Ask A Scientist© Chemistry Archive

name         Winifred
status       other
age          50s

Question -   We have several quart containers of coins covered 
with residue.  These coins were at the bottom of a water fountain and were
given to a charity; not for a collection, but to cash in for face
value.  Do you have an easy solution to removing the residue? 

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Ms. Johnson,

Since you did not describe the residue, I shall assume it is either algal or
hard-water scale. Swish the coins around in Clorox solution. That will 
kill any
algae residue. Follow that treatment with a swish in hot water to which a 
bit of
dish detergent has been added. Rinse with plain water.

If the residue is from hard water you can remove the scale by rinsing the 
coins
in (full strength) white vinegar followed by plain water.

Regards,
ProfHoff
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Well, I cannot say what will work for sure because I do not know the chemistry
of the water or the composition of the residue.  Chances are that the
residue is scale from hard water, which will mostly comprise calcium and
magnesium carbonates.  The best remedy for that would be to soak the coins
in white vinegar for 24 hours or so, and then to rinse them thoroughly.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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