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In addition to hides, bones are a material highly valued by the glue boiler. Chemically speaking, the framework supporting the fleshy tissues of the animal order, and which we call bones, is a combination of phosphates of lime and magnesia, carbonate of lime, and alkaline salts, united with fatty and cartilaginous matter. To the latter we look for our yield of glue; to the fatty matter for the fat, and to the phosphates for the basis of fertilizers.

Bone cartilage is composed of carbon, hydrogen, oxygen and nitrogen, the percentage composition being practically constant, whether the cartilage be from an old or a young animal. The bones of the young are, however, much richer in cartilage than those of the old. This is reversed in the case of the inorganic or mineral matter, the old having the greatest yield of phosphates.

Then again, the fatty matters are more in evidence in full-grown animals than in youth or age; also in the thigh and leg bones the yield is higher than in the heads, ribs or shoulder blades, the latter averaging 12 to 13 per cent., whilst the former runs 18 to 19 per cent.[1]

[1] Bone Products and Manures. By Thomas Lambert. London, 1901.

Bones being less subject to putrefaction than skin-stock, they are not brought into commerce in a prepared state. They are mainly bought by contract from various dealers within easy access to the works. The rates are generally fixed for a certain period, and cover all classes of common bones, whether fresh butchers’ or a mixture with partly boiled bones. Bones differ considerably in their value. A fresh bone will yield the highest percentage of fat and glue. On the other hand, partly boiled bones may contain only 6 per cent. fat with 30 per cent. water. In buying bones the manufacturer should exercise great care, as the dealer sometimes finds ways and means of including hoofs, horns, iron, beefy matter, and even pieces of brick. Naturally they form weight, but, excepting the horns, have no value.

To separate the different classes of bone coming into the works, and arrange them according to the amounts they would produce of fat and glue, is no doubt a desirable object, but in practice it is seldom carried out. However, if the manufacturer wishes to undertake this tedious work, it is recommended to make the following distinctions:

1. Bones of young animals, sheep, calves, dogs, cats, etc., being readily disintegrated, are thrown into one pile, and also the light bones of oxen, such as skull bones, shoulder bones, the vertebra of the tail, etc.

2. A second pile is made of the foot bones of goats, sheep and cattle, provided they can be had, as is the case in the United States and England, in sufficiently large quantities.

3. Scraps and shavings from bucks’-horn from turners and button-makers.

4. Thick bones of oxen, horses, etc., which must remain longer in the lime-bath, together with waste of hard bones from turners.

5. Where large quantities of bones are handled it is advisable to sort out the bones of the upper thigh, as they can be more advantageously used for the manufacture of piano-keys, handles for tooth-brushes, etc. Hoofs, which are frequently found, should be thrown out, as they yield no glue and can be utilized for other purposes.

The further manipulation of the bones for the manufacture of glue requires first of all their crushing or grinding in a stamper or mill. By this crushing or grinding of the bones two objects are attained, namely, they are more readily deprived of their fat and present more points of attack to the corrosive agents to be used later on. The crushed bones are put in a large boiler, and for a few hours subjected to the action of steam. Leg bones, as well as horns, should not be boiled, as they contain no fat, and would lose too much glue-yielding substance. After boiling, the bones are placed in a lime vat for 8 to 14 days. The water used for boiling the first portion of bones may be used for a second one.

The extracted fat amounting to 4 or 5 per cent. of the quantity of bones used, is taken off the surface of the cold liquor and the latter may be utilized as a fertilizer, or fed to cattle.

For crushing the bones, a stamping mill is generally used, it yielding, when properly constructed, material for the manufacture of glue, as well as granulated bones which form an excellent product for the preparation of animal charcoal.

Since animal charcoal in pieces of quite even size is now in general demand, it is recommended to manipulate the bones in the above-described manner, to sell the granules to the manufacturer of animal charcoal, and use for boiling glue only the completely-crushed portions and the porous bones which are not at all suitable for the manufacture of animal charcoal.

Fig. 6.

Fig. 6 shows a stamping mill very suitable for the crushing of bones, the illustration showing the mill open on the left side and closed on the right. It is furnished with 16 stamps, D, each stamp being provided with a cast-iron shoe. The stamps are lifted by means of a cam shaft in such a manner, that the height of fall of the outermost pairs of stamps is least and that of those in the centre greatest. Between the inner stamps is a sieve H with meshes of sufficient size to allow the largest pieces, which can be produced by granulation, to fall through.

Underneath the sieve is an Archimedean screw K for carrying off the pieces of bone passing through the sieve.

Fig. 7.

Fig. 8.

