The first evidence of gluing was found in an Egyptian tomb dated 1400 b.c. and there is little doubt that glue was used for some hundreds of years before this date. Pictorial evidence exists of Egyptian craftsmen boiling bones and hides to produce glue. In the Middle Ages the use of glue fell into a decline. The joints of furniture before a.d. 1500 were pegged and sometimes strengthened by iron straps. The first glue plant was founded at the beginning of the eighteenth century in Holland.
Owing partly to the development of aircraft construction, the laminating of woods, and the growth of the plywood industry, animal glues have been improved and new adhesives evolved. Unless the carver is also engaged in furniture making or other wood constructional work, his use of glue is occasional. I do not, therefore, propose to write at length on the subject which is already covered in many excellent books on carpentry and woodwork.
The most famous of these, Scotch glue, made from the bones and skins of cows, is in substance similar to glue used by the Egyptians three thousand years ago, and has been in constant use in many countries for nearly three hundred years. It has great strength, is easy to prepare and is suitable for all interior work. Manufacturers do not advise the use of animal glues under tropical conditions.
This is made from milk and it takes 7,500 gallons to produce one ton of casein. It is produced in the Argentine, France, Poland, South Africa, and New Zealand. Casein glue is in white powder form and is mixed with cold water immediately before use. Chemically this is an alkaline glue and can cause brown staining on hardwoods. Casein glues are partly water and heat resistant but the joints need protection from the weather in exterior work.
Synthetic resins (accelerator-set)
These glues have a chemical setting action and must be used in conjunction with an accelerator or hardener catalyst. Synthetic resin glues are water and heat resistant.
Synthetic resin emulsions
These are strong and very easy to use. They have a quick action, are non-staining and are made to a number of different formulas to cover all types of work. These glues are not advised for use under humid and tropical conditions.
The gripping time of glues varies. If you are in any doubt about the length of time the joint should be cramped, leave it overnight; in the case of Scotch glue, twenty-four hours is not too long. As all the new glues are bought with full instructions for use, I will only describe the method of making Scotch glue.
You will need a glue pot or double vessel. This can be improvised by using two tins, one that will stand inside the other, or a saucepan and a tin. The standard glue pot is a better proposition as it retains the heat well and will last a lifetime. You will also need a good quality 1 in. paint brush—soak it overnight before use. The glue is bought in slabs.
Wrap one of these in a cloth or sacking and break it up into small pieces by hitting it with a hammer. The cloth prevents the pieces from flying about the room. Tip the glue into the inner vessel with just enough water to cover it. Put water in the outer vessel. Heat the pot, stirring the glue until it has melted to an even consistency. Do not let the water boil over into the glue. If the glue is watery, it is too thin; it should run off the brush like thin syrup. If it is too thick or lumpy, add a little hot water. Remove any scum on the surface.
Scotch glue must be used hot. It is therefore necessary to work quickly. Apply the glue liberally to the wood surface and cramp without delay.
A glued joint, well made, is not weak or unsightly. It is often stronger than the wood itself. If you have ever dismantled mahogany furniture, you may have noticed that the wood sometimes breaks but leaves the joint intact. Many of Grinling Gibbons’ carvings are made up of 2 1/2 in. planks of lime wood, glued together.
Pressure must be applied to joints at the time of gluing, either by the vise cramps or by machine press. It is most important that the surfaces to be glued should be clean. Also that they should be flat and true. If you cannot use a plane, find a carpenter who will do this part of the operation for you. A slightly convex surface will give an open joint. Sometimes the surface is planed or shot very slightly concave in the center, in order to give a good joint at the edges. In this case the cramps are placed in the middle of the wood. The surfaces can also be scored by an old saw or the edge of a rasp, to give an extra bite for the glue.
You can test the flatness of a surface with the aid of a metal straight edge or by holding the edge of your plane across the surface. Arrange to have the window or light behind the edge; it is then quite easy to see whether the surface is true. Swing the edge at different angles on the surface in order to check every part. Planing a true surface is not easy for the amateur and needs practice. Should the carver wish to prepare the wood himself, I do advise that he get general woodwork instruction either from the many books on the subject or at a class in carpentry.
A rub-joint can be very satisfactory if the wood is small enough to hold in the hand. Fix one piece in the vise with the surface to be glued uppermost, apply the glue to both pieces of wood. Press the second piece of wood hard on the piece in the vise, rubbing at the same time to expel the air and surplus glue. In a few seconds the wood holds firm and the joint is made.
The spring dog (Fig. 1) is very useful when gluing up delicate work or for repairs. It can be made from the type of upholstery spring illustrated.
When the carver wishes to join a limb to the body, a strong jointing method must be adopted. The mortise-and-tenon, or the dovetail, are both good methods but not advisable unless you have some skill in general woodwork. The dowelled or pegged joint, however, can be easily mastered by the amateur. It is frequently used by carvers in all sorts of ways.
When making a dowelled joint, the dowels must be of the same diameter as the holes. A brace and bit is used for drilling. If the dowel is driven in from the outside of the carving (Fig. 2, I), the operation is very simple but the dowel end will show and this is not always desirable on small work. The other method is to use the dowel on the inside of the joint (Fig. 2II). Make sure that the holes are deep enough to take the dowel at the same time avoiding a large gap at the end.
You can gauge the depth by putting an elastic band round the bit at the length required. Make a saw cut down the length of the dowel (Fig. 2 IV) to take the surplus glue, otherwise too much surplus glue may prevent the joint from closing. Make sure that the holes in both pieces to be joined are in alignment. This latter problem varies with each carving. If the sides of the joint are straight, measurements can be taken. But if the shape is free, as in the case of an arm and shoulder, a template of the joint section with hole centers marked will be found very useful.
I have used a wood template with the holes drilled in the appropriate places. The thickness of the template acts as a guide and allows the holes to be drilled in perfect alignment.
The easier method of drilling right through from the outside was used in the Risen Christ (Fig. 3). In a large work of this kind the dowels are hardly visible, especially if the color is well matched. The arms in this figure are heavy and free from the body so that a strong joint was necessary. Three 1/2in. dowels were used on each side and a 4 1/2 in. screw. The screw was used to draw the joint tight before the dowels were driven in.
Cramping is not easy on a work of this size but sash cramps can be used for large work as shown in figure 10.
Jointing methods used by the Egyptians
The Egyptians used jointing methods for figures in both wood and stone. For wood the mortise-and-tenon joint was in common use. The tenon was fixed by wooden pegs as shown in figure 2, III. Their wooden statues were covered with gesso and painted.