Re: [Harp-L] Treating reeds for stress

["Vern Smith" <jevern@xxxxxxx>]

----- 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
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