The further washing of the cartilage is effected by repeatedly pouring water over it, the operation being continued until the water running off shows no acid reaction. Washing has to be done very carefully, since glue-solution obtained from cartilage containing but a very small quantity of acid will not congeal. It is, therefore, advisable to add to the last wash-water 1 per cent. of soda, this quantity being quite sufficient for the neutralization of the last traces of acid.
Sulphurous acid process. In this country sulphurous acid is largely employed in the manufacture of glue derived from bones. When ordinary bones are treated with a current of moist sulphurous acid gas, they absorb from 10 to 12 per cent. of their weight of the gas in the course of 12 hours. The amount may increase to 15 or 20 per cent. on longer treatment, but the excess will then disappear on exposure to air. Messrs. Grillo and Schroeder of Düsseldorf, who patented this process in 1894, believe that this is simply due to the calcium phosphate present in the bones, and remark that an absorption of 11 to 12 per cent. on the gross weight amounts to 16 or 17 per cent. of the inorganic constituents, and corresponds to the equation:
Ca3(PO4)2 + SO2 + H2O = 2CaHPO4 + CaSO3,
the sulphurous acid simply acting in the same way as sulphuric acid does in the manufacture of superphosphate, but being a milder acid than sulphuric, the alteration of the organic constituents which are available for glue-stock can be almost entirely avoided. The acid phosphate is soluble in water, therefore the bones after treatment are easily disintegrated by boiling water when a large portion of the lime remains in the sediment, while the gelatine is dissolved.
The process as commercially conducted is very similar to the well-known sulphite method of treating paper pulp, and is carried on in iron cylinders or better in close wooden vats lined with lead.
Fig. 40.
The gas is usually generated in an impure form, with a large admixture of air and carbonic acid, by combustion of pyrites and coal, of crude sulphur, or even of only highly pyritous fuel.
On the other hand, since it is well established that the absorption of a diluted gas is less ready, and is more wasteful than that of a gas in a pure state, the employment of a definite quantity of sulphur dioxide in a concentrated state, either prepared by the regulated burning of sulphur, or the decomposition of sulphuric acid, yields more regular results, and a product of better quality. Liquid sulphur dioxide, which is now obtainable at a moderate price and in quantity, has the advantage that it yields a continuous current of pure gas of any required rapidity by simply opening a valve, and that the exact amount used can be ascertained by taring the containing vessel before and after the operation (S. Rideall).
The washed bones are brought into the above-mentioned cylinder or vat and treated with a saturated solution of sulphurous acid. The duration of the action of the acid varies according to the condition of the material and can only be determined by experience. The result of the process is a liquor almost as clear as water, which, after evaporation in the vacuum pan, is equal as regards clearness and lustre, to the best quality of glue prepared from waste of hide and skin. The fat extracted from the bleached bones is lighter in color and has not the disagreeable odor of ordinary bone fat, and consequently brings a better price.