The Physics of a Wooden Bow
In Traditional Archery, Not All Bows are Created Equal
In order to understand the material used in building a bow, one must first understand the basic physical principles by which it operates. A bow is essentially a spring. As the archer draws back the bowstring, s/he stores tension in the curvature of the bow's limbs. When the bowstring is released, the tension stored in the limbs as potential energy is converted into kinetic energy which is used to propel the arrow forward.
The materials used in a bow's limbs must be flexible enough to withstand repeated bending to its maximum degree without permanently losing shape or snapping. It must also be sufficiently rigid to require great force to bend it. Otherwise the tension stored in the limbs would be insufficient to launch the arrow with any considerable force.
The two requirements, flexibility and rigidity, are clearly diametrically opposed, and finding the best compromise in satisfying both is the crux of the science of archery materials.
Wood is generally light and flexible, and it is easily tooled into almost any shape, using anything even as simple as a sharpened stone. More than anything however, wood exists in a readily usable state. It requires only to be cut and formed.
Although technically any type of wood may be fashioned into a bow, a few types stand out as having particularly desirable traits, while many others are barely workable. Elm, apple, cherry, hickory, ironwood and ash to name a few will all produce a bow of adequate performance. Two types however, have been prized for centuries: yew and osage orange.
Yew and Osage Orange
Yew is the wood that was most commonly used in Europe to construct the bows that medieval armies fought with. To the archers of the Middle Ages a solid, knot-free piece of yew was a sought-after commodity.
Yew is a medium-hard wood tree found mostly throughout Europe and, to a lesser extent, along the American western coast. Its natural characteristics are such that its capacity for energy storage (i.e., its elasticity) approaches that of spring steel. This makes for a bow with a tremendous ratio of flexibility to strength.
Osage orange occurs more frequently in the Americas and yields a wood that approximates the characteristics of yew. Both yew and osage orange however, are somewhat difficult woods from which to obtain long, straight sections and both are very expensive to purchase in suitably large pieces.
For these reasons a bowmaker might opt to use a different wood whose properties may fall slightly short of the two discussed above, but are less cost-prohibitive. Elm is one example of a more easily acquired hardwood which can still give birth to a bow of exceptional quality.
Problems With a Wooden Bow
The drawbacks of wood are numerous, nonetheless. Wood is very moisture-absorbent, which will cause it expand and contact as it becomes wet and re-dries. Moist wood will allow more flexibility, but less tension. Dry wood is more rigid, but also more brittle.
A bow that changes in its performance features is obviously not conducive to consistent shooting. Over time, this repeated cycle of wetting and drying will weaken wood fibers and eventually cause the wood to split.
Modern wooden bows are often protected by a lacquer finish or a fiberglass laminate to increase the wood's longevity as well as the bow's overall attractiveness. A handsome, handmade wooden bow may still garner a price tag in the hundreds or thousands of dollars. If left bare, a wooden bow should be regularly coated with oil to keep as much moisture out as possible.
The Arrival of the Composite Bow
The need to protect the wooden bow from the elements gave rise to the invention of the composite bow: the integration of different materials to enhance the bow's longevity and performance.
In an article written for the Journal of the Society of Archer-Antiquaries in 1959, C.N. Hickman discussed one of the earliest known composite bows, which was unearthed in Egypt at Thebes in the late 1800's. Its date of manufacture is estimated at earlier than 600 BC. Materials used in its construction include at least two different types of wood, strips of animal horn for reinforcement, layers of animal sinew to bind the assembly together, and an outer wraparound layer of birch-bark as weatherproofing.
The concept of using multiple, separate materials to manufacture a weapon of superior quality would probably not have been new, but it signified a great leap forward in archery technology and laid the groundwork for all future generations of bows.
Two Medieval Archers
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