Shaku Crack

Crackability

Will your flute crack? Who knows. It'll depend on a lot of things but mostly the moisture balance in the bamboo. If moisture gets too out of balance for that particular piece of bamboo, it'll crack. There's bamboo you can oven dry and it'll never crack, other bamboo will freak out at the first dry breeze. It has to do with how hard the wood is and how quickly it can transmit moisture. Since bamboo contains silicon, which is necessary for bamboo growth, hardness can vary greatly from culm to culm.

Although there are those who flinch at bamboo being referred to as wood, in most areas the two are very similar. Both can be heat bent, both are hygroscopic, both have similar cell structures, both contain lignin, both can crack and so on. As far the purposes of engineering go the two materials are similar. And perhaps more similar than the dissimilarities among woods. Although the physical structure of bamboo and trees is markedly different the material isn't.

Bamboo is somewhat analogous to people. Young bamboo is healthy and stupid. Old bamboo is smart and infirm. Culms grow old and die after maybe five to 8 years--figure 12 human years to a bamboo year. During a culm's life moisture content declines, the walls develop strength and much of the sugar converts to starch. So what's the ideal age for flutes? What's the ideal age for people? To further this analogy: Who do you want to have to depend on in a pinch? The person raised with a silver spoon in their mouth or the one from a more hard scrabble existence? Thusly, bamboo from poor soil, a cooler climate, etc. is probably better for flutes. The tougher culm makes the tougher flute. Since you probably had little to do with the selection of bamboo used in your flute the rest of this page has information which can used for your defense.

Below is information for the construction of a simple, organic Crack Meter.

Let's start with a basic fact: adding moisture to wood makes it expand, removing moisture from wood makes it shrink. Now consider the two examples below: a solid stick of wood and a hollow tube of wood. With the solid, water vapor can only escape from the outside surface thus the outer surface is always shrinking at a greater rate than the interior surface. With the tube, vapor is escaping from both the inner and outer surfaces. If the wall of the tube is very thin the areas of the inner and outer surfaces are nearly the same, thus the contraction is nearly the same. The solid tends to split and the tube doesn't.

Notice that we've uncovered a principle here. The solid cracked and the tube didn't. In the solid, the drying (shrinkage) was very uneven and in the tube, inner and outer shrinkages were nearly identical. So here it is: It isn't drying that creates cracks; what causes cracks is uneven shrinkage (or expansion). Understand this single principle and most cracking problems can be avoided. If the wood in your flute loses (or gains) moisture at a uniform rate it won't crack--it can't.

Just insure that moisture loss or gain is as uneven as you can make it. How to do that? Make sure your flute has a vapor barrier on the inside of the bore (lacquer or some such), make sure the vapor barrier on the outer surface is either missing or insufficient and then stick the flute in a hot oven. Moisture will quickly leave the outer surface but be retained by the inner. The outer circumference will shrink and the inner circumference won't. Think of a standard piece of bamboo with an inner diameter of 20mm and an outer diameter of 32mm. The inner circumference is about 63mm and the outer about 100mm. Reduce the outer circumference by 1% while keeping the inner size the same and you've got a one millimeter crack. That's where cracks come from--not from drying per say, but from uneven drying.

Have both surfaces (inner and outer) transmit water vapor similarly. An even better solution is to create inner and outer surfaces which are both strongly impermeable to water vapor. That way it doesn't matter if your flute dries over time as the shrinkage will be very slow and therefore even. The best thing you can do isn't to buy humidifying stuff and fuss and worry, the best thing you can do is apply a good vapor barrier to the outer surface of your flute, while making sure the bore is either lacquered or has a finish identical to the outside. Then you can take your flute anywhere and not worry. It won't matter whether you take it into a hot seamy shower or to the Sahara in the summer. Both surfaces will transmit water vapor very slowly thus there will be no uneven expansion or contraction. You won't have to worry about dry air in the winter or any of the other things which torment those who attempt to monitor their bamboos's humidity. As long as the moisture content (whatever it is) is uniform throughout the bamboo it'll remain stable.

It's a far better method to control the rate of vapor transmission rather than trying to maintain a specific level of moisture in the wood of your flute. Don't try to control the humidity but flow with it, only at your rate--very slowly. What we're saying is don't worry if your flute dries out, worry if the moisture content of the wood in your flute becomes uneven--that's when it'll crack.

The goal isn't a specific moisture content, the goal is a uniform moisture content.

The moisture content of unfinished wood kept indoors in most parts of the United States fluctuates seasonally (summer to winter) from 14% to about 4%. The moisture content of wood which has been well finished with lacquer or varnish oscillates in a much narrower range--7-9%.