The base of the stamping mill consists of iron plates so arranged as to form steps, the plates lying towards the center of the mill constituting the lowest steps. Every two stamps standing alongside one another rest upon such a step. When the mill is set in motion, the bones reaching the stamping trough from the right and left fall upon the steps, and are crushed by the descending stamps.

As a rule, the bones to be stamped are not brought directly into the stamping trough, but are first passed through a crushing mill and the coarser pieces thus obtained are subjected to the action of the stamping mill.

Figs. 7 and 8 show a well-constructed bone crusher. It consists essentially of two cast-iron rollers A and B, furnished with case-hardened cutters. The bones are introduced through the hopper B, and the rolls set in motion by means of cog-wheels a and b. The bearings of the roll B run in a carriage which can be shifted by the lever-construction f i. The object of this contrivance is to allow of the roll B giving way in case a harder material than bones, for instance, a stone, passes between the rolls.

Fig. 9.

The Crosskill bone mill, Fig. 9, as described by S. Rideal, is intended to be driven by a strap from the fly wheel of a common portable engine. It consists of a pair of strong rollers made of wrought-iron with case-hardened cutters, and a revolving or oscillating riddle for separation of the ground bones as they fall from the cutters; the whole carried by a substantial cast-iron frame. The mill will grind from 6 to 16 hundred-weight per hour with a three to eight horse-power engine.

For sorting the crushed bones into pieces of equal size, a sieve, Fig. 10, is used consisting of a drum constructed of narrow boards covered with wire-netting of different degrees of fineness. The upper portion A of the drum consists of narrow-meshed net and through this falls the fine meal which is conveyed by the Archimedean screw F over the frame F G H into vessels serving for its reception.

The lower section, B, of the drum is furnished with netting, the meshes of which become gradually wider towards the lower end, and, hence, the smallest particles of bone fall through the funnel, D, the medium-sized ones through E, and the largest ones through F. Pieces which cannot pass through F, leave the drum at G.

In factories manufacturing glue as well as animal charcoal, the larger pieces are steamed by themselves to obtain their fat, and then charred, while the small pieces and the meal are utilized for glue.

Fig. 10.

The lime-bath used for bones should be of the same strength as that for skin-stock. After removal from the lime vat and washing, the bones are put in a tank of stone or wood (brick pits should not be used) containing cold hydrochloric acid of 70° Bé. or 1.05 specific gravity (= 10.6 per cent. HCl) for thick bones, or half that strength for thin bones, and are thus left to digest for 8 to 14 days, being frequently stirred and fresh acid added. By the action of the acid the calcium phosphate is dissolved and the bones become cartilaginous, flexible and transparent. The phosphates can be precipitated by ammonia, or the whole evaporated with charcoal or silica, and distilled to make phosphorus.

When sufficiently softened, the stock is washed in wicker baskets or a washing drum to remove adhering acid. They are then placed for one day in the lime liquor, again washed, and then either dried or stored away for future use, or boiled at once to glue, while in a moist state.

Leg bones, horns, and other soft bones which contain scarcely any fatty matter are not steamed for the reason previously stated, but in all other respects are treated like steamed bones.

It is of the greatest importance that the bones should be thoroughly freed from acid, since even the smallest quantity remaining behind exerts an injurious effect upon the finished glue. It is therefore recommended to test the water draining off, or the bones themselves, with litmus. If the tincture turns red, it is a sure indication of the presence of free acid, and the washing must be continued until the blue color of the tincture remains constant.

Gerland’s suggestion, to use dilute sulphurous acid in place of hydrochloric acid for dissolving the phosphates of the bones, and to evaporate the sulphurous acid by heating, whereby the phosphates are precipitated in an insoluble state, has now been quite generally introduced.

For the preparation of gelatine from bones, Jullion and Pirie’s process may be recommended. It requires a somewhat expensive plant, but saves hydrochloric acid and time. The process consists essentially in dissolving the phosphates of the bones in vacuum. A box of wood, or better of granite, which can be closed air-tight, is required for this purpose. The box is filled with bones, and acid of the previously-mentioned strength poured over them. The box is then closed and the air pumped out by water or steam power. The smallest cracks and pores of the bones are thus freed from air, and the latter is replaced by hydrochloric acid, which in this manner acts rapidly and is completely exhausted. The remaining crude glue is then further worked in the usual way.

Bones honey-combed by putrefaction, exposure to the weather, or burial in the ground are of little or no value to the glue-boiler, as nearly all the glue-yielding substance has been destroyed; they should therefore be thrown out in buying stock. The ammonia which is formed when putrefaction sets in, colors the glue dark.