As the use of glue enters largely into the construction of all patterns, some instruction as to its selection and the manner of using is necessary. When building up patterns, the connections should in all cases be made by gluing.
Nails should never be used except when they can be so placed as to be entirely removed from all danger of contact with the tools used in turning and shaping the pattern, and when so employed should be used in conjunction with glue. The only advantage in their use is the hastening of the work, because they take the place of hand screws or clamps while the glue is drying. The use of nails, however, is always unsatisfactory, for when the point is passing through the upper piece, small thin slivers are broken from the under surface, which have a tendency to separate the two surfaces instead of exerting the required pressure as when hand screws are used.
For pattern work select only the very best quality of cabinetmakers' glue, or better still, the best quality of white glue. This white glue can always be had in two forms: (1) clear; and (2) opaque. The first is the glue without the addition of any foreign substance. The second looks much whiter than the first, because of the addition of whiting, or other mineral, to the glue. This addition does not in any way lessen the adhesive qualities of the glue; on the other hand, it sets more readily and dries more quickly, but for this very reason it is harder to use on large surfaces, as the first brushing on one part of the work will begin to set before the entire surface can be covered. As this objection does not apply to small or moderate-sized work, however, the opaque white glue is to be preferred in such cases.
Good glue will keep in a dry room of any temperature for an indefinite length of time, but when cooked in the gluepot it deteriorates very rapidly. Each successive reheating and boiling lessens its adhesive qualities, hence it should always be used fresh, or nearly so. A greater quantity of glue than is likely to be used in two or three days should not be cooked at one time.
The cooking and preparing must be done in the regular gluepot, made for the purpose, and sold in all hardware stores. No rule can be given for the relative quantities of glue and water to be used. Some glues, especially the cheaper grades, require much less water than the better and finer qualities. As a general rule, however, pack the glue firmly in the pot and add sufficient cold water to cover it. Fill the outside kettle with cold water and boil until thoroughly cooked, so that it will run smooth and clear from the brush or paddle. It should run freely without returning and gathering in bunches or clots at the end of the paddle, but must not be so thin as to be weak and watery.
If the glue is too thick, no amount of pressure will bring the two glued surfaces in close contact, and if too thin there is danger that the joint will not hold. Always use cold water for cooking and dissolving fresh glue. Hot or boiling water will make the glue stringy and will require a much longer time to cook to an even and smooth consistency. Great care should also be taken to keep the outside kettle, which surrounds the gluepot proper, full of water. If allowed to boil dry the glue in the inner pot will be scorched or burned, and will then be entirely useless. It must then be thrown out, the pot washed or boiled out clean, and fresh glue again cooked. The hot water in the outside kettle should in all cases be used for thinning the glue to the required consistency. Cold water chills the glue and necessitates reheating.
In cold weather the precaution must be taken, unless the room is warm and entirely free from drafts, to heat the pieces of wood before applying the glue, else the latter may be chilled and fail to set. The time required for well-made joints to dry so that the hand screws can be removed is from 4 to 6 hours.
Sometimes a difficulty will arise in the case of large surfaces on thin material. When the glue is applied it moistens and expands the surface upon which it is placed, causing the edges to curl up and pull away from the adjoining piece which has a tendency to move in the opposite direction. In such cases never moisten the back of the thin pieces with water from the outside kettle, as is sometimes directed, but, working quickly, spread the glue rapidly, and then place between two thick stiff pieces of board, previously dressed true, prepared and heated for the purpose. Use as many hand screws as can be conveniently placed on the work, and allow it to remain in these clamps until all moisture from the glue is absorbed by the two outside heated boards. Twenty-four, or better 48, hours should be given to this process, if possible.
All such gluing of thin pieces should in every case be done first and allowed to dry while the other parts of the pattern are being constructed. Under no circumstances use water on any surface of seasoned wood. The reseasoning or drying out of such water will invariably distort, curl, and warp the pieces so treated, after being glued together. Even the water contained in the glue is objectionable, while unavoidable, and can be most satisfactorily removed only as directed above.
In all cases where end wood is to be glued, or where the grain of the wood runs diagonally to the plane of the joint so as to present the open end wood pores for the glue, this end wood, or partially end wood joints, should be first sized with thin glue - glue about half the thickness of that used for gluing - and allowed to dry. This will raise the grain and roughen the surface of the joint, which, when dry, must be lightly and carefully scraped off with a sharp chisel, when it will be found that the open pores of the wood are filled with dried glue. The joint may now be glued, and the glue will hold as in ordinary jointing.