[TheForge] ( bronze "silverware"

[Bruce Freeman]

Peter,

I think you've got the right idea.

The problem is not metallic copper.  That's pretty inert.

The problem is the copper oxides - the tarnish.  Copper tarnishes
pretty quickly in air, especially when damp.  Most of this tarnish is
the oxide, but other salts can form as well.  Copper salts are readily
soluble in mild acid (like vinegar, especially in the presence of
chloride, e.g., table salt).  If this gets into your food, you're
taking a chance you needn't take.

Someone else posted that a toxic dose of copper would be about 10 mg.
I do not know that to be true.  However, it may be well to communicate
just how much volume 10 mg corresponds to.  Think of an average water
drop from an eye dropper or such.  That's about 1/20 mL = 0.05 mL =
0.05 g = 50 mg.  I would guess that a copper salt will weigh 2 to 3
times water, so that that same volume of a copper salt (solid) would
be 100 to 150 mg.  So a volume corresponding to 1/10 of a drop of
water would be the toxic level, if all these assumptions are true.
That IS a lot to get off a teaspoon, so this is probably not a
tremendous worry.

However, why live dangerously?  Put a couple ounces of vinegar into a
cup, add a sprinkle of table salt, and stir with the copper teaspoon.
Voila, the copper will dissolve in the acid.  Rinse with fresh water
and use the teaspoon for anything you like (except, don't leave it in
the food for any prolonged period because it could tarnish while in
use.)  Repeat this procedure before every use and you're safe.  You
can even reuse the salted vinegar, which eventually will turn blue
from the copper salts.

I could have approached this from the other direction and allowed you
to estimate the copper coming off the spoon into the salted acid, but
that would require me to look up extinction coefficients, etc., and
it's too late at night to do that!


On Wed, Dec 30, 2009 at 2:45 PM, Peter Fels & Phoebe Palmer
<artgawk at thegrid.net> wrote:
>
>
> Knowledgeable opinions please;
>
>
> Some friends sent me an stylishly elongate, red copper alloy, teaspoon
> from their dining room set...with the question;
>
> Are they safe to use?
>
> They were made in Thailand, where such " silver" is common, they said.
> My first reaction , was to say "no", especially don't leave it to soak
> in acidic food.
> They have a new baby and copper oxides can be pretty nasty.
> Sure would appreciate any more informed advice on this.  pf
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-- 
Bruce
NJ