Dr. Bruno Terne’s apparatus (Fig. 40), previously described, may also be used for the generation of sulphurous acid for bleaching purposes.
[CHAPTER VIII.]
DIFFERENT VARIETIES OF GLUE AND THEIR PREPARATION.
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.