Gelatine, like glue, is produced from hides and skins, and bones. It is distinguished by its purity, has a slight yellowish tint, and is very hard and elastic. In cold water it softens, swells up, becomes opaque, but does not dissolve. In hot water it dissolves completely, and on cooling for several hours, an almost colorless, transparent and very firm jelly results. This property of becoming jellied is in part lost if the solution is for some time exposed to a temperature higher than 212° F.
The chemical constitution of gelatine is entirely changed by concentrated sulphuric or nitric acid. Concentrated acetic acid, on the other hand, renders softened gelatine transparent, and then dissolves it; the solution does not become viscid, but preserves its adhesive property. Dilute acids have no appreciable effect either on the coagulating or the adhesive power.
Tannin is a valuable and delicate test of the presence of gelatine. When added to a solution containing only 1/5000th part of gelatine, nebulosity is immediately apparent. When more concentrated gelatinous liquors are treated with tincture or infusion of nut gall, a dense, white, caseous subsidence occurs which, on desiccation, becomes brownish-yellow, agglutinates, and forms a hard, brittle mass easily reduced to powder.
Gelatine is much used for culinary and medicinal purposes, and for fining beer, wine and other liquids. Considered medicinally, it is emollient and demulcent, and for this end is dissolved in water or milk, and rendered palatable by the addition of acid and sugar. In pharmacy, it is used for the formation of capsules intended to conceal the nauseous odor and taste of medicinal preparations inclosed in them. It is likewise employed for coating pills.
SKIN GELATINE.
But few changes have been made in the process of manufacturing skin gelatine since the method introduced and patented, in 1839, by George Nelson. This patent relates to the preparation of a transparent gelatine from waste of calf skins, and of an inferior variety from other skins freed from hair, wool, and fleshy and fatty matters. The mode of procedure is the same in both cases, and is as follows: The cuttings being washed are macerated in solution of caustic soda or caustic potash at a temperature of 60° F., until they are partially softened. Ten days is the average period required to effect this. They are then placed in closed vessels and permitted to remain until a thorough softening is effected. They are now washed in a revolving cylinder, through which a current of water passes to free them from adhering alkali; exposed in a well-closed chamber to the action of sulphurous acid, and finally submitted to pressure to remove the adhering water. The softened mass bleached by sulphurous acid is then placed in a suitable vessel and subjected to the action of steam until it is, as far as possible, dissolved. The liquor is then strained and set aside at a temperature of 100° to 120° F. for the impurities which may have remained to subside. The clarified solution is poured upon slabs of slate or marble to the depth of about half an inch and allowed to remain there till sufficiently solidified, when it is cut, and washed to remove all traces of acid. It is subsequently redissolved by means of a steam bath at a temperature of 95° F., finally again solidified, and dried by exposure to dry air upon nets.
Messrs. J. and G. Cox, of Edinburgh, patented in 1844, a process by which a perfectly pure substance, superior to that prepared from isinglass is obtained. Shoulders and cheeks of ox-hides are preferred by the patentees. They are thoroughly cleansed in water, after which they are cut into pieces by a machine similar to that used for cutting straw, and then subjected to the action of a paper-maker’s pulp-mill. By this process the gelatinous fibre is well washed and cleansed, as a stream of water flows through the mill during the whole operation, carrying off all the impurities. The comminuted material is next pressed between rolls, mixed with fresh water, sufficient to effect its solution, and heated to a temperature varying from 150° to 212° F. The resulting gelatine-solution is then allowed to cool to 150° F., and mixed with fresh bullock’s blood—1 part of the latter to 700 parts of solution. At a somewhat increased temperature the albumen of the blood coagulates and rises in the form of foam to the surface, or subsides in the shape of flakes, carrying with it the impurities, and thus clarifying the liquor. The latter is allowed to stand for some time, when it is poured upon stone slabs and allowed to solidify.
G. P. Swinborne’s improved patented process for the preparation of gelatine from hides, skins and glue pieces, relates mainly to the cleansing of the raw material. The latter is reduced by means of suitable instruments to shavings or slices, and soaked in cold water, which is drained off and replaced by fresh water three times a day, until no odor or taste is perceptible. The shavings are then heated with water, not above boiling, strained through filter cloths, and the liquor is then run on to slate or other material to dry.
The modern process of preparing skin gelatine is, according to Thomas Lambert, carried out as follows: The first treatment the cleansed skins undergo is the “steeping” process with caustic soda or milk of lime. In some factories a mixture of caustic (slaked) lime and soda ash is used, in the proportion of 6 lbs. soda ash and 6 lbs. slaked lime to every hundred-weight of skins treated, the chemical change being that the whole of the carbonate of soda (soda ash) is converted into caustic soda by its equivalent of caustic lime, the excess of the latter remaining as such. The equation representing this is—
| Na2CO3 Soda ash. | + | CaH2O2 Caustic lime. | = | 2NaHO Caustic soda. | + | CaCO3 Carbonate of lime. |