It's a mistake to think that the moisture content of bamboo/wood is the same as the relative humidity of the air. Wood/bamboo always remains hygroscopic, which means that it responds to changes in atmospheric humidity. As the relative humidity drops, it loses bound water; as the relative humidity increases, wood/bamboo regains bound water. For a given RH level, a balance is eventually reached at which the wood/bamboo is no longer gaining or losing moisture. When this balance of moisture exchange is established, the amount of bound water contained in a piece of wood/bamboo is called the equilibrium moisture content of the wood.

At room temperature and a RH of 75% the equilibrium moisture content of wood/bamboo is about 14%. For 30% RH it's around 6% as seen in the graph below.

Science Fair Project

Let's do some experiments which might be suitable for the junior high science fair. Bamboo has openings (tubes) in its structure which run the entire length which the plant uses to transport water/sap. For the most part, sap is sugar water and the sugar is, for the most part, glucose (C6 H12 O6). As a point of interest, cellulose is glucose minus one water molecule (C6 H10 O5).

First Experiment: Demonstrate the tubes

Cut a couple pieces of bamboo 2” long.
Put 1/2” of water in one paper cup and 1/2” of alcohol in another.
Place bamboo sections in each cup.

Watch carefully, as the alcohol will make it to the top in a few seconds. The water will take minutes if at all.
Why? Water is more viscous than alcohol, but more importantly, the albumen in the cells hydrate and swell, closing the tubes.
Albumen is the clear stuff in eggs. And it can be hardened by heat, as in frying an egg. Albumen is present in almost all biological cells including bamboo. It is kind of a gel when hydrated (think egg again) and shrinks when dry.
Leave both samples in their cups for 30-60 minutes, then remove and examine the bottoms. The water sample swelled, the alcohol didn't.
The water sample started to soak in the ‘bark’ layer, the alcohol didn't—and never will no matter how long you leave it. The top of the alcohol sample wetted almost immediately and the water sample didn't and probably didn't even after an hour.

Second Experiment: Evaluate water/alcohol as filling agents

Cut a couple pieces of bamboo 1/2” long. With an exacto knife or chisel, split one side of the bamboo. The sample should spring open, leaving a gap in the bamboo. The size of the gap tells you the degree of tension the bamboo was under just before you split it. So, one can saw a small sample off the end of a culm, split it and have an immediate general indication of the tension the culm is experiencing, thus the crackability of the culm.
Put 3/4” of water in one paper cup and 3/4” of alcohol in another.
Place bamboo sections in each cup.

For bonus points, notice what percentage of the bamboo is below the water's surface when you first place it in the cup. This percentage is the bamboo's specific gravity. The specific gravity of maple is 0.54, oak 0.66, walnut 0.55, hickory 0.74. Getting the picture? Contrary to ordinary intuition, bamboo is a very dense material compared to woods. Place the sample in the upright position to get a good reading.

By watching the gaps of our samples we have a very good, real time indicator of what's going on in the wood. The gap of the water sample will expand for maybe 30-60 minutes and then begin to close. In a few hours it'll be closed. The gap in the alcohol sample will do very little and the gap won't close.
Should you want you can repeat this gap experiment with different solutions (acetone, mineral spirits, oil, etc.) they'll be about the same as alcohol.
The conclusion is that water is filling cells that other solutions don't and that as long as a gap remains the potential for cracking remains.
Now take the samples out of their solutions and watch the gaps during their drying cycles. This single experiment conveys a tremendous amount of information about cracking in general, the potential of any particular piece of bamboo to crack during service and the cellular structure of bamboo.

What's needed is some treatment of bamboo where the gap closes and remains closed. Once that's achieved, the bamboo will have no (zero, nada) propensity to crack.

Bamboo Flute Crack Meter

This gap method (as described above) is very instructive, but further, it can serve as an aid in keeping your flute healthy.

If your flute maker were to cut a small sample from the top of the culm of that root-end shak you paid thousands for and did the same (any treatments, lacquer, humid box, oiling, etc.) to it as the flute, you would have a real time indicator of the stresses in the flute. That the gap sample came from the same bamboo as your flute would guarantee identical bamboo morphologies—thus identical responses to treatments, moisture, humidity, etc. The idea would be to keep the Crack Meter with the flute, thus it would experience and reflect the same conditions and indicate the same stresses.

However, this isn't likely to happen as your flute maker doesn't want you to know the stress your flute is experiencing. But you can make your own crack meter and although it isn't from the same bamboo as your flute, it'll reflect ongoing humidity conditions--a glance at the width of the crack will let you know what's going on and the stresses your flutes are currently experiencing. Wide gap indicates high cracking stresses, narrow gap lower stresses.

Maybe it's never dawned on you but bamboo flutes are experiencing stress their entire lives. Some days are worse than others but the stress to crack is unremitting. Will your flute crack? A Crack Meter is probably your best indicator.