Shaku
Design
The Structure of Bamboo

Updated 4/24/08

The growth structure of bamboo is entirely different than that presented by trees. The coarse, open cell structure so characteristic of the various woods is totally absent. In its place are found bundles of microscopic fibers laid parallel to each other, much like the strands within the sheath of a bridge cable. The spaces between adjacent bundles or cords of the bamboo are filled with lignin, a thermoplastic resin. Toward the outer surface of the hollow shell the number of these fibrous strands increases rapidly. Just below the outer protective enamel, bamboo is composed almost entirely of these cellulose fibers. Thus, considered from the outermost surface inwardly, the fiber structure, density, and strength of bamboo vary enormously.

There is no known method of infallibly appraising bamboo. Two factors, however, are readily apparent: (1) weight (density), and (2) appearance of the end grain. In viewing the end grain, notice in particular the character of grain and fiber structure at the outside edge, just under the enamel or cuticle. In most cases this is an exceptionally dense, close-grained substance. Taking note that the cut end does not occur in the immediate vicinity of a node, this dark amber-colored layer should extend, with slight diminution in density, inwardly at least one-third or more of the total wall thickness. Farther is better. If it is at all possible, examine a number of culms in order to gain a good degree of perspective. It is far simpler to isolate a single excellent specimen from a number of fair ones, than to appraise a single culm without reference to others.

The graphic above is two samples of end grain structure. The darker areas are the 'fibers' and the lighter areas comprise the matrix--primarily lignin. Notice the difference in distance, from the outer wall inward, measured to the point where the fibers begin breaking up into four segments. Thus the sample on the right is the stronger, denser, harder piece of bamboo. One can see these fibers exposed on the edge-cut of a shakuhachi and make a cursory assessment of the bamboo. It's a little harder to make an assessment on a slant cut rather than a cross-section, but as a general rule, one wants more of the dark brown fibers and less of the tan lignin.

Contrary to common intuition, bamboo is a very dense wood. It's density runs 2 to 3 times that of pine and usually is more dense than oak. On density alone bamboo would be listed well up in the hardwoods--above teak, mahogany, maple, walnut, oak and ash. In general, bamboo has about the same density as hickory. Density is the single most important indicator of strength in wood and may, therefore, predict such characteristics as hardness, ease of machining, and resistance to penetration. Dense woods generally shrink and swell more and usually present greater problems in drying.

For those who doubt the density of bamboo, go ahead and measure the specific gravity on your own:

1. Take a bamboo chopstick and cut to 150mm
2. Square both ends and cut so the stick is uniform in crossection
3. Mark at 100mm and every 10mm to 150
4. Place in a tall drinking glass and fill with water until the stick floats
5. Divide the length of the submerged portion by the overall length (in this case, 125/150 = 0.83)

The result is the specific gravity of the bamboo. Everything that floats has a specific gravity less than 1.0. The SG of ice is around 0.92, thus a little less than a tenth of a uniform icicle will float above the surface of the water.

It's not unusual to find bamboo that will just barely float--specific gravity of 0.95-1.0.

Here's a list of specific gravity for different materials and woods. Turns out beeswax is more dense than ordinary bamboo, about the same as ebony.

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