[CHAPTER V.]
MANUFACTURE OF BONE-GLUE.
The manufacture of bone-glue differs chiefly from that of skin-glue in the processes employed for the conversion of the glue-yielding tissues. This conversion may be effected by boiling the bones with water, or subjecting them to the action of steam, or by first extracting their mineral constituents with acid, and boiling the remaining cartilaginous mass with water until dissolved.
When the finest quality of all varieties of glue, namely, colorless gelatine, is to be manufactured, the bones should not be comminuted in a stamping mill, because in consequence of the unavoidable development of heat, they acquire a slightly empyreumatic odor which adheres to the gelatine prepared from them, and cannot be removed.
In factories working on a small scale, the bones are comminuted by hand, being placed upon a grate-like support of heavy iron rods and crushed with a wooden hammer, the face of which is studded with big-headed nails. In larger establishments the crushing rolls previously described are used, and in order to lessen the effect of heating as much as possible, the crushed bones are allowed to fall directly into a vessel filled with water.
Fat being a very valuable constituent of bones, it should be gained as completely as possible, by boiling or steaming the bones, or by extracting them by means of a solvent, such as benzine or carbon disulphide.
1. BOILING BONES.
This is the older and more incomplete process of extracting the fat. The bones are placed in a boiler, covered with water so that it stands a few inches deep over them, and the whole is boiled over an open fire. The melted fat collecting on the surface of the water is skimmed off. By boiling, a portion of the glue-yielding substance is, of course, converted into glue, and passes into the water. In order not to lose this glue, the same water is repeatedly used for boiling fresh quantities of bones, and is finally used for feeding pigs. By this method 4 to 5 per cent. of fat is at the utmost obtained.
The bone fat obtained by direct boiling of the bones, is, if entirely fresh material has not been used, of very inferior quality. It is dark yellow to deep brown and of a disagreeable odor. It is only fit for certain purposes, and to be utilized in the manufacture of soap has to undergo a special process of purification, whereby it is rendered white and odorless.
2. STEAMING BONES.
In order to obtain a larger quantity of fat than is possible by boiling, the bones are preferably steamed, i. e., subjected to the action of high-pressure steam. This is effected in a closed cylinder of thick boiler-plate, into which steam of ½ to 1 atmosphere pressure is admitted. The cylinder is provided with a perforated false bottom upon which the bones are placed. By steaming for two or three hours, all the fat is extracted from the bones and collects, together with the condensed water formed by the steam coming in contact with the cold bones, underneath the false bottom. However, by the continued action of high-pressure steam upon the bones, a considerable portion of the glue-yielding tissue is converted into glue, which passes into the resulting liquor. This, however, is no drawback if only fat and glue are to be obtained from the bones, since by continued steaming, a liquor still richer in glue results, and need only be evaporated. But, as a rule, the greater portion of the bones, especially the granulated parts, are to be utilized in the manufacture of animal charcoal, and, hence, great care has to be observed in steaming.