The object is more easily effected by bleaching the raw materials previous to boiling them to glue.
This is accomplished by placing the glue-stock, thoroughly limed and while still moist, in a bath of chloride of lime, not too strong, as otherwise the solution of the materials becomes difficult. A bath of the proper concentration is made by dissolving about 9 ozs. of chloride of lime in sufficient water to cover 110 lbs. of glue-stock. After one hour add sufficient hydrochloric acid to obtain an acid reaction, which is recognized by litmus-paper dipped in the bath turning red.
Although the glue-stock is not bleached entirely through by this process, the thin portions and outsides of the thick material acquire a lighter color, and the first run of glue solution will have a light color and can then be treated further without much difficulty.
Sulphurous acid has been successfully used for the production of colorless glue without the necessity of boiling.
Waste of hides and skins is the only available material for this process. Place the waste in water until putrefaction sets in. When this is the case wash the material in a bag or wicker basket in running water. Then pour 2½ parts of sulphurous acid over 12 parts of wet material, mix the whole thoroughly and let it stand in a hermetically closed vessel for 24 hours. Now draw off the acid, and after washing the material thoroughly repeat the operation. When the vessel containing the mixture of material and sulphurous acid is opened for the second time the foul odor should be entirely superseded by that of sulphurous acid, this being a sure indication of the correct execution of the process. Wash the material, and, after squeezing, throw it into a vat large enough not to be filled by it more than two-thirds full. After filling the vat with water allow the mass to digest at a temperature of 109.4° F. for 24 hours. The result will be a gelatinous solution, which is drawn off and converted into glue. The undissolved residue is transformed into gelatinous solution by pouring water over it and allowing it to stand at a somewhat higher temperature.
For carrying out this process and that of bleaching with chloride of lime it is best to use a vat provided with a stirring apparatus, somewhat like a hollander used by pape-rmakers, as being most suitable for washing, disintegrating and mixing the material.
Glue-liquor may also be successfully bleached with sulphurous acid, and in speaking later on of the manufacture of bone glue, a very practical apparatus for this purpose will be described.
Glue-liquor bleached by sulphurous acid clarifies very readily and is protected from spoiling. The resulting glue remains, however, quite acid, and cannot be used for all purposes, especially not in combination with colors, chemicals, etc., upon which the acid has a destructive effect.
3. FORMING OR MOULDING THE GLUE.
After clarifying, the liquor is run into moulds of deal wood or sheet iron, lightly joined and of a rectangular form, slightly converging towards the bottom so as to allow the more ready detachment of their contents. They are about 3.25 feet long, 10 inches wide at the top, and 7¾ inches at the bottom, and 5 inches deep. When very regular cakes of glue are desired, cross grooves of the required shape are cut in the bottoms. After being well cleansed and ranged upon a level the boxes are filled to the brim through large funnels with strainer cloths affixed to their barrels. It is best to place them upon perfectly clean stone flagging slightly inclined towards a reservoir for the reception of such portions of their contents as may run over. The apartment in which the work is performed should be clean and airy, a dry cellar being the best for the purpose. In place of a large number of boxes, a shallow vessel lined with sheet-iron and capable of holding the entire quantity of liquor is sometimes used, from which the solid jelly is cut out in cubic masses, which are further divided.