved much grease, is run off, and a fresh lot of solvent is added and heated up. After four such extractions only about ¼ per cent. of grease remains in the bones. To remove the remainder of the solvent high-pressure steam (80 lbs.) is blown through the bones. The extractor is then opened and the degreased and somewhat dried bones are mechanically conveyed to the cleansing mill. The grease solutions obtained are subjected again to steam with a view to removing the solvent and obtaining it for repeated use in this sense. The efficient distillation and recovery of the solvent is indeed an essential element in the success of the process.

The greases obtained, whether by the use of fat solvents or by skimming off during extraction, or in any other way, are mixed together as is appropriate to their origin and purity, and subjected to further purification, the object of which is to remove gelatinous and albuminous matters, and to decompose lime or soda soaps. The precise methods of purification are, of course, dependent mainly upon the impurities known to be present, but the readiest method is to give the grease further steaming or boiling with water, and so effect by washing and by solvent action the elimination of non-fatty matters. In many cases it is found advantageous to employ mineral acids or oxidizing agents to assist the process. The process may be repeated as often as is desired.

The recovered and purified greases are often of a high standard of purity, and the best are quite fit for edible purposes. The large extension of the margarine industry in this country has indeed caused a larger proportion than ever of this bye-product to be so used. In some cases it is found commercially advantageous to submit the grease to action of the filter press, and so to separate it into solid and liquid portions, the former containing a much larger proportion of stearin, and the latter of olein. Much of the grease from the gelatine trade is also found suitable for soap manufacture, and is therefore a valuable source of glycerine.

Other Raw Materials.—Whilst hide pieces and fleshings, and animal bones, comprise the principal raw material for the manufacture of gelatine and glue, there are also minor sources of raw material which, though often not suitable for gelatine manufacture, will yield a satisfactory glue. Thus the skins, bladders and bones of fish form the source of "fish glue." Sole skins, indeed, when deodorized by chlorine and decolorized by animal charcoal, are made into gelatine. The bladders of some fish (e.g. the sturgeon) are washed, purified and dried with rolling to make "isinglass," a form of natural gelatine in which the original fibrous structure is retained. There is a limited demand for this material for clarifying purposes by brewers, wine merchants and cooks.

Leather waste may sometimes be used to make a low-grade glue. Vegetable-tanned leather offers much difficulty unless very lightly and recently tanned. The tannage must be stripped by drumming with weak alkalies, e.g. borax, sodium sulphite, or weak soda. Chrome leather may be stripped easily and completely by Rochelle salt and other salts of hydroxy acids (Procter and Wilson), and also by ammonia acetate, oxalate and similar salts (Bennett), also by certain organic acids (Lamb). Processes are patented by which chrome leather is digested with lime to make glue, the chromium hydrate being insolubilized. Viscous and tenacious substances are also obtained from some vegetable matters and are called "glue."

REFERENCES.
"Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed.; Skin Gelatine and Glue, pp. 25-48; Bone Gelatine and Glue, pp. 59-66.
"Gelatine, Glue and their Allied Products," T. Lambert, pp. 11-52.
"Encyclopedie chimique," Fremy, tome x.

SECTION III.—EXTRACTION

The term "extraction" is applied to that essential process by which the gelatinous matter from whatever raw material is used, is actually dissolved in water and removed from the rest of the material. Extraction is often termed "boiling" or "cooking." Whether one is treating hide fleshings and pieces or whether one is dealing with raw or acidulated bones, the general principles of extraction are much the same, and most of this section is equally applicable to any class of material.