The osseous cartilage may be removed from bones, by suspending them in a vessel with weak nitric or hydrochloric acid (1 part acid to 9 parts water) at 10° (50° F.). The acid causes an effer vescence, by acting on the carbonate of lime, and dissolves out the whole of that and the other earthy constituents without affecting the cartilage, which, while retaining the form of the bone, soon becomes soft and translucent. It is then washed in tvvo or three changes of cold water, to retnove all traces ot' acidity. It shrinks and darkens on drying, becoming hard and strong, but somewhat brittle, and losing none of its transparency. It now forms gelatine. It has been proposed to obtain gelatine from bones, &c., by means of benzine and other hydro-carbons. Some times it is found advisable to treat the material with lime before adding the hydro-carbon, but only rarely. After the bones have remained for a considerable time under the influence of the hydro carbon, the fatty matters are dissolved, and the pure gelatine is found at the bottom of the vessel. The two products are thus easily separated, in order to be treated in the usual way, and the hydro carbons are recovered by evaporating with steam, and condensing.
The following is a German plan for preparing gelatine from bones. The bones are exposed to the sun and air for about six weeks, and, in dry weather, are moistened several times daily with water. Quantities of 10 to 15 cwt. are put into vats and soaked with a solution of hydro chloric acid at 4° (? Beaume), which is drawn off when saturated, and replaced by a fresh solution, repeating till the bones are softened. These are washed in fresh water and placed for fourteen days in a solution containing a small atnount of lime, then taken out and thoroughly rewashed in fresh water, and laid out upon large plates to dry in the air. The product at this stage is raw gelatine. About 300 lb. of this is laid in running water for twenty-four hours, which makes it soft and easily broken up ; it is then left for several days exposed to the open air, after which it is put into an immense kettle with 40 gallons of river water ; a fire is made, and it is slowly cooked, the mixture being stirred every half hour, and 4 oz. of alum added, which helps to liberate the fatty particles, and thu.s materially to purify the gelatine. After cooking for eight to ten hours, according to the state of the mass (Which may be tested by filtering some through a linen cloth, from which it should come clear and free from all impurities), the whole is put into a vat containing 3 gallons of fresh water acidulated with sulphuric acid. It is stirred, two quarts of acetic acid are added, and the mass is left to stand for one hour, when it is again filtered through linen cloth and put into wooden vessels, where the mass gradually attains a solid state. Before becoming thoroughly hard, it is cut by machine into thin sheets and laid out to dry in an airy and dry spot under an awning. The very best brand of gelatine is said to be made in this way. Should it be desired to produce coloured gelatine, the following modification is needed : On completion of the last filtration through linen cloth, a small quantity of gall is added, and then the required colouring matter. The
most common colour is carmine, dissolved in aqua amrnonice, and stirred into the mass. Aniline colours may also be used. The proportions are generally 1 oz. of colouring matter to 4 lb. of liquid gelatine, the former being first thoroughly cleaned by repeated straining through linen cloth, then added to the diluted gelatine, aud the whole well mixed while warm and poured out on large frames or sheets of glass placed in a cool, dry, airy place. The sheets are taken off when dry, or just before if they are to be stamped with patterns.
In the method of manufacture known as Rice's, the bones aro placed in dilute phosphoric acid, by which the earthy matters are dissolved and removed from the cartilage, which latter can be turned into gelatine by any ordinary process. The acid is recovered from the earthy matter for re-use in the following way :—About two-thirds or more of the solution of acid phosphate of lime is submitted to the action of sulphurous or sulphuric acid, which precipitates the lime as sulphite or sulphate, either being easily removed, and leaving the acid or acid phosphate (according to the amount of acid used) in an available condition for further use on fresh bones. By extracting the phosphates originally held in the bone, this process yields an actual surplus of phosphoric acid, so that it is claimed that almost 50 per cent. can be gained on each treatment. The residues are used for manure. The cost of proddtion is said to be greatly reduced by this plan. Phosphoric acid alone is found to be best ; but it may also be used in conjunction with other acids, in such proportions that the mixture will dissolve and remove the earthy matters.
Efforts have been made to obtain white gelatine from low products, and to whiten dark coloured gelatines, such as those got as a secondary product in the manufacture of neat's-foot oil, and which sell with difficulty. One proposed way of surmounting the difficulty consisted in digesting the raw material—hoofs, bones, &c.—in water or superheated steam at a pressure of three atmospheres. After three hours' digestion and half au hour allowed for settling, the strongly ammoniacal solution of gelatine is concentrated, the supernatant oil having been previously removed. A dark, brittle gelatine was thus obtained, which it was tried to bleach, without success, by means of sulphurous acid or a sulphate in presence of hydrochloric acid. The duration of the digestion was then diminished, and, instead of drawing off all the liquid at the end of three hours, it WEL8 drawn off three times, from hour to hour. The solution was then supplied with some wood oharcoal mixed with 25 per cent. of animal charcoal, and after standing for twelve hours, was treated as above. The product was a gelatine of good quality, appearing yellow only iu large masses, tasteless, scentless, and fit for all purposes. The solution requires 4 per cent. of the charcoal mixture.