Re: [Harp-L] Treating reeds for stress




----- Original Message ----- From: "Cliff Hall" <12barz@xxxxxxxxx>
To: <harp-l@xxxxxxxxxx>
Sent: Wednesday, September 24, 2008 10:22 AM
Subject: [Harp-L] Treating reeds for stress



Could somebody explain.......... what the "stress" in the reeds actually
consists of? Is it some kind of difference in molecular arrangement? What
kind? Is it measurable? So far, it seems to be relieved by baking but
also by freezing. I wonder if playing Mozart to the reeds might help? Or
maybe some "smooth jazz".


Cliff

Stress is the force-per-unit-area that tends to stretch the convex surface and compress the concave surface of a deflected reed. Stress in a vibrating reed is necessary to its function as a spring and is bad only if it is excessive. Tuning notches can cause stress to be excessive by causing it to be concentrated in a small area.


Because reeds are very thin and because they are free to take any shape, I would expect the amount of residual stress to be very low.
For this reason, I would not expect that heating them to low temperatures would affect their life.


I don't think that you can draw valid conclusions about reed life from just a few reeds. I think that you would need to campare large numbers of treated and untreated reeds by blowing them on a machine, measuring the time-to-failure of each one, and then doing a statistical analysis.

Copper alloys (brass and bronze) do not respond to heat treatment the same way that iron alloys (steel) do. The properties of brass and bronze are changed by work-hardening which breaks the metal crystals up into smaller pieces. Most brass and bronze springs (good springs make good reeds) are deliberately work- hardened by rolling. This is not a bad thing.

If you get a copper alloy above a certain temperature, it will anneal. That means that the larger metal crystals re-form. This is normally done to increase its ductility. Ductility is the ability to flow in a high-stress plastic state without breaking. Pure gold has high ductility and glass has almost none.

Shell casings are annealed to increase their ductility, not their strength. When the gun is fired, the ductile brass stretches/flows to allow the forces to be absorbed by the gun chamber. Firing the gun work-hardens the brass and makes it more likely to split the next time in the gun.

For the above reasons, I posit that annealing a harmonica reed will not improve it and may very well harm it. Heating to temperatures below the annealing temperature isn't likely to do much at all.

I blame impurities in the metal and stress concentrations at tuning notches for premature reed failures. Failures after long use are probably metal fatigue. Stainless steel is most likely less subject to fatigue than brass and bronze.

For the above reasons, I would never cook my reedplates.

Vern
Visit my harmonica website www.Hands-Free-Chromatic.7p.com






This archive was generated by a fusion of Pipermail 0.09 (Mailman edition) and MHonArc 2.6.8.