Caustic soda turned intto black slime...

[Karl D]

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??

Thanks - I'm just curious....!

K

 

[Andrew Gabriel]

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.

 

[The Natural Philosopher]

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.

 

[John Rumm]

Soda diz gerizzmerzz
Happy new ear
To all the people living in fear.

Think I will have a pint of what he is drinking!

 

[Marcus Fox]

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 will dissolve in hydroxide (alkali) to form zincates [Zn(OH)4]2- and
[Zn(OH)3]-. It is amphoteric, and will also dissolve in acid, to release
hydrogen gas.

Marcus

 

[Marcus Fox]

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.

Marcus

 

[The Natural Philosopher]

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.