The vats are refilled with water, and a second digestion made, the liquor running to the clarifying vats at a strength of 12 per cent. In the third digestion the temperature is raised a few degrees to practically exhaust the gelatinous matter, and can either be clarified to form an inferior gelatine, or concentrated for size. The exhausted matter is taken to the manure-shed for mixing purposes. The clarifying of the first and second liquors—and if used for gelatine the third liquor—is done separately in the clarifying vats. The clearing agent used is alum, to the extent of ½ per cent., or a little blood diluted with water; both are mixed with a small portion of the hot liquors in a bucket, and then well stirred in the vats. The liquors are raised to 177° F. to coagulate the impurities, and then lowered to 149° F. and allowed to stand for two hours. During this time the coagulated matter rises to the surface, and is skimmed off. On leaving the vats, the liquors are filtered through fine copper gauze into a receiver from which is fed the vacuum pan.
Gelatinous liquors being peculiarly sensitive, especially as regards color, to high temperatures, it is found advisable to evaporate them in vacuo; and this method is adopted by all continental makers. The three grades of the liquors, concentrated to the required strength, are run on to squares of glass, 4´ × 4´, fixed in a wood framing, to a depth of ½ inch for cake and ¼ inch for leaf gelatine, then placed perfectly level on racks for jellying. In 24 hours the jelly is firm, and can be easily cut to the desired size of cake or leaf wanted. A very fine gelatine is produced by cutting the jelly into small pieces, washing well with cold water, and remelting at a temperature of 176° F., and poured again on the glasses for jellying.
The drying is carried on by exposing the cut cakes, on nets fixed to a framework of wood, to a rapid current of dry air in the tunnels, as described in the drying of glue.
The materials used for this purpose should be of the best description, the most suitable bones being calves’ feet, waste of turners and button makers, the bony cores of the horns of the ox and cow. Such bones do not require comminution, but if large bones of oxen, horses, etc., are to be used, it is recommended to break them as small as possible by means of a wooden mallet, and to avoid the use of iron stampers, as the bones become heated by the heavy blows and friction to which they are subjected during the process, and acquire an empyreumatic odor which is retained by the gelatine.