Re: dirt build-up on the reeds...

[JOHN THADEN <JJTHADEN@xxxxxxxxxxxxx>]

>Hmmm interesting thought John Thaden has.  If the oxide is brass and you 
>remove it won't the pitch change also?  Doesn't the oxide form uniformly 
>along the reed surface.  If it does why would the reed nesecarrily always 
>go flat?

Yes, when I remove it the pitch changes, always upward as I recall.  Even
if it forms uniformly along the surface, think of the demands made on the
reed at different points.

At the tip, the demand is that the reed weigh a certain amount; 
copper oxidation will probably more often increase the weight since it 
recruits oxygen and/or nitrogen and, unlike iron rust, forms a coating rather
than crumbles off.  With more weight, the pitch goes down.

At the "hinge", the demand is that the reed have a certain
strength and springiness; copper oxidation may increase weight there also,
but maybe it also disrupts the crystalline structure of the metal and thereby
weakens it.  At the tip, that has no effect.  At the hinge, it flattens the 
pitch.

>How much mass is really being added to the reed? Is it possible 
>that metal fatigue is a factor rather than oxides?  FJM 

Good question.  I'll let you know.  I'll clean a reed that broke off of a harp
till it is bright, weigh it, then let it corrode and reweigh it.  You may well
be right that the strength loss at the hinge -- corrosion-induced metal
"fatigue" -- is the dominant factor.  

Your response also got me hitting the books . . .
I'm no metallurgist, but after a quick scan of Merck Index, I'd guess that
the verdigris (boss word, huh?) that forms on both copper and brass (which
is mostly copper I think, with nickel?) is a mixture of blackish-brown or
black copper oxide (CuO), blue-green copper nitrate (CuN2O6), and the nitrate
tri- and hexahydrates (CuN2O6.3H20, CuN2O6.6H2O).  Also, if any electrolysis
occurs between copper parts in the presence of a salt solution like salive,
then red copper oxide can form (Cu2O), which can actually appear red, yellow or
brown depending on the crystal size.  It gradually oxidized to the black oxide
in moist air.

Both oxides are practically insoluble in water and alcohols.  The black is 
soluble in dilute acids, alkali cyanides (DON'T TRY IT), solutions of ammonium 
bicarbonate, and slowly, in ammonia.  The nitrates are readily soluble in water.

Acids will etch the uncorroded metal, so probably not great for cleaning,
but an ammonium bicarbonate soak sounds like a winner! 

Regards,
John Thaden