(1) Glue is, undoubtedly, the most important cement used in the arts. It serves to unite wood, paper, and almost all organic materials. The carpenter, the cabinet maker, the bookbinder, the hatter, and numerous other trades use it extensively, and in some cases to the exclusion of everything else. Good glue, properly prepared and well applied, will unite pieces of wood with a degree of strength which leaves nothing to be desired. The fibres of the hardest and toughest wood will tear asunder before the glued surfaces will separate, and certainly anything more than this would be unnecessary. Bevan found that when two cylinders of dry ash, each l 1/2 in. diameter, where glued together, and then torn asunder after a lapse of 24 hours, it required a force of 1260 lb. to separate them, and consequently the force of adhesion was equal to 715 lb. per sq. in. From a subsequent experiment on solid glue, he found that its cohesion is equal to 4000 lb. per sq. in. This would indicate that our methods of applying this substance as a cement are capable of improvement, and it is undoubtedly true that great care and skill must be used if the best results would be obtained.
Good glue is hard, clear (not necessarily light-coloured, however), and free from bad taste and smell. Glue which is easily dissolved in cold water is not strong. Good glue merely swells in cold water, and must be heated to the boiling-point before it will dissolve thoroughly. Good glue requires more water than that which is poor. The best glue, which is clear and red, will require from one-half to more than double the water that is required with poor glue. From careful experiments with dry glue immersed for 24 hours in water at 60° F. (15 1/2° C), and thereby transformed into a jelly, it was found that the finest ordinary glue, or that made from white bones, absorbs 12 times its weight of water in 24 hours ; the glue from dark bones, 9 times, while the ordinary glue made from animal refuse, absorbs but 3 to 5 times its weight of water.
The quality of glue may, to a certain extent, be estimated by breaking a piece. If good, it will break hard and tough, and when broken will be irregular on the broken edge. If poor, it will break comparatively easy, leaving a smooth straight edge.
Glue is insoluble in alcohol, though a small quantity of alcohol may be mixed with the solution without difficulty; but if too much alcohol be used, the glue separates from the water and falls to the bottom of the vessel in the form of a white viscid substance. Neither does it dissolve in ether, or in the fixed or the essential oils, although oily matters of all kinds may be incorporated with the solution of glue, forming a sort of emulsion. These facts will enable readers to judge of the value of those recipes in which they are directed to dissolve glue in alcohol or in oil, for the purpose of making a glue which will remain liquid at all times. A little alcohol may be added, but if the amount of alcohol be sufficient to produce any marked effect, the glue is apt to separate. One of the most marked characteristics of good glue is its property of gelatinizing. By this is meant the fact that a moderately strong solution of glue which is quite fluid when hot, forms a stiff jelly when cold. This property is no bad test of the quality of glue. The firmer the jelly, the better the glue.
In ignorance of this principle, some persons have made great efforts to get rid of this property, and acids and various salts have been added to the solution of glue for the purpose of preventing its gela-tinization, and thus retaining it in a liquid form that would be ready for use at any moment. But by those who have devoted the most careful attention to the subject, the fact stands unquestioned that the strongest glue is that which is purest and which gelatinizes or sets most completely.
Glue being an animal substance, it must be kept sweet, and free from putrefaction; to do this it is necessary to keep it cool after it is once dissolved, and while not in use.
The most serious defects in glue are the mixture of extraneous matters and incipient putrefaction. There are other substances, beside gelatine, present in the matters from which glue is prepared, and unless these substances are carefully separated, the glue will prove of inferior quality. Hence, in selecting glue, choose that which is transparent and free from clouds or flocks in its substance. Very clear and colourless glue is by no means the best; but, whatever be the colour, see that it is clear. It is true that in some cases very finely divided powders have been added to glue with the avowed object of rendering it stronger. Phin is inclined to believe, however, that such additions serve merely to cloak defects in the glue itself, or in the mode in which it is applied. Peter Cooper is said to add very finely divided Paris white to his glue, and it is claimed that the glue is improved not only in appearance but in actual strength. White lead added to glue is said to make it waterproof as well as to strengthen it, and from the well-known relation of white lead to oils and animal substances it is not impossible that this may be the case.
The other and by far the greatest and most common defect in glue is incipient putrefaction. This may occur either at the glue-factory or in the workshop of the mechanic; in either case it is fatal to the strength of the glue. It may often be detected, however, by the smell and taste. The odours of good glue and of that which has begun to " decay are so different that, once observed, they will never be forgotten. Glue which has begun to putrefy at the factory may not exhibit any odour so long as it is kept perfectly dry, and it may even have been so deodorized by bone black or the action of chemicals, that it does not exhibit a bad odour when first moistened. But when allowed to stand for a very short time, especially in warm weather, putrefaction again commences, and the odour is quite perceptible, while good glue will remain sweet and sound under the same conditions.