Marcus Fox wrote:

"The Natural Philosopher" wrote in message
...

Karl D wrote:


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??

Mezac probably is the metal, and that would be summat like zinc hydrozde
or oxide perhaps.

Use an acid to cleam it of. Star with vinegare, work our way up thrugh
decsalers and bolied rhubarb leaves to brck acid.


No. Zinc dissolves in acid and releases hydrogen gas.


Yes, but slower thn the hydroxides and zincates will.


The idea is to clear rhe sludge and etch teh actual metal a bit to
brighten it up.

Ive used this trick on brass. Works well.



Marcus