It is best to start with a slow fire to give the stock time to soften and thus prepare it for solution. When somewhat softened, the mass is brought to boiling and the latter kept up, gently and uniformly, until solution is complete. Solution is promoted by careful stirring, but care should be had not to disarrange the straw upon the grid and on the sides of the boiler as this would interfere with proper filtration of the glue solution.
The duration of cooking depends on the nature of the raw materials. Scraps of skin from young animals, antlers, sheep trotters, etc., dissolve in three to four hours, while waste from ox and horse hides, or bones from old animals, require six to eight hours.
The progress of the operation is readily ascertained by pouring a small sample of the gelatinous fluid in half an eggshell, and setting it aside for a few minutes to cool. If a clear and consistent jelly be obtained, boiling has been carried on to a sufficient extent, and the liquid is drawn off. Any undissolved glue-stock remaining upon the straw filter can be boiled by itself, and the resulting gelatinous liquor utilized in the next boiling.
It is evident that quick and uniform solution of the materials, which enhances the quality of the glue, is promoted by comminuting the glue-stock either by grinding, stamping, or mechanical means.
The succeeding clarification of the glue is much facilitated by removing while boiling the scum, consisting of fat, coagulated albumen, lime-soap, accidental admixtures, and other impurities. Before drawing off the gelatinous liquor it is advisable to withdraw the fire and allow the contents of the boiler to rest for fifteen minutes.
The residue remaining upon the straw filter consists of hair, lime-soap, undissolved particles of hide and bones, lime, etc., and is utilized, after repeated boiling, as fertilizer or for the manufacture of gas.
The mode of glue boiling above described is the oldest and at present is only in use in small establishments. Fig. 11 represents a convenient apparatus for the purpose. It consists of three boilers upon as many different levels. The lower boiler, b, serves for the settling and clarification of the glue. It communicates with the second boiler, a, which contains the material to be acted on, by means of a pipe provided with a stopcock, and is sufficiently heated by a small fire to keep the glue liquid without allowing it to reach ebullition. The upper boiler, c, which is heated by the waste heat of the chimney, serves as an economical reservoir for hot water. The end of the discharge-pipe of the settling boiler is provided with a filter of woven wire. As the sides and bottom of the second boiler are lined with straw, which acts as a preliminary filter, the glue runs off quite clear from the settling boiler.
When this mode of manufacture is adopted, two boilings can be made per day, under favorable circumstances, so that, if the boiler has a capacity of 220 lbs. of stock, which will yield from 110 to 132 lbs. of dry glue, the daily fabrication will be about 220 lbs. of finished product.
Fig. 11.
