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Re: Crack Prevention

Author: JRC
Date: 2010-11-23 14:41

The temperature and moisture contribute to expanding and contracting of the wood. I do not have the data to back it up but I would venture to guess that temperature affects more overall. Moisture (wetness) would affect more "locally" where wood surface is in direct contact with water and saliva. This observation is based on the fact that grenadilla wood is dense and does not absorb water like a blob of sponge would. But the wood surface would become "wet" and some local absorption is inevitable. Molecules of any wood will expand when wet. It is especially troublesome when the temperature difference between the blowing air and the wood surface is "high"; more condensation. If you are salivating over wonderful music you are playing, it will get even worse. I would not worry too much about moisture absorption into the wood from the moist air. It would be much less effective in getting into the wood than water in direct contact

So, the internal surface of the oboe bore would expand relative to the rest of the wood by higher temperature from warm blowing air and the water absorption by direct contact. The wood will experience stress from this local expansion of internal surface. The level of stress is higher if the temperature difference is higher and wetness level is higher. The wood can stretch to relieve the stress, one would hope. Bamboo wood would stretch much more than grenadilla wood would, so, grenadilla wood is more likely to crack under the stress than bamboo to relieve the stress.

If you look into the bore from the bell after playing about 15 minutes, you will see moisture starts to condense all along the internal surface. Depending on how dry the room, it may already be dripping water. But you will also see that there is more water closer to the reed socket. The dripping water channels down along the bottom wall. Now you know why all the tone holes are located along the top side of the wall so the water will not plug up the holes.

The closer to the source of the warm and moist air from your reed, the more condensation and the warmer the internal surface would be. So, the upper top joint would experience more stress from temperature and moisture. It also happens that the ratio of diameter to thickness is the largest there also. The stress experienced by the wood would go with this ratio. Now you can see that the top of the top joint will be stressed the most.

The crack would most likely happen at the weakest point. Holes and posts contribute to the stress on wood and therefore weakness. Just look at the top joint of the oboe. There are high concentration of weak points around the c# and d trill holes. There are metal posts, tone holes, and octave holes. Guess where the most frequent cracks point in oboe would be? The stress has to be relieved and crack will be the natural course.

Now let us review what we have here. Higher temperature differential and direct contact of water with wood surface provides the cause and high concentration of holes and post provides the failure point of the cracks. I did not mention any thing about cracks around the metal piece at the top of the middle joint and the bell. Phenomenology is very different there and they are much much less serious problem.

What should we do to minimize the possibility of top joint crack? I can think of 3 things.
1. Reduce the temperature differential as much as possible.
a. Warm up the oboe, especially the top joint before blowing into it, like putting it under your arm inside the coat or holding it with your warm hands (hopefully your hands are warm)
b. Avoid blowing hot air into the cold oboe. No No No! It would be better to blow from the bottom instead of from the top, if you really want to blow into it.
2. Minimize direct contact of water with the wood surface. You can actually stop this altogether by applying water repellant over the wood surface. Use your favorite magic portion but consider the following;
a. Do not use any thing that will leave film. Thompson’s water seal is no no.
b. Avoid using things that would damage wood in long term. Animal fat tends to turn acidic in time (you should check this with credible chemist to confirm this). I was told that acid is not a good thing to wood (you should check with experienced carpenter especially custom cabinet designer/makers to confirm this). On the other hand, if you keep oiling it before it turns acidic, that might solve that problem. I was told that high unsaturated oil content (they call it anti-oxidant) would lessen the probability of turning acidic. There is something called Emu oil from Australia that has very very high omega 9, a form of unsaturated oil. (This is a good place for a chemist to jump in.)
c. If the magic portion can be absorbed into the wood and permanently seal the surface from water, that would be perfect. BUT, too much of it could change the acoustic properties of the wood surface and could affect the timbre. Do not forget you paid an arm and a leg to buy that oboe for that wonderful sound.
d. Lose that feather to spread water all around the surface! Bad idea! You should remove water not spread around to make the situation worse. Use something that absorbs water. Use something that does not rub the surface, which would also rub off the water repellant, then you will have to apply it again and again, and who knows, it may change the tone color of the oboe.
3. Spread the stress points and reduce the density of the holes and post. The trill holes do not need to be next to each other right next to the post and right under the octave hole. On the other hand we, the players, have no option. I hope the manufacturers understand the physics and do something about this. They can also condition the wood to repel water especially the top joint as a part of the manufacturing process. Telephone phone pole and railroad tie manufacturers do it. Even some lumber woods are treated for long term. Why not the oboe manufacturers?

By the way, if any one wants to try lanolin, I bought one from Amazon. http://www.amazon.com/gp/product/B00028MLKC/ref=cm_cr_rev_prod_title

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Crack Prevention new

RobinDesHautbois

2010-11-18 10:31