Gap materials (acorn nut warning)

Here's a late tidbit of information relating to the now-expired thread on
gap materials.

For my RSG rotating electrodes, I used 10-32 brass acorn nuts.  The arcing
surface rapidly became black and pitted, and I assumed that this was the
normal wear and tear to expect.  I had bought several more nuts than I
needed, and I later noticed that a couple of them looked slightly different.
The ones on my rotor appear to have been cast, while two odd ones not used
had machined surfaces.  It turns out that the cast ones are in fact made of
zinc or some similar alloy (I'm assuming this as this is the only
non-magnetic alternative I can think of), and copper or brass plated.  The
machined ones are solid brass.

Lessons here are:
1)	Zinc is a poor electrode material in that it wears quickly.
2)	When shopping for acorn nut electrodes, bring a file to scrape the
tip with.  Don't assume that if it's non-magnetic it's solid brass.

Regards, Gary Lau
Waltham, MA USA