Wood that is used for furniture or interior work should have a moisture content of from 5 to 6 per cent for satisfactory gluing. Lumber for outside use can be glued successfully if its moisture content is about 12 per cent.
Glues most commonly used in making joints in wood are animal glue, casein glue, liquid glue, and waterproof resinous glue. These four types differ in some respects, such as consistency, rate of setting, strength, and water resistance.
The consistency of animal glue is variable from thin to very thick, depending on the changes in temperature. Its rate of setting is rapid. The strength of the glue is very high when gluing conditions are ideal. Its resistance to water is low, and it should therefore never be used for outside work or on work that may be exposed to dampness or moisture.
Liquid glue varies in consistency from thin to medium. It does not set as rapidly as animal glue. The strength is high, but its resistance to water is low. Casein glue varies from medium to thick in consistency. It has a rapid rate of setting, its strength is very high, and its water resistance is rather high.
The conditions under which glue can be applied are extremely variable, and depend on the properties of the glue, the method of preparing it, the quantity of glue that is spread, and the length of time elapsing between spreading the glue and applying the clamps. The clamps are used to squeeze the glue out into a thin continuous film and to hold the pieces in close contact with one another until the glue has set.
The consistency of the glue controls the amount of pressure required by the clamps. A thin glue should have a light pressure applied by the clamps, but a thick glue requires a heavy pressure. The elapsed time between the spreading of the glue and the application of the clamps will depend on the rate of setting. The more rapidly a glue sets, the shorter must be the elapsed time. A glue that requires heat in its preparation, such as animal glue, should be applied in a warm room because it chills and thickens very rapidly.
Casein and liquid glue, which are applied cold, do not require a controlled room temperature. The clamps should be left on the work until the glue has set and there is no danger of the joint's opening. This takes from 4 to 8 hours, depending on the thickness of the stock and its ability to absorb the moisture present in the glue.
Satisfactory glue joints can be made only between edge-grain surfaces. It is impossible to produce a strong joint by gluing two pieces end-to-end or end-to-edge. This difficulty is overcome by the use of a cut joint, such as the lap joint and the mortise-and-tenon joint.
Any moisture changes in the wood after it has been joined by glue affect the strength of the joint. Animal and liquid glues, which have very little water resistance, should never be used when there is the slightest chance that the pieces which are being joined will be exposed to dampness or moisture.
Casein glue, which is fairly high in water resistance, will resist the many changes in moisture content; but when exposed to these changes over an extended period, it will break down. In order to protect the glued joint after it has been made, the outer surfaces of the wood should be coated with a sealer, such as paint or varnish. This closes the pores of the wood, preventing any moisture from entering.
Preparation of animal glue
Animal glue must be prepared in a double boiler. (This consists of a small pot in which the glue is mixed, held in a larger one with water in it.) Animal glue comes in the form of cake, flake, and powder. The cake, if used, must be broken into small pieces. The glue is mixed with water in the proportions recommended by the manufacturer.
The mixture should be thoroughly stirred to allow a uniform absorption of water and to prevent the formation of lumps. The mixture should stand several hours until the glue is thoroughly soaked and softened. When the mixture is ready, it is heated over a water bath at a temperature no higher than 150° F. This is done by pouring water in the larger pot of the double boiler and plating the smaller one in it.
When the glue is warmed, it is applied with a brush to the surfaces to be joined. When preparing animal glue, mix sufficient for the day's work only. The reheating of glue reduces its strength and results in weak joints. The glue pot should be cleaned after the day's work, for glue is a breeding place for bacteria which decompose and weaken it.
Liquid glue requires no preparation, for it comes ready for use.
Casein glue is obtained in powder form, and prepared by mixing it with water. It contains alkalies which corrode most metals; therefore it should be prepared in a glass or porcelain container and mixed with a nonmetallic mixer. The glue-water ratio is determined from the manufacturer's recommendation and should be strictly adhered to. The working period of a casein-glue mixture is no more than several hours, after which it begins to set. For this reason it should be prepared in small quantities.
The most modern of all glues is the synthetic-resin glue made of chemicals. There are two types, the urea-aldehyde combinations and the phenol-aldehyde combinations. They are obtainable in powder form (to be mixed with water), in liquid form, and in a dry film form; the latter two are ready for use.
The urea-aldehyde adhesives in liquid form require the addition of a hardener, or catalyst, just before using. This catalyst is supplied with the adhesive, and is added to it just before using. When preparing a liquid urea-aldehyde glue by adding the catalyst, prepare only enough for immediate requirements; once the hardener is mixed, the adhesive begins to set, or harden. Some of the urea-aldehyde adhesives in powder form have the catalyst already added. When this type is prepared with water, the action of the catalyst begins. For this reason, urea-aldehyde adhesives in powder form should be kept in closed containers to keep out moisture.
In general, the phenol-aldehyde adhesives require simultaneous application of heat and pressure. This operation is known as hot pressing. A joint made with such an adhesive properly applied has a very high resistance to moisture. The majority of urea-aldehyde adhesives also require hot pressing, although the temperatures required are generally lower. There are some ureaaldehydes that can be spread and pressed at room temperature. Like the phenol resins, the urea resins, when prepared and applied as directed by the manufacturer, are highly waterresistant.
The period between application of glue and need for pressure exerted by clamps is relatively short; therefore, you should check and arrange the clamps to receive the pieces before gluing is begun. If there is any danger of the clamps' marring the wood, place a piece of stock between them for protection.
Dowels are cylinders of solid wood used to reinforce joints. They are usually made of close-grained hardwood, such as birch or maple. The size of a dowel is determined by its diameter; sizes range from-3/16" to 1". Dowel lengths are a standard 36". Shorter lengths, ready for use, can also be obtained, each with the ends chamfered and a straight or spiral groove cut along its length. The purpose of the groove is to permit the excess glue and air to escape from the hole in which the dowel is placed when assembling the work. The diameter of a dowel should be about half the thickness of the smallest member being joined; the length of the dowel should be four times the diameter.
Laying out dowel holes
When using dowels to reinforce edge-to-edge butt joints, such as would be required to make up a wide panel. In order to line up dowel holes, they must be located simultaneously on both pieces. The boards are placed on the bench with the butting edges in their relative position and marked. They are next placed face to face, with the edges which are to be joined flush with one another and held together by means of handscrews.
The number of dowels to be used must be determined before their location can be established. The dowels should be placed from 6" to 8" apart, depending on the length of the edge; but in any case no less than two dowels should be used nor should they be placed any closer than 1" from either end. When the number of dowels to be used has been decided, the laying out of their location should be started, from the center rather than the end. A center line is squared across both edges, using a trysquare; then from this center line the dowel-hole locations are marked.
The handscrews are removed and the marking gage is set for half the thickness of the stock. A gage line is scribed across the dowel-hole locations, keeping the head of the gage against the work face.
Boring dowel holes
Dowel holes are bored with an auger bit of the same size as the dowel to be used. The depth of the hole should be 1/16" more than half the length of the dowel. A bit gage is extremely helpful when boring holes to a required depth. When a bit gage is placed on a bit, it should be slipped on from the tang end. When the gage is set to bore a hole of a predetermined depth, the measurement should be made from the lip of the bit to the end of the gage. It is important to keep the bit square to the surface into which the hole is being bored; otherwise the dowels will not line up with the dowel holes when the two pieces are assembled.
The laying out of an end-to-edge butt joint is done
in the same manner, except that the holes are located and bored in the
edge of one piece and the end of the other. A minimum of two dowels should
be used, placed no closer to the edges of the wood than twice the dowel