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In article ,
(Karl D) writes:

I recently obtained a cast metal ashtray that looks like it is cast in
zinc. It'ss actually an admiralty ashtray from HMUDE (Her Majesty's
Underwater Defence Establishment. It had some stains inside it so I
sprinkled some caustic soda crystals in there (since they just
happenned to be at hand) and topped up with water. 30 minutes later, I
was astonished to find the thing full of black slime! Some sort of
chemical reaction, I'm guessing.... but what? Anyone here know enough
chemistry to tell me how/what/why??

Zinc is amphoteric, which means it will dissolve in both acids
and alkalis. If it is zinc, then you created sodium zincate.
If it is aluminium (or an aluminium alloy -- some look like zinc),
that is also amphoteric.

--
Andrew Gabriel