Glue differs from many other adhesives in that it is of animal origin. The varnish gums, shellac and resin, all have adhesive qualities and are all derived from trees of this day or of past ages. None of these last, however, are suitable for joining wood in the manner commonly known as gluing.
Diversity of Adhesives
All the vegetable gums are reduced to powder under shock or pressure. They are not soluble in water. Most of them are slow-drying. They also lack the quality of penetrating the surface of wood readily when used thick enough to form a good cement for uniting the parts. All of the gum or resin classes of adhesive materials depend upon the oxidation of certain ingredients or the evaporation of special oils or other volatile fluids to harden them so as to form a binding material.
There is still another class of adhesives to which belong the materials from which the various adhesive pastes are made. These include dextrine (Dextrin T.W. is wheat starch. Dextrine is used as a thickener for different textile techniques. Needs to be heated (cooked) to generate full viscosity. ) and the various starch compounds. But all of this class are from their nature unsuitable for making joints in woodwork. The starchy nature of the paste prevents its penetrating the surface of the wood so as to get a good grip and the joint formed is not flexible or strong. This is due largely to the fact that as the paste hardens it does not possess elasticity. As the moisture dries from paste, the latter always tends to crack.
Let us consider wood and see what is the nature of the material we have to contend with in the glued joint. Wood does not change much as to length but it
Relation of glue to the surfaces on which it is used, flexibility of glue, veneering, the effect of the seasons and climate, selecting a suitable glue, and the preparation of the materials to be joined
may change a surprising amount in width or thickness. The grain of most trees is not straight and so when the board is cut from the log it is separated from the rest of the structure that helped to hold it in place. Strains are at once set up and they continue as long as the stock seasons. If the stock is so piled as to hold it straight and is thus left for a number of years, the parts of the piece finally come to rest and stop fighting among themselves. The board is then said to be seasoned.
Even seasoned lumber if it is subjected to a change in the amount of moisture in the air will shrink or swell and may warp proportionately.
But the foregoing is not the modern method of seasoning lumber. The tree of today is to be the furniture of a period only four to eight months removed from the time the log started for the mill. Our modern dry-kilns remove the moisture but they do not season the stock in the old way. The removal of the moisture, including the sap from wood, sets up strains in the stock that are adjusted only as the wood finally comes to have the same amount of moisture in all parts, and also when the tension in the different parts finally comes to a balance.
On this account the makers of high-grade furniture and chairs usually dry the lumber in the plank, cut it to the proper sizes for making the individual parts and then dry it again. This reduces the trouble by drying the second time when the stock has been cut to small units.
Bent stock is always given a good drying after bending and in some cases after the first drying a certain amount of machine work is done on it and the material dried again. From this we see that the manufacturer does all he can to remove the strains from the various parts of a piece of furniture before the glue is applied. Even then, when two pieces are united, moisture is introduced with the glue and it takes time for the parts to become adjusted.
Flexibility of Glue
A good glue, as has been said, must be flexible. This is necessary on account of the fact that the pieces united may in some cases change in width or thickness, but the glue must form a continuous and hence flexible joint. There are some instances where greater flexibility in the glue is required than in others, but as a rule one grade of glue will do for a certain line of furniture.
When glue is applied the stock should be warm so as to prevent chilling the glue before it has time to penetrate the pores of the wood. By the advantage of this precaution we thereby establish a firm bond between the glue and wood. The warming of the stock for gluing naturally has a tendency to extract the moisture from the surface of the wood and this in turn introduces strains in the pieces. The excess of moisture in the glue is absorbed by the stock and this introduces moisture in one side of each piece. Those who do veneering know that if one side of a piece requires veneering it is necessary to veneer the back of the piece. Certainly, this is advisable if permanent work is to result. The back or inside is generally covered with a cheaper veneer. There are two reasons why both sides of the stock have to be veneered. First, making a glued joint on each side of the stock establishes uniform conditions on both sides of the stock at the time the gluing is done. Second, the covering of both surfaces with veneer with its underlying glue joint establishes equal conditions on each side so that any change in the amount of moisture in the air of the room where the piece is will have equal effects on both sides of the piece.
If one side were not veneered there would be uninterrupted pores or ducts leading from just under the varnished surface completely through the main portion of the stock from the side that was not veneered. On the other side these conditions would be broken by the glued joint under the veneer. The result would be that the stock would warp if exposed to such changes in the amount of moisture in the air as occur in an ordinary office or home during the course of a year.
Influence of Furniture Design
The manufacturer of the glue is likely to be hastily blamed for many troubles that in reality arise from the carelessness in manufacture or from faulty design in furniture. Sonic suggestions are therefore offered in regard to due care in gluing. For very accurate and delicate work it is of great importance that the grade of the glue used be uniform, that when applied it always contains the same amount of moisture, and that the temperature of the glue and the stock be kept the same at all times.
There is liable to be more trouble from irregular work in the glue-room in warm summer weather when the windows are open than in the winter when the plant is closed and the temperature and moisture conditions are kept more uniform from day to day.
In selecting a glue suitable for a given class of work the treatment the finished piece is to be subjected to should be taken into consideration. If the stock is thin and is to be subjected to bending strains a very strong and flexible glue is required. If the parts are heavy or massive and not subjected to sudden strains or rough handling a strong glue is required but it does not need to be so flexible.
In selecting the glue, it is also necessary to take into account the character of the stock to be glued. There are certain hard fine grained woods that do not take glue well as it cannot enter the surface of the wood sufficiently to affect a firm base for the joint. On the other hand, there are some woods the grain of which is so open that the glue is immediately absorbed and conducted away from the surface before it has a chance to form a joint. This is always the case when trying to glue most woods on their end grain. In such cases the surface should be given a priming coat. It is usually possible to use a lower grade of glue in the case of coarse-grained woods. The manner in which the surface to be glued is prepared is also of great importance. If some woods are sanded the pores on the surface become so filled with the wood dust that the glue will not adhere. The stock to be glued should be left with the grain open. In the case of stock that is cut with knives as in planing it is usually best to glue soon after cutting. If the planed stock is left for some time the ends of the pores seem to become stopped up. For many classes of work material direct from the saw can be glued better than stock that has been planed. In this case the grain of the wood appears to be open and there are many almost microscopic irregularities in the surface that tend to bond the wood to the glue and make a secure joint.