Glue is made from refuse animal matter, and also from parts of fishes, the latter being known as fish glue. It comes in sheets or cakes or flakes, to be dissolved and used hot, or already prepared in liquid form.

The majority of practical mechanics prefer the former (i. e., "hot" glue) for nice work, although the use of liquid glue has increased much of late years. Hot glue is probably preferable if all the conditions are just as they should be, but if not so, liquid or "cold" glue may be better.

Buy the best grade. It is the cheapest for good work, and you will not use enough to make the price much of an obstacle. The only sure test by which to buy glue is to get a little and see how it holds. A good way to prove the quality of your glue is to soak it over night, or as long as may be necessary, in whatever quantity of water you think it will absorb. The more it will swell without dissolving, the better the quality. Poor glue will dissolve.

You cannot positively tell good glue by the colour, for there are many kinds (and for more than one purpose) and many makers, but whatever the colour, the glue should be clear looking and not cloudy or muddy. Do not use glue that has a mouldy or otherwise disagreeable or offensive smell or a bad taste.

To prepare hot glue, break the glue into small pieces, and soak it in all the cold water it will absorb for perhaps twelve hours, when it will have become swollen and softened and will look and feel somewhat like jelly. Then put it in the inner glue-pot (see Glue-pot) and cause the water in the outer vessel to boil for several hours. It is quicker to dissolve the glue at once without soaking, but the result does not seem to be quite as good as by the former method. The glue to be right for use must be thin enough to drip from the brush in a thread or stream, without collecting in drops like water, and you can tell something about its being in condition to use by testing it between your fingers. Do not weaken its strength, however, by diluting with more water than is necessary.

It is important to keep the glue and the glue-pot clean, and if the odour from your glue becomes offensive at any time clean out the glue-pot and make fresh.

Glue loses strength by repeated meltings, so do not dissolve too much at a time, and after heating it over two or three times throw away any that is left in the glue-pot, cleaning the latter thoroughly. On work which you are very particular about mix fresh glue each time.1

'Two pieces properly glued are often stronger than one solid piece - that is, the glued joint is stronger than the wood itself, as you will probably discover some day when you have occasion to break apart a piece of good gluing; but after a long time the glue is apt to deteriorate in adhesive or cohesive

You can make a good glue-brush of a stick of rattan. Soften the end in hot water and pound it with the hammer until the fibres separate. For corners, cracks, holes, and the like use sticks, which you can whittle to any required shape.

Although apparently too simple an operation to need much explanation, and often ignored in books on woodwork, as if anyone could of course glue two pieces together, the operation, to be really successful, calls for more knowledge of the principles involved than beginners or amateurs usually possess. Do not daub a thick layer of lukewarm glue on the pieces, and then slap them together as you would make a sandwich, after the usual domestic fashion. Done in this way the pieces often stick for a while, but there is nothing certain about it.

We have seen that wood is full of little holes (pores, as they are commonly called), or spaces between the fibres (see Fig. 6). The glue becomes worked into these little pores and that is what gives it such a firm hold on the wood, somewhat as plastering is forced (purposely) into the cracks between the laths.

So you must have the glue thin, that it may fill these little cavities and get a "grip" on the wood; you must have it hot, that it may the more easily penetrate these open spaces before it becomes chilled; you must have the wood warm, that the glue may not be chilled and begin to set before it has a chance to penetrate the interstices of the wood; and you must press the pieces together so hard as to expel the body of glue from between them, forcing it into the pores and squeezing outside what will not go in, to be wiped or scraped off afterward. For what you want is not to have the two pieces held together by a layer of glue between them, lightly sticking to each surface and separating force, particularly if the joint has not been protected by paint or varnish, so do the best work you can if you wish it to last. Nevertheless, in important work it is usually safest to take a whole piece when you can, rather than glue up two or more pieces, except in cases, perhaps, where the matter of warping, etc., is concerned, when it may be better to build up the desired shape of pieces selected for the purpose, the two in proportion to the thickness of the layer; but to have the two surfaces as close together as possible, held so by the tenacity of the glue reaching from the cavities of one surface to those of the other. The closer the surfaces are forced together the better, as the glue will be less exposed to the atmosphere.

