Posted by Chuck(G) on December 19, 2002 at 17:34:59:
After disassembling another vintage instrument, I discovered one more set of copper-plated brass valve pistons. I know that Joe S. says that this is a bad thing, but these looked pretty free of corrosion and were still pretty tight after several decades.
So I got to wondering...
The usual procedure for refurbing valves is to employ a coppėr "flash" coat, then to plate nickel. But it seems that while this is more corrosion-resistant than plain copper, it carries a penalty. Given that the surface of the casing and that of the piston will be rubbing together, it's the softer brass casing that's going to be worn away, more so than the hard nickel-covered piston. And it's the casing that's least replaceable.
With copper-plated pistons, the copper will wear more than the brass casing--and a new plating of copper is easy to apply.
But if a hard corrosion-resistant surface is a plus, why stop with nickel? Why not rhodium- or chromium-plate pistons?
If stainless or monel pistons are the cat's meow, why aren't valve casings made of the same stuff? My old Orsi Verdi tuba has nickel sleeves inside of the valve casings, so this can't be anything new.
I'm wondering if there was a good reason for copper-plating pistons in the old days. This particular instrument dates fromt he 1920's, where it seems that everything was nickel-plated, even my grandma's pot-bellied woodstove, so it can't be a matter of inaccessible technology.
Anyone got any thoughts on this?