Gluing, when properly done, is the strongest known means of fastening pieces of wood together. Joints made with glue are stronger than those made with nails, dowels, screws, clamps, or straps, because glue spreads in a uniform film that firmly binds together every part of the surfaces to be joined. Since its discovery, this property of glue has destined it to an intimate association with wood in the innumerable products of joinery, veneering, and cabinetmaking, for which it provides joints that can resist high stress and violent shocks and that permit the combining of wood into economical thicknesses and into useful and ornate shapes otherwise impossible.

Although the bond of all glues, except the synthetic resins, can be destroyed by prolonged wetting, with customary care in use the service value of a good glue joint is reliable, a fact that unfortunate experiences in home gluing, the result of faulty surfaces, preparation, and methods, should not obscure.

Animal glue, most of it made by cooking hides, fleshings, tendons, or bones of cattle in water, was long the world's principal wood adhesive and is still in common use.

The development of new glues began with the need of modern industry for adhesives that were cheaper or that would provide greater water resistance and thereby longer durability under the more severe service conditions of new and potential uses for its increasingly diversified products.

The first of the new glues, introduced about 1905, were vegetable or starch glues, a large part of which were derived from tropical cassava root, which easily yielded its large starch granules for the purpose. Despite their low resistance to moisture the cheapness and prolonged workability of these glues in the cold state early recommended them for quantity manufacture of plywood and veneer products. The present production volume of vegetable glues equals or exceeds that of animal glues.

Present-day emphasis upon water resistance in glues began during the First World War, when this property became important in aircraft construction. At the end of that war, casein glue, of ancient but vague history, had won for that purpose a recognition which it still commands in such manufactures as doors, plywood, furniture, pianos, and trucks.

Casein, the dried and ground curd of milk, is relatively inexpensive. After it is dissolved in water, water resistance is commonly imparted by adding slaked lime to form with it a jelly that will set permanently and not redissolve upon wetting. Other chemicals, usually sodium salts, are added to provide satisfactory working properties. The mixture, applied cold, sets to a hard and cementlike solid, whose bonding strength approaches that of animal glue. It is commonly marketed as a dry mix that contains all essential ingredients except water.

Blood-albumin glue, which is made from blood from the packing house, was a forerunner of the resins in its requirement of a hot press to obtain proper setting and bonding of its joint. In its dry state it ranks somewhat below casein in adhesive strength, but it has better moisture resistance. It has been largely replaced by synthetic-resin adhesives.

Amid all the magic claimed for the soybean, the recent development of a practical glue from it is not surprising. Soybean glue is cheap. It can be applied cold. It has inherent water resistance comparable to casein glue, although somewhat lower in strength. Made from the meal residue of soybean-oil extraction, the glue has a rather mushy texture, but sets to a firm bond in the cold press. It has won a commanding place in the great Douglas-fir plywood industry and in other fields like wooden-box construction, because its inexpensive water-resistant joints permit mass production never before attained. Because of its relatively high alkalinity, which may cause staining, the glue is not adapted to fine veneers.

Synthetic resins, the newest adhesives, impart to the glue joint the highest water resistance yet attained. In contrast to the earlier glues that at best could withstand only a moderate amount of dampness, a first-class synthetic-resin glue appears to withstand direct and repeated wetting almost indefinitely. In tests at the Forest Products Laboratory for more than a decade, certain synthetic-resin glues did not appear to soften or hydrolyze even on continued soaking of bonded wood specimens. These glues are not attacked by molds or decay fungi and maintain their hold as long as there is any wood left to test. Thus the bond that cannot be destroyed without destroying the wood appears to have been realized, although even yet the resins do not promise that their bond with wood will be spontaneous, for the production of a good joint generally requires high pressure, with or without added heat.

Adhesives: Mixing, Application, And Requirements For Best Use Of Glues In Common Use
Glue	Mixing and application	Temperature requirements	Water resistance	Common uses	Available from
Animal	Soaked in water and melted; applied warm by hand or mechanical spreaders.	Control of temperature of gIue, of room, and of wood important.	Low	Furniture, cabinet, and millwork.	Retail sources; specific grades usually obtained only from manufacturers.
Starch	Mixed with water and alkali usually with heat; applied cold by mechanical spreaders; too thick for hand spreading.	Used at ordinary room temperatures.	Very low	Plywood and veneered panels for furniture; not well suited to home use.	Manufacturers.
Casein	Mixed with water at room temperatures; applied cold by hand or mechanical spreader.	do	Medium	Used in gluing lumber, millwork, and plywood.	Retail sources.
Urea-resin: a. Room - temperature-setting; b. Heat-setting.	Powder form mixed with water at room temperatures; liquid form mixed with hardener at room temperatures.	a. Not recommended for use below 70° F. b. Requires heat for curing; hot presses commonly used.	High	a. Furniture, cabinet, and millwork. b. Plywood.	a. Retai1 sources. b. Manufacturers.
Phenol-resin	Powder form mixed at room temperatures with water or water-alcohol mixtures; liquid form may require addition of hardeners; film form used as received.	Requires heat for curing; hot presses commonly used but kilns have been employed.	Very high.	Plywood and laminated wood products for severe service; not well suited for home use.	Manufacturers.
Resorcinol-resin	Resin usually supplied in liquid form with which a powdered or liquid hardener is mixed at room temperatures.	Not recommended for use below 70° F.	do	Millwork and laminated wood products for severe service.	Retail sources.
Melamine-resins	Resin powder mixed with water at room temperature.	Requires heat for curing; hot presses commonly used.	do	Plywood; not well suited for home use.	Manufacturers.

Phenol-formaldehyde and urea-formaldehyde are the most widely used synthetic-resin glues. Melamine and resorcinol glues, among the discoveries during the Second World War, promise good performance, the resorcinols particularly so because highly durable bonds can be obtained without hot pressing. The resins are available as powders, solutions, or prepared films. Their special advantage is that they reduce surface swelling and the other changes caused by the water in the less concentrated adhesives, particularly in furniture and other fine veneer work in which they are being adopted.

THE GLUING OF WOOD is not a simple, infallible procedure, because wood species vary chemically and physically, and glues vary in source, methods of preparation, and use.

Findings at the Forest Products Laboratory after years of research lead to five general recommendations for applying glue.

It is usually unnecessary, and often detrimental, to roughen the wood surface; in fact, the mating surfaces should be machined to a smooth, true fit.

Animal glue must not be overheated. Heating the wood is generally unnecessary or detrimental.

Glue should be thick, rather than thin, in consistency when it is pressed.

A relatively heavy pressure should be applied to bring the surfaces to be joined into firm contact until at least partial setting has occurred.

Minor details of procedure can be varied in any way that will assure a proper jellylike consistency of the glue at time of pressing.