David Rijckaert, Man Sleeping, ca. 1649.
It’s late, and you’re still awake. Allow us to help with Sleep Aid, a series devoted to curing insomnia with the dullest, most soporific texts available in the public domain. Tonight’s prescription: a chapter from Glue, Gelatine, Animal Charcoal, Phosphorous, Cements, Pastes and Mucilages, a 1905 book by F. Davidowsky.
Besides the broadly distinguished forms of skin- and bone-glue, the trade recognizes a large number of varieties, distinguished either by their value or their fitness for special purposes.
Joiner’s Glue.—This variety is without doubt the oldest in use and most in demand, and its principal requisite is its great adhesive power. It is used for joining wood, leather, paper, etc., and varies very much in quality and price.
The best variety is prepared from scraps of hide and skin. A light color not being especially demanded, there existing rather a prejudice in favor of a dark-colored article, waste of cattle and horse skins and tendons can be used for its manufacture.
Joiner’s glue, which is generally preferred in thin cakes, is chiefly manufactured in regular glue factories, though to be able to compete with the bone-glue turned out by the large establishments, the glue-boiler generally mixes skin and bone-glue, and is thus enabled to turn out a tolerably good quality. The price paid for the different varieties of joiner’s glue varies very much, being generally higher in winter than in summer, and is frequently more regulated by the external appearance of the article than by its actual value. Glue without gloss, very much warped and of a very dark color, may, notwithstanding its faulty appearance, possess excellent qualities.
Nothing need be said about the manufacture of joiner’s glue, since what has been said about the manufacture of glue in general suffices for the purpose.
How to make and use glue. Break the glue into small pieces, put it into an iron kettle, cover it with water, and allow it to soak twelve hours; after soaking boil until done. Then pour into a box which can be covered air-tight; leave the cover off until cold, then cover up tight. As glue is required, cut out a portion and melt in the usual way. Expose no more of the made glue to the atmosphere for any length of time than is necessary, as the atmosphere is very destructive to made glue.
All glue, as received from the factory, requires the addition of water before it will melt properly, and every addition of water (while the glue is fresh made) will, up to a certain point, increase its adhesiveness and elasticity. Some glues will bear more water than others, but all will bear more water than usually falls to their share, and that, too, with a greater improvement in the quality of the work. For glue to be properly effective, it requires to penetrate the pores of the wood, and the more a body of glue penetrates the wood the more substantial the joint will remain. Glues that take the longest to dry are to be preferred to those that dry quickly, the slow-drying glues being always the strongest, other things being equal. Never heat made glue in a pot that is subjected to the direct heat of the fire or a lamp. All such methods of heating glue cannot be condemned in terms too strong. Do not use thick glue for joints or veneering. In all cases work it well into the wood in a manner similar to what painters do with paint. Glue both surfaces of your work excepting in the case of veneering. Never glue upon hot wood, as it will absorb all the water in the glue too suddenly, and leave only a very little residue, with no adhesiveness in it whatever.
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:
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.
By this incomplete treatment with hydrochloric acid, a certain quantity of the phosphates remains in the cartilage and is inclosed in the glue prepared from it, the finished product acquiring thereby a dirty whitish color, which is by many considered an evidence of its quality. This mechanical admixture of phosphates, however, does not affect the adhesive power of the glue, neither increasing or decreasing it. Such white and opaque glue is manufactured to answer the demand in certain quarters of the trade, and, as above mentioned, heavy white substances are often intentionally incorporated with skin glue, as well as bone glue, to give it the appearance of Russian glue. These heavy powders add to the weight of the product, though when incorporated with it in small quantities do not injure its adhesive power, but large quantities render the product weaker.
Patent glue. This term is applied to an indefinite number of preparations, but particularly refers to a very pure variety of bone glue of a deep dark-brown color not showing net marks. It is very glossy, and swells up much in water. To satisfy the demand for thick cakes, they must be cut from very concentrated jelly to insure their drying.
Gilder’s glue is found in commerce in very thin, pale yellow cakes tied up in packages weighing about 2 lbs. each. It is a variety of skin glue bleached with chloride of lime, and dissolves with difficulty in water. The first runnings from the boiler are used for its manufacture.
A very superior article of gilder’s glue is obtained by cutting rabbit skins into fine shreds and boiling in water, then turning the mixture into a basket through which the liquid passes, leaving the refuse behind. About 100 grammes (3.52 ozs.) of sulphate of zinc and 20 grammes (0.705 oz.) of alum are then separately dissolved in pure boiling water and poured into the first-mentioned liquid, and the whole well stirred together while hot. The mixture is then passed through a sieve into a rectangular box, in which the jelly remains twenty-four hours in winter, or forty-eight in summer. The solid mass is taken from the box, cut into slices of proper thickness, and dried upon nets.
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