Glue is the cement used for joining different pieces of wood; it is a common jelly, made from the scraps that are pared off the hides of animals before they are subjected to the tan-pit for con-version into leather. The inferior kinds of glue are often contaminated with a considerable portion of the lime used for removing the hair from the skins, but the better sorts are transparent,
* By Mr. Thurston, of Catherin-street especially the thin cakes of the Salisbury glue, which are of a clear amber colour.
In preparing the glue for use, it is most usually broken into small pieces, and soaked for about twelve hours in as much water as will cover it; it is then melted in a glue-kettle, which is a double vessel or water bath, the inner one for the glue, the outer for the water, in order that the temperature applied may never exceed that of boiling water. The glue is allowed at first to simmer gently for one or two hours, and if needful it is thinned by the addition of hot water, until it runs from the brush in a fine stream; it should be kept free from dust and dirt by a cover, in which a notch is made for the brush. Sometimes the glue is covered with water, and boiled without being soaked.
Glue is considered to act in a two-fold manner, first by simple adhesion, and secondly by excluding the air, so as to bring into action the pressure of the atmosphere. The latter however alone, is an insufficient explanation, as the strength of a well-made glue joint is frequently greater than the known pressure of the atmosphere: indeed it often exceeds the strength of the solid wood, as the fracture does not at all times occur through the joint, and when it does, it almost invariably tears out some of the fibres of the wood: mahogany and deal are considered to hold the glue better than any other woods.
It is a great mistake to depend upon the quantity or thickness of the glue, as that joint holds the best in which the neighbouring pieces of wood are brought the most closely into contact; they should first be well wetted with the glue, and then pressed together in various ways to exclude as much of it as possible, as will be explained.
The works in turnery do not in general require much recourse to glue, as the parts are more usually connected by screws cut upon the edges of the materials themselves; but when glue is used by the turner the mode of proceeding is so completely similar to that practised in joinery works, that no separate instructions appear to be called for, especially as those parts in which glue is required, as for example in Tunbridge ware, partake somewhat of the nature of joinery work.
When glue is applied to the end grain of the wood, it is rapidly absorbed in the pores; it is therefore usual first to glue the end wood rather plentifully, and to allow it to soak in to fill the grain, and then to repeat the process until the usual quantity will remain upon the face of the work; but it never holds so well upon the endway as the lengthway of the fibres.
In glueing the edges of two boards together, they are first planed very straight, true, and square; they are then carefully examined as to accuracy, and marked, to show which way they are intended to be placed. The one piece is fixed upright in the chaps of the bench, the other is laid obliquely against it, and the glue-brash is then run along the angle formed between their edges, which are then placed in contact, and rubbed hard together lengthways, to force out as much of the glue as possible. When the joint begins to feel stiff under the hand, the two parts are brought into their intended position and left to dry; or as the bench cannot in general be spared so long, the work is cautiously removed from it, and rested in contact with a slip of wood placed against the wall, at a small inclination from the perpendicular. Two men are required in glueing the joints of long boards
In glueing a thin slip of wood on the edge of a board, as for a moulding, it is rubbed down very close and firm, and if it show any disposition to spring up at the ends, it is retained by placing thereon heavy weights, which should remain until the work is cold: but it is a better plan to glue on a wide piece, and then to saw off the part exceeding that which is required.
Many works require screw-clamps and other contrivances, to retain the respective parts in contact whilst the glue is drying; in others the fittings by which the pieces are attached together, supply the needful pressure. For instance, in glueing the dove-tails of a box, or a drawer, such as fig. 19, page 55, the dovetails if properly fitted, hold the sides together in the requisite manner, and the following is the order of proceeding.
The dove-tail pins, on the end B, fig. 19, are first sparingly glued, that piece is then fixed in the chaps of the bench, glue upwards, and the side A, held horizontally, is driven down upon B by blows of a hammer, which are given upon a waste piece of wood, smooth upon its lower face, and placed over, the dovetail pins, which should a little exceed the thickness of the wood, so that when their superfluous length is finally planed off, they may make a good clean joint. When the pins of the dove-tails come flush with the face, the driving block is placed beside them to allow the pins to rise above the surface. The second end, D, is then glued the same as B, it is also fixed in the bench, and A is driven down upon it as before; this unites the three sides of the square. The other pins on the ends B and D are then glued, and the first side, A, is placed downwards on the bench, upon two slips of wood placed close under the dove-tails, that it may stand solid, and the remaining side, D, is driven down upon them to complete the connexion of the four sides.
The box is then measured with a square, to ascertain if it have accidentally become rhomboidal, or out of square, which should be immediately corrected by pressure in the direction of the longer diagonal; lastly, the superfluous glue is scraped off whilst it is still soft with a chisel, and a sponge dipped in the hot water of the glue-kettle is occasionally used, to remove the last portion of glue from the work.