Different parts of articles are connected or jointed partly by glue, nails, or screws, and partly by the special adaption of the parts themselves, as in mortising and dove-tailing.
The simplest way of jointing two pieces of wood is to introduce between them a connecting medium in liquid form, i.e., glue.
Glue is made from the refuse, clippings, etc., of tanneries and glove manufactories. After being subjected to a boiling process, these materials are reduced to a viscous fluid, which solidifies on cooling into a stiffish jelly, which is then cut into thin slices and dried upon nets stretched on frames.
Good glue is known by its light brown or brownish yellow colour; its sparkling transparency; its hardness and elasticity ; by the way it breaks off in flakes and whitens in the line of fracture; and by its power of resistance to the dampness of the air. It swells if steeped in cold water, but does not melt even after one or two days' immersion. The ultimate test of good glue is, however, its cementing power.
1. The Preparation of Glue.
The cakes of glue, either entire or in pieces, are first soaked in cold water. After the glue swells it is put in a glue-pot (Fig. 86) and melted by heat. The glue-pot consists of two pans usually made of cast-iron or tin-plate. The larger of these, the outside pan, is, when in use, half filled with water, and the smaller one, the inside pan or glue-pot proper, in which the glue is placed, rests upon a rim or flange round its mouth. This inner pan should always be lined with tin. The water in the outer pan prevents the glue from burning, (an accident which must always be carefully avoided), and as the contents of the glue-pot are surrounded by warm water, they may be kept fluid and fit for use a considerable time after the pan has been removed from the fire.
If glue is wanted in a hurry, the cakes may be put in a towel or a similar piece of stuff to keep the glue from being scattered about, and broken to pieces with a hammer. The pieces are then put into the glue-pot and stirred during boiling, to prevent unmelted glue sticking to the bottom. This mode of preparation is quite as good as the preceding.
Fig. 86. Glue Pot Outside Pan. 1/6.
(inside pan) and Brush.
Glue is applied with a strong brush, of which there should be two sizes, one for large surfaces and one for small surfaces, e.g., mortise holes, etc.
Liquid Glue. - The addition of acetic acid to melted glue prevents putrefaction, and, without lessening its cementing power, keeps it liquid at ordinary temperatures. " Liquid glue " may be made as follows : - Four parts of good glue are melted in four parts diluted acetic acid, in the outer pan, or on the top of an oven. One part spirits of wine and a small quantity of alum are then added, and the mixture is kept in a wide-mouthed bottle, the cork of which has a hole to admit the brush.
This glue remains liquid at + 14° to 18° C, and does not solidify until + 8° to 12° C.; it is very convenient for small articles, as it is always ready and in good condition, and its cementing power is quite equal to that of glue prepared in the ordinary way. Its only drawback is that it dries more slowly.
In the case of articles exposed to moisture, the addition of 10 per cent, of boiled linseed oil is advantageous. The glue to which it is added should be hot and strong, and should be stirred till the varnish has been thoroughly mixed. The wood to which this wood-cement is applied should be dry and warm, and the pieces should be firmly pressed together until the glue dries.
The process of glueing is very simple, but it must be carefully performed to ensure a strong inconspicuous joint. The general rule holds good that the layer of glue shall be so thin that the seam can hardly be seen, and this presupposes that the pieces fit accurately (see page 146), that they are kept in sufficiently close contact while the glue is drying, and that the glue itself does not cool before they are put properly together.
To keep the glue from cooling, the wood should be warmed as well as the glue, and the operations of applying the latter, putting the pieces together and applying the required pressure, must be rapidly performed. Generally speaking, it is sufficient if one of the wooden surfaces is warmed : thus in dovetailing and slotting the pins only are warmed; in blocking, the blocks only, etc.
Warming the wood.
The glue, which must be neither too thick nor too thin, is laid evenly and quickly, in as small a quantity as possible, over the surface of the wood with the brush.
Laying on the glue.
In the case of pins for mortising, the glue should be thicker than for jointing boards, and the glue is generally applied to the hole as well as to the previously warmed pin, though sometimes only to the latter.
Screwing together is performed either in the bench, which is the simplest method, or in hand-screws, or in a press with wedges. The article must remain under pressure till the glue dries. If the glue is too thick or the wood cold, or if the glue cools before screwing up, the joint will show, and will not be good. A joint of this kind does not look well, and is less durable than one properly glued together.
The bench pegs or the hand-screw should always be in order before glueing, to save time. Just before the final tightening of the screw, the work should be carefully examined to see if the parts are in their right places. If not they must be made to fit. If the staves of a barrel are not in the same plane, the screw must not be loosened, but the stave which is not flush must be hammered into place, and the screws tightened. The work must not again be disturbed till the glue has hardened.
Making a joint.
In screwing up finished pieces of work, bits of wood must always be put between the work and the bench-pegs or the point of the screw, to prevent marks. When large plane surfaces are glued together, it is necessary to use several cramps to obtain strong enough pressure.
The glue which exudes from the joints of objects which are finished off before glueing, e.g., the inside of a drawer, must be carefully wiped off with a clean sponge or rag dipped in warm water immediately after glueing together, before it completely dries. Care must be taken not to wet the wood unnecessarily.
Removal of superfluous glue.
The better the glue penetrates the pores of the wood, the stronger the joint. Consequently, glue holds better in loose-fibred than in close-grained wood, which presents a hard, smooth surface. Broad surfaces of the latter description are roughened a little before glueing, by drawing a coarse file over them.* Glue which dries slowly is stronger than that which dries quickly.
A well-fitting joint made with good glue is so strong that, when long boards are joined together, the wood itself generally gives way before the joint. This, however, is not the case when end pieces are joined together, or when the wood is very hard or close-grained.
Strong and weak glue joints.
Two pieces of wood may be glued together without cramping or screwing together, e.g., a block of wood on a plank. The block only is warmed, but glue is laid upon both. The former is then pressed upon the latter, and rubbed backwards and forwards to get rid of the superfluous glue, until it begins to adhere. Care must now be taken that it is in its right place, and is not further disturbed. The two pieces adhere by atmospheric pressure.