On Thu, 27 Apr 2006 11:06:05 -0400, Andre Langevin wrote:
I have found this:

5% Cupper sulfate CuSO4 is a good solution for that, where it forms black
color with zinc and red with steel.

So i will test with some on a cotton swab and check which color it turns,
black is zinc.

You're aware, I hope, that gas tanks that are "empty" can have enough
residue in them to stay explosive, sometimes for years, right? I went
to an emergency call a few years back where a fellow was cutting an old
gasoline tank, had been empty for as long as anyone could remember.
Took him a couple days to die of his injuries. This isn't the time for
random experimentation. Take it to someone who knows how to handle it.