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.