Frequent Use - Selecting Glue - Preparation - Employment - Preservation - Liquid Glues - Brush.
In making up any piece of furniture, it is well known that glue is largely used, so largely, indeed, that the cabinet-maker cannot do without it, although there are theorists who object to it. Theory, however, is one thing and practice another, for the custom of using glue is so well established that no one interested in the manufacture of furniture could think of dispensing with it. As the use of glue is sometimes deprecated on the alleged ground of its instability, it may be said that glue, or, what is much the same thing, some kind of cement, has been used from time immemorial by woodworkers, except when it is obviously unsuitable, and in construction exposed to the weather. For ordinary furniture, glue is and may be used freely, though perhaps it may be well to say that it is not reliable in damp, tropical climates.
The cabinet-maker is strongly recommended to use only the best quality of glue, and not be tempted by low prices, for there is plenty of it made and sold which, even if it may be suitable for other purposes, is not fit for good and durable furniture. A very fair idea of the quality of glue may be got by noticing its appearance, and generally this will be a sufficient guide to the novice when purchasing. A very slight acquaintance will enable him to discriminate between good and bad, and the following hints will be of assistance in doing so. The principal feature, without going into more minute tests, which could only be appreciated by an expert, is the colour and transparency of the cake. It should be of a good clear brown or tawny tint when looked at by transmitted light. Inferior qualities are dark and muddy looking. Very light-coloured glues, as a rule, are not as strong as the clear brown, though they are occasionally useful for special purposes. For ordinary use, however, they are not so suitable, and the beginner may as well leave them alone. If he can, let him get the best English or Scotch glue, for there is nothing better, though some of French or German origin may look nicer. Foreign glues are not much in favour with English artisans, though the best qualities are infinitely superior to much of the black opaque stuff which is sold.
Not of less importance than the original quality of the glue is the way in which it is prepared and used, for a joint made with the best of it will be weak unless it is properly employed. This fact is often ignored by amateurs, and the contents of the ordinary domestic glue - pot are seldom satisfactory. Before proceeding further, it may be well to explain how to prepare the glue for use. As is well known, the recognised form of glue-pot consists of two vessels, the outer one containing water and the inner one the glue, which should never be heated in anything brought directly in contact with the fire. If it is, the glue is very apt to be burnt and rendered worthless. With water surrounding it, it is impossible to overheat the glue. Before, however, the glue can be melted by heat, it must be softened in water, and while doing this further notes of its quality can be taken. Sufficient should be broken up in small pieces and covered with cold water, in which it is allowed to stand for some hours till thoroughly soft. If the glue dissolves in the cold water it is poor. It should merely gelatinise and become soft without dissolving, and speaking roughly, the more water it will absorb the better its quality. Of course the water causes the glue to swell up, so that it should be well covered. When quite soft, the surplus water may be poured off and the glue be melted over the fire in the glue-pot, care being taken to keep plenty of water in the outer pot. When the glue is melted it is ready for use, and is at its greatest strength, for - and I particularly wish to impress this fact on the novice - the oftener glue is melted the weaker it becomes. This is the reason why household glue is so often defective and will not hold. The old glue is melted and remelted whenever it is wanted, till at last it has become worthless as an adhesive. It must not, however, be understood that glue should not be melted more than once, for if good originally it may be heated several times without apparent depreciation, though this is constant. For this reason, therefore, no more glue than can be used within a reasonable time should be prepared at once.
If the glue when hot seems too thick, as it very likely may be, it can easily be thinned by putting a little hot water to it. As that from the outer kettle is very handy, it should always be clean and not allowed to remain in impregnated with rust. The proper consistency of glue is of some importance, but it is impossible to give any definite directions about it. Perhaps the best rough test is to dip the brush into it and notice how it drops from it. If it does so in lumps or in a very thick stream, some more water is required; but if it runs like oil of medium consistency, it will be about right. It should be thin enough to be rubbed freely into the wood without being lumpy. A thick coating is not necessary, for all excess in a joint must be pressed out, as the object is to get the pieces of wood as close together as possible. There is no greater mistake than to suppose that the larger the quantity of glue in a joint the stronger it will be, for the reverse is the case.
Perhaps of equal importance to the freshness and consistency of glue is the necessity of using it as hot as possible and bringing the glued surfaces together while it is so, for the adhesion otherwise cannot be good. It will be noticed that as the glue cools, either in the pot or on the wood, it changes from a liquid to a stiff jelly. In this condition it may be kept for a considerable time if the air is excluded from it so that the moisture cannot evaporate. By taking advantage of this, the trouble of frequently waiting while the hard glue is soaking may be avoided by those who only use it occasionally or in small quantities. All that is necessary is to pour some glue while hot into a tin, from which sufficient for use can be cut out from time to time as required. This is the nearest approach which can with safety be made towards keeping glue in a usable condition, for the various methods which are sometimes mentioned for keeping it in a liquid state cannot be recommended; the best properties of glue are nearly always injured by such treatment. Leland, in his Manual of Wood Carving, says that if about a teaspoonful of nitric acid is added to half a pint of hot glue this will be improved as well as remain for a considerable time in a liquid state. I have not used this preparation, but name it, as the author who gives it is generally reliable, and the knowledge may be useful to some. Good glue prepared in the ordinary way, however, is satisfactory for all purposes of the cabinet-maker.
It should be mentioned that as the water evaporates from the glue in the pot it is necessary to add a little occasionally to supply the deficiency. In case some readers prefer to have a liquid glue which can be used at all times cold without any preparation, it may be said that Lepage's fish glues are very good, and have excellent adhesive qualities. They do not set or become hard so quickly as the ordinary kinds, so that they allow of a little more time being taken. As hot glue sets quickly, some more so than others, one that allows more time when gluing large surfaces is sometimes an advantage. I do not think even the fish glue is quite so strong as the ordinary kind when this is of the best, but it is at least equal to the average, and better than inferior qualities, and so far as I have seen is uniform. Unless bought in large quantities it is dearer than the hard kind, though as there is little waste with it the ultimate cost is perhaps not much greater.
In the absence of a proper glue-pot a very efficient substitute may be found in an empty jam-jar and small saucepan, anything indeed that will hold the glue and permit of its being melted in a water-bath.
It seems almost unnecessary to say that the glue is rubbed on the wood with a brush. Almost any kind will do, though by preference it should be moderately stiff, and of course in size proportioned to the work. A good useful brush may be made with a piece of cane. The hard, flinty skin is cut off for a short space at one end, and this is then hammered till the fibres are sufficiently loose.