by Edward Dix
As is typical of most students of aikijutsu, I spent many training hours caressing my bokken and jo but, as a professional botanist and forester, I had more than typical affection for wooden weapons.
There is a wide variety of imported and domestic equipment available, but much of it is unsuitable for serious training. Even if most of your training is solitary kata, a serious practitioner wants more than a replica. More to the point, in paired practice of kumitachi or kumijo, you’re going to strike weapons together, hard. Nobody wants a weapon that turns into a pile of kindling on impact. In trying to determine which woods make the best bokken and why, I consulted the Wood Handbook: Wood as an Engineering Material, a publication of the U.S. Forest Service and their Forest Products Laboratory in Madison, Wisconsin.
The weapon I picked up most often was a Bizenzori-odachi, manufactured in Japan and purchased through The Kiyota Company, Inc. of Baltimore, Maryland. It is made of a cream-colored wood, listed as “white oak” in English, and called shira-kashi in Japanese. Shira-kashi is hard, heavy and tough wood.
A strong wooden weapon will always feel heavy for its size, indicating a high density. The reason for this is in the basic structure of wood. As you may remember from high school or college biology, all living things are made up of cells, and the wooden bodies of trees are no exception. The biochemical substances forming the cell walls of wood, the polymers cellulose and lignin, are actually heavier than water. Dry wood of most tree species floats because a good part of the volume of wood is cell cavities and pore space. Trees vary in their cell wall thickness and open space, causing some species to contain more wood substance per unit volume do than others. In technical articles comparing the wood of different species, measures of density are reflected in tables of specific gravity.
Specific gravity is a ratio of the density of a substance to the density of pure water. Any material with a specific gravity greater than 1.0 sinks when placed in water. The reason I bring this up is because specific gravity is a good index of the amount of wood substance contained in a sample of dry wood. And, allowing for variations due to the presence of gums, resins, or other extracts, specific gravity has been demonstrated to be a good index of wood strength. By comparing the average specific gravity of samples from different tree species, we can get a clue to the relative strengths of weapons made from their wood.
Other clues to the quality of a wooden weapon can be seen in the grain. The grain is formed by the combination of growth rings and rays. Rays are lines of cells that run outward from the center of a tree, perpendicular to the rings, like the spokes of a wheel. Growth rings are more complicated. Trees growing in temperate climates emerge from their winter sleep with a spurt of growth that produces large, thin-walled cells called earlywood. Through the summer, growth slows down, producing smaller, thicker-walled cells called latewood. The alternating layers of earlywood and latewood form the growth rings familiar to anyone who has looked at a log or stump and counted the rings to determine the tree’s age.
If you look at the butt end of your bo, jo, or bokken, you can probably find the growth rings (although you may need a magnifying glass). A higher proportion of the denser and usually darker latewood makes for a stronger weapon. Now, study the grain in the sides of your weapon. A clean, straight grain, without any curls or slope relative to the long axis, indicates that the wood fibers run parallel to this axis, giving the greatest strength, while defects that weaken wood are revealed by curves or slope in the grain. Visual inspections of this type are often performed by manufacturers in separating the stock used for top-grade weapons from that used for the economy-grade models.
Using shira-kashi as my standard, I set out to compare the qualities of this wood to North American species. The key to finding more technical information about wood or any plant or plant product is to find the scientific name. The scientific name for shira-kashi is Quercus myrsinaefolia, an evergreen oak found in Japan and China. With the scientific name in hand, I requested information on the engineering properties of Q. myrsinaefolia from the U.S. Forest Service Forest Products Laboratory. A return fax provided excepts from a Chinese journal article with the average strength data translated.
The specific gravity of clear, straight-grained samples of Q. myrsinaefolia averages .71 or about 43 lbs/ft3. For comparison, North American white oak (Quercus alba), commonly encountered in commercial lumber, averages .68 (a density of 42 lbs/ft3). Interestingly, live oak (Quercus virginiana), an evergreen oak which grows from Virginia to Florida and west to Mexico, in approximately the same climate zone as Q. myrsinaefolia, is the densest of our oaks, with specific gravity averaging .88 (55 lbs/ft3). Other strength values, called the Modulus of Rupture and Modulus of Elasticity (which describe how a sample bends under stress), are higher for shira-kashi than for North American white oak or live oak.
But these were only species averages and I suspect the manufacturer of my bokken had selected above-average stock for his product, because both of my Bizenzori-odachi felt much heavier than any oak I encountered elsewhere. As an example of the problems inherent in relying on species averages, I once purchased a bokken manufactured from hickory. The average specific gravity of true hickory species (as opposed to Pecan hickories) is the equivalent of oaks, being .60 – .75 (37-47 lbs/ft3). But I found this particular bokken disappointingly light for its size. Collisions with shira-kashi bokken left it seriously dented. I expect that high-quality bokken could be made from carefully selected hickory, because this wood demonstrates great strength under impact loads — it is preferred for tool handles which require high shock resistance. This deserves more research. The measures of hardness and impact bending strength usually applied to small, clear, straight-grained samples of wood in order to calculate the species averages, might be applied to product samples to compare domestically produced bokken with imported weapons.