After disposing of the 30 bucketfuls in the manner mentioned, the remainder of the gelatinous solution is evaporated. This is accomplished in the box e´, which is filled 8 centimeters (3.15 inches) deep with gelatinous solution, and steam is then admitted into the tubular bottom pieces. To promote evaporation and keep the fluid constantly in motion, the cock i´ is only opened far enough to keep up gentle ebullition. During evaporation the solution should be frequently stirred with an implement resembling a rake. The nearer the required degree of concentration is approached, the greater care must be exercised to prevent the solution from boiling too strongly. The proper degree of consistency is obtained when half a saucer full of the solution placed in a shady place in the air acquires in a short time such a consistency that when touched with the finger no impression remains. The cock i´ is then closed, and the jelly is drawn off into the box d´, which contains the 30 bucketfuls of clarified jelly, care being had to mix the two solutions as quickly as possible. After evaporating all the gelatinous solution and mixing it in the box d´, the whole is heated to 158° F., by admitting steam through the cock k´, care being had not to forget closing it as soon as the above temperature has been reached. The solution is then thoroughly stirred and permitted to settle for three hours to allow of the precipitation of the lime salts decomposed by the alum. The fluid, which is now perfectly transparent and of a beautiful dark-yellow color, is then drawn off into wooden cooling boxes 2 to 2.5 meters (6.56 to 8.2 feet) long, 20 centimeters (7.87 inches) wide, and 16 centimeters (6.30 inches) deep. The following day the gelatine is cut into leaves 25 centimeters (9.84 inches) long and 12 centimeters (4.72 inches) wide, which are dried upon nets. When quite dry, the drying process is finished by bringing the leaves into the drying-room which is heated by the pipe g. (Fig. 59.)
The bone flour remaining in the box d´ still contains much jelly, which is extracted by pressure. This is accomplished immediately after running off the gelatinous solution into the evaporating vessel. The liquid which has drained through the cloth frame previously mentioned, is drawn off by opening the cock p´, Fig. 62, while the residue in the box d´ is placed in coarse bags and the jelly extracted by subjecting the bags to strong pressure under an iron screw-press. Before mixing the extracted fluid with the solution in the evaporating vessel, it is recommended to allow it to settle, as it is always more or less turbid. The residue remaining in the bags is an excellent manure.
The modern process of preparing bone gelatine, which is now almost in general use is as follows: Clean bones are selected for the purpose, and the operation commences with the extraction of the fat by means of benzine, though some manufacturers prefer carbon disulphide as a solvent, because it is claimed that by reason of its low boiling-point it does not injure the gelatine in the same manner as benzine, and besides it leaves no smell in the degreased bones. It is advisable to bleach the degreased bones, spread out in thin layers and kept constantly moist, by exposure to the action of air and light. The bleached bones are conveyed to large vats for the purpose of extracting their mineral constituents by digestion with hydrochloric acid. If the gelatine is to be used as an article of food, or for medicinal purposes, only the purest hydrochloric acid obtainable should be used, while the ordinary article suffices for that for technical purposes.
The vats are filled three parts full with bones and the latter covered with a solution of hydrochloric acid of 10 per cent. strength. Digestion is carried on until the bones become soft, flexible and semi-transparent. The acid water is now drained off, and a supply of fresh water added, and then discharged. This is repeated until the last water is entirely free of acid, which is known by adding a few drops of silver nitrate, the absence of any white precipitate indicating that the water is entirely free of acid.
The bones are now bleached in the manner as described under skin-gelatine, preferably with solution of sulphurous acid, the process with gaseous sulphurous acid being difficult to carry out and considerable time is required for the gas to completely permeate the cartilaginous mass. The bleached bones are then conveyed to the boiling vats and the resulting liquors treated in the same manner as described under skin-gelatine.
As bone-gelatine solidifies with a larger content of water than the product from skins, evaporation of the liquors may be early interrupted and the resulting jelly cut into thin leaves which are finally dried.
COLORED GELATINE.
Cakes or leaves not entirely colorless may be utilized for colored gelatine which is employed for various purposes. Coloring is effected by simply dissolving and distributing uniformly a suitable quantity of coloring matter in the clarified gelatine liquor previous to solidifying; of course only water-soluble coloring matter should be used.
Colored gelatines are frequently used by confectioners and in the household in the preparation of jellies, and the use of poisonous colors should under all conditions be avoided. Sufficient attention is not paid to this, because many of the aniline colors which are now so much used for the purpose are at least open to suspicion, while others, especially picric acid, which yields a beautiful yellow color, are decidedly poisonous.
The following coloring matters are perfectly harmless and yield good results in coloring gelatine: