The techniques used to join pieces of wood together evolved significantly between the 12th and 15th Centuries. This evolution was in response to both the technical problem of making strong joints that would resist wood movement and the demands of the customer base. This evolution in technique was part of the general evolution of woodworking from the relatively crude and utilitarian objects of the early Middle Ages to the highly decorated furniture in use at the dawn of the Renaissance.
The Technical Problems
Wood is a living material composed of a mixture of cellulose, which makes up the cells, and lignin, which joins the cells together. As the cells absorb or release water from the environment, they expand and contract, causing the wood to move. Due to the orientation of the cells, movement perpendicular to the grain of the wood is usually several times that parallel to the grain. When end grain is joined to long grain, the differential wood movement can cause the joint to fail. (see Figure 1)
A second problem is caused by the need to make a mechanically strong joint without the use of external fasteners. Medieval hide and casein glues were not especially strong and metal fasteners, such as nails, were expensive. Complicating the matter is the fact that nails do not hold well in end grain.
The Butt Joint
The Butt Joint, shown in Figure 2, is the simplest way of joining two pieces of wood together. It is also one of the weakest since is prey to the greatest amount of wood movement. Due to the lack of mechanical strength, the butt joint needs to be nailed. Despite the presence of more sophisticated joinery techniques, the butt joint persists due to its inherent simplicity and ease.
The Mortise and Tenon
One of the earliest solutions was adopted from the timber framing used for large buildings. As shown in Figure 3, the tenon fits into the mortise providing a mechanically strong joint. Wood movement can be coped with by leaving the mortise slightly longer than the tenon, thus giving the tenon room to expand. Mortise and tenon joints used in the Middle Ages are frequently pegged. This technique is found on furniture pieces starting in the late 11th Century.
While the mortise and tenon works well on relatively narrow pieces of wood, this technique becomes cumbersome when attempted on wider pieces. The medieval joiner�s solution to this problem was the dovetail joint shown in Figure 4. While somewhat demanding to cut, dovetails provide a mechanically strong joint that is unaffected by wood movement. More advanced forms of the dovetail can be cut so that the joint is concealed from one or both sides.
Frame and Panel
An alternative to dovetailed joinery is the frame and panel technique shown in Figure 5. This provides an elegant means of covering a large area. The panels "float" in grooves in the frame, allowing them to expand and contract as necessary. Medieval craftsmen frequently used simulated frame and panel construction on chests, covering a dovetailed or butt jointed frame with thin pieces of wood to simulate the presence of frames and panels.
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