Holding power of glue. 1. Glue exerts a far greater hold on surfaces of wood cut across the grain than on those that have been split, or cut with the grain.
2. When two surfaces of split wood are laid together, the hold of the glue is the same whether the fibres are laid parallel or crosswise to each other.
3. The holding power of glue on different woods estimated in kilogrammes per square centimeter (0.155 square inch) is as follows:
| Cut across the grain. | Split. | |
| Beech, | 155.55 (342.21 lbs.) | 78.83 (173.42 lbs.) |
| Hornbeam, | 126.50 (278.30 lbs.) | 79.16 (174.15 lbs.) |
| Maple, | 87.66 (192.85 lbs.) | 63.00 (138.6 lbs.) |
| Oak, | 128.34 (282.34 lbs.) | 55.16 (121.35 lbs.) |
| Fir, | 110.50 (243.10 lbs.) | 24.16 ( 53.15 lbs.) |
Cologne glue. The variety of glue known under this name is prepared from selected scraps of hide and skin, and is consequently very pure, and possesses great adhesive power. It is of a light-brown color, and comes into commerce in short thick cakes of great hardness. It is an excellent quality of glue, and is preferred to all others by bookbinders, workers in leather, etc. There are many imitations of this variety, bone-glue being frequently sold as Cologne glue.
The genuine article is manufactured from refuse of hide, which, after liming, is carefully bleached in a bath of chloride of lime, the concentration of which depends on the darker or lighter color of the glue-stock. For 220 lbs. of glue-stock, it is generally customary to use 1 lb. of chloride of lime mixed with sufficient water to cover the stock.
After thorough impregnation of the glue-stock, which generally requires about half an hour, add sufficient hydrochloric acid to impart an acid taste to the bath of chloride of lime. To be able to mix the mass thoroughly, it is best to use a vat provided with a stirring apparatus. After allowing the acid to act for a quarter of an hour, remove every trace of it by careful washing.
To obtain a jelly as clear as possible, the gelatinous liquor is drawn off as soon as the thin portions of the glue-stock and the outside of the thicker ones are dissolved, they being more thoroughly bleached than the rest. The residue is worked into darker glue.
Russian glue. This variety is of a dirty white color, and, like Cologne glue, is brought into commerce in short, thick cakes. Its color and opaqueness are imparted to it by an addition of 4 to 8 per cent. of white lead, chalk, zinc white, or permanent white (sulphate of baryta). It has been claimed that the superior adhesive power of Russian glue is due to this addition of mineral substances, but the results of many experiments fail to substantiate this claim. In case the glue turns out turbid, it may be of advantage to make it opaque by an addition of coloring matter, but the quality of the glue remains unchanged. The best time to add the coloring matter is shortly before drawing the glue-liquor from the clarifying vats into the cooling boxes, as the jelly is then of sufficient consistency to prevent the substances from settling on the bottom. Skin-glue, as well as bone-glue, is sold under the name of Russian glue.
Quite a considerable quantity of Russian glue brought into commerce in the form of brownish-white sheets is prepared from bones, the latter being degreased by boiling, steaming or extraction, and the solution of the mineral constituents effected by means of hydrochloric acid. The treatment with acid is, however, continued only till the bones commence to become soft and flexible. The solution of phosphates is then drawn off, and the softened bones are washed and in the usual manner worked to glue.