Chapter XLVIII: Section II: Bye-Products of the Gelatine and Glue Trades

From the skin gelatine and glue trades the most valuable bye-product is the grease, which is obtained from the "fleshings" of the adipose tissue. These fleshings are themselves a bye-product of the leather trades. The recovery and purification of this grease has been dealt with previously (Part V., Section II.). In the skin glue trade the only other bye-product is the residue from the extraction process (Part V., Section III.). This residue is known usually as glue "scutch," and is composed of the proteins of the skin which are insoluble in hot water. These insoluble portions are obtained from all layers of the skin. There is much hair often in scutch, the hyaline or glassy layer (grain), and the elastic fibres of the corium are also insoluble, and a proportion is derived from the fibres of the adipose tissue on the flesh side. All these portions are fairly rich in nitrogen, and the scutch has, therefore, considerable value to makers of fertilizers. It is liable to contain also a percentage of grease, which is usually removed by steaming under hydraulic pressure. This process recovers a valuable bye-product and increases the manurial value of the scutch. There is always left in scutch some of the gelatinous skin substance which, strictly speaking, should have been removed during extraction. There is, however, a practical limit beyond which it does not pay to do this. When this limit is reached the cost of steam in extracting, and also in evaporating and drying, together with the loss of time and labour involved by occupation of the plant, is greater than the value of the possible product.

From the bone-glue industry, the grease is similarly a valuable bye-product, but there is also another of equal importance, viz. the phosphate of lime, which comprises about half the raw material. As previously described in Part IV., Section II., this is usually extracted after the grease, by solution in weak hydrochloric acid. The solution is neutralized in lead-lined vats with milk of lime, a precipitate of di- and tri-calcium phosphates being obtained. Calcium chloride is left in solution, and the precipitate should be, therefore, well washed if it be desired to have dry phosphate. The bone-glue industry is, generally speaking, much more intimately connected with the fertilizer trades than the skin-glue trades, indeed the extraction of the bones for glue is not always advisable, in which case the protein matter as well as the phosphatic matter of the bones are employed for making "bone manures." For details of this industry the reader is referred to a companion volume in this series on "Chemical Fertilizers."

REFERENCES.

"Chemical Fertilizers and Parasiticides," S.H. Collins, M.Sc.

"Bones," Part II., Section V., p. 72.

"Precipitated Bone Phosphate," Part III., Section III., p. 157.

"Bone Manures," Part III., Section V., p. 173.

"Gelatine, Glue, and Allied Products," T. Lambert.