You will see from all this that gluing should be done in a warm room of an even temperature.

While with hot glue it will not do to change the relative positions of the pieces after putting together, you can have considerable time to get them in position if the liquid or cold glue is used. Where several places in the same piece of work have to be glued together at the same time, it is frequently very hard to get around with the hot glue before that first applied has begun to set, unless you have help. In such cases, cold glue is a great convenience. If your shop is not warm or if you cannot have your glue hot, you had better use the liquid glue. It takes much longer to set than the other. In cold weather it should be slightly warmed. It can be thinned with vinegar or acetic acid, or what you wish to use at once may be thinned with water. Do not pour water into the can of glue, as it will not keep so well.

You will also readily see that it is much easier to make good glued joints in soft wood than in hard, for the former is more readily squeezed to a fit by the clamping, while with the latter it is quite essential that the pieces should fit with extreme accuracy before clamping (see note under Clamps).

Before beginning to glue have everything laid out, fit the pieces together, clamp them up just as if you had put on the glue, and see that everything comes together right - i.e., rehearse the gluing process before using the glue itself. This is a very important point, particularly when there are several pieces to be glued, for you will have no time to waste after you have begun to use the glue.

Do not spread the glue on too thick. Take the dirt off both pieces, then, while putting the glue on one, have the other warming slightly at the fire. The moment the glue on the brush leaves the glue-pot it begins to cool. If it fairly begins to set before you get the two pieces together, your joint will not be good. You will have to take it apart, scrape off all the old glue, and begin over again. So you will see there is no time to be lost when once you begin and it will be too late then to correct any mistakes in the fitting of the woodwork. Good workmen always put the work together and take it apart again before gluing.

Do not wipe off the glue which squeezes out from a glued joint (unless for some special reason) nor wash it off with water. Let it harden, and clean it off after the joint has set. It helps protect the joint.

Do not be in haste to unclamp your work. When to release it depends on the kind of wood, the kind of work, and the circumstances under which the gluing is done, and no exact time can be set. If for some temporary and unimportant purpose and in soft pine, for instance, you can unclamp in a few hours or even less, but for important work, which is to hold permanently, twelve hours is scarcely time enough even for soft wood and hot glue, and twenty-four hours is none too long, for though the glue dries quickly to the touch, it takes considerable time to get thoroughly hard. It is safer to allow more time for hard wood. The thickness of the stock makes a difference also. Large junks and blocks and boards glued flatways require more time that 1/4" stock, thin strips, or little splinters. You can tell something by the condition of the glue that is squeezed from the joint. Liquid glue sets much more slowly, and twenty-four hours is soon enough to release the work under average conditions. The warmth and dryness of the air make a good deal of difference. Under unfavourable conditions more than forty-eight hours may be required.

If for any reason you cannot clamp a joint, after applying the glue rub one piece back and forth upon the other a few times.

Rub wax, soap, or tallow on any part which must not be stuck by surplus glue which may exude from a joint, as in the case of a panel which may become stuck by the glue used in fastening the frame (see Doors and Panels).

To glue two pieces where the surface is to be planed or trimmed at the joint, do not glue them together after they are planed or trimmed, but glue them first, and plane or trim them afterwards, taking care to have the grain of the pieces run in the same direction (see Jointing).

To glue pieces end to end, or as in a mitre, - that is, " end wood,"-first size with thin glue to stop the pores, else the glue will be quickly soaked up. Then, after allowing this coat to stand, glue in the ordinary way. But glued joints in end wood are seldom good and are to be avoided.

A great deal of glued work comes apart, and a great many mistakes in putting work together are caused by not understanding, or not bearing in mind, the way wood expands and contracts and warps and winds from heat and cold, dryness and moisture. This is an important matter if you wish to do good gluing. Do not think that all that is necessary is to have your wood dry and that then you can glue the pieces together in any relative positions. Veneers or thin pieces are sometimes successfully glued with the grain of the pieces running at right angles, as seen in chair seats, but as a rule avoid gluing wide pieces together with the grain running at right angles. See Laying out the Work, in Chapter IV.; also Jointing.