"
Zinc works well for a spark gap material, providing the physical size
and design are good, for the power you plan to run. It probably isn't
so effective that it would pay to go out of your way to locate a big
chunk for the purpose. I used a couple of 3/8" round pieces with good
results at 350 watts."
In the "good old days" prior to WW1, zinc was a preferred material for
spark gaps. (At up to 1 kW or so.) Does give a very nice blue color.
Ed