Fundamental Construction Techniques for Furniture & Wooden Objects
Wood is everywhere. Think about it ... nearly everything we touch and use has a tree source. We use trees as shelter, food, fuel, furniture, sports equipment, synthetic cloth, huge ultra-modern windmill propellers, patterns for metalwork, and a myriad of other uses. The history and decline of civilizations can be traced by their use and sometimes overuse of wood resources.
This guideline will describe some of the ways that wood is processed into familiar forms, such as joined furniture. This description will lead to a better understanding of the various technologies: from the ancient to the modern. You will see that there is much in common through time and across cultures. In fact, the joinery of wood can be reduced to a fundamental set of principles in evidence the world over. By understanding the basics of joinery, we can also understand and predict the utility and ultimate degradation of some furniture.
WOOD AS A BUILDING MATERIAL
Wood is a plant stem tissue, and the two most important functions are support and conduction. The crown (leaves and branches) of the tree is supported by the stem, which must resist the force from the weight of the branches and leaves, as well as the loads imposed by wind, rain, and snow and ice. The stem of the tree also conducts water and nutrients to and from the crown and root system. Even more fundamental, the wood structures are typical of a "cellular solid." In the case of wood, this means that the stem is comprised of hollow tubular plant cells bound together by an intercellular glue. Most of the volume of wood is air-filled space. Wood is light-weight, but incredibly strong: pound-for-pound stronger than steel. The greatest strength of wooden components is found parallel to the same direction as the original stem (imagine the stress of a tree blown by high winds). That is why the long direction of boards runs in the same direction as in the tree. We have used these characteristics for centuries to create strong, durable tools and structures. Early people not only observed the great strength of the tree, they used the best features in their own construction. Centuries-old wooden ships and buildings stand as a testament to the second life of the tree.
There are really only a few ways to successfully join wood, whether building a house or a chair. The greatest challenge, beyond fashioning a log into boards, is joining the wood components at right angles to one another. The earliest archaeological examples all exhibit typical joinery - no matter where in the world they are found. Joints do more than make use of small pieces of wood. They make frames, increase length, and make large surfaces of solid wood. Many of these ancient methods were still found after the advent of metal fasteners (nails, screws, etc.) simply because the joints had proven so strong.
The frame: joining at right angles
Lashing the parts together, usually with ropes, is the simplest method. However, this can be a weak structure, and bulky because the members overlap. Any joint will be only as strong as the weakest component, and rope or leather can not match the strength and durability of wood.
The strongest method for joining wood at right angles is the mortise-and-tenon. This ancient joint is found in Egyptian furniture thousands of years old. The joint is like a squarish peg (the tenon) fitted precisely to a squarish hole (the mortise). There are literally hundreds of variations on the mortise-and-tenon joint, each suited to particular purpose or craft tradition. The most common tenon is rectangular in cross-section, as is the mortise. This gives great resistance to twisting forces. You can probably guess that a round mortise-and-tenon is not as strong. The tighter the fit, and the longer and taller the tenon, the stronger the joint will be. The so-called through-tenon, with the tenon completely penetrating the mortise-bearing member, is the strongest of all. It is important that the tenon not slide out of the mortise, whether the joint is for furniture, house, or ship. The most common means to secure the tenon is a peg, which fits into a hole near the opening of the mortise. In some cases, such as portable furniture, lashing is also used in combination. Wedges which spread the tenon in the mortise are sometimes seen. This also prevents the tenon from being pulled out of the mortise.
Probably the next development in joinery was the dovetail joint, which is often seen in box or drawer construction. The joint is comprised of a wedge-sheped tenon (the "tail") on one component which overlaps a corresponding wedge-shaped slot in a second component. The portion of wood surrounding these slots is called the "pin." Except in the case of decorative joinery, all the pins are on one board, all the tails on another. The term "dovetail joint" can refer to one tail, or many in a row, such as on a drawer side. As in the case of morties-and-tenon, the strongest dovetail joint is made when the pins and tails go all the way through the joint.
In the best mortise-and-tenon and dovetail joinery, no glue is required.
The panel: edge joints
The edge joint, or those which join the thin, long edge of boards together to make a panel, is another ancient technique. These joints increase the width of the wood surface, such as for a table top. Usually, the deges are simply glued together, but sometimes a more elaborate joint is used. Simple edge glueing requires that absolutely straight and square surfaces be prepared. Glues prepared from the skin of animals were the most common, and are still used today. Door panels and violin soundboards are made using edge joints. More elaborate joinery, such as tongue-and-groove ( a modified mortise-and-tenon), are used only for alignment of the mating surfaces. Veneering can also be thought of as a specialized form of edge glueing.
Increasing length: splining
Joints which increase length are called spline joints. These have been used whenever the wood being available is not long enough, such as in house building. However, splining is also used for special properties, such as for greater strength of wooden ship masts. Splining is not often seen in furniture though, because is unnecessarily complicates construction.
An example of these fundamental joints can be found in 18th century case furniture construction, such as a "high boy." Such a cabinet might have a box made of a set of planks joined by dovetails at the corners. A morties-and-tenon frame with legs would lift it off the floor. Doors of frame and panel construction would enclose the case. These are frames joined by mortise-and-tenon, with panels (perhaps two or more edge glued boards) fitted into a groove of the inside edge of the frame. There might be drawers; typically, these would also be of dovetail construction.
MODERN WOODEN CONSTRUCTION
Modern furniture construction - say, from 1840 to present - is marked by a number of innovations. While some see the modern period as the end of high quality hand-built furniture, this is not so. Furnishing an entire house was possible for only the wealthier citizens. Hand-made furniture was made the most efficient way possible, but it was still a labor-intensive and expensive craft. Modern manufactur made stylish furniture accessible to nearly everyone. Innovations have included high-speed machine wood preparation and joinery, spring upholstery, and the use of plywood and other modified-wood products. In the machine-age, dowel joinery largely replaces mortise-and-tenon, and dovetails are machine-cut and joined in seconds. Modern adhesives are stronger than wood, set rapidly, and withstand adverse environments and intense use. Nails, screws, and other fasteners - once made individually by hand - have become inexpensive as well as sophisticated. They now replace and even improve the strength of some joinery in new construction. In much modern furniture, joinery that was traditionally unseen is shown for decorative effect.
In spite of these innovations, high quality hand-made furniture has always been available. In the late 19th century, the highest-quality furniture was custom designed and hand made. The cost of this class of work was, and still is, merely reflective of the skillful maker and the use of fine materials. And in the late 20th century, we can add an additional factor: the mystique of the craft.
FOR FURTHER READING...
Bramwell, Martyn, ed. The International Book of Wood, 1982, London: Artist House.
Hayward, Charles H. Woodwork Joinery, 1979, New York: Sterling.
For more information, please contact Melvin J. Wachowiak, Jr., Senior Furniture Conservator, SCMRE.