Making malachite on copper coins

I blogged before about cleaning some two pence coins with vinegar, and how they got a turquoise tinge after washing if we didn't rinse off the cleaning solution.  On one of the repeats of our two pence cleaning - which he remembers periodically and we clean a few more coins - I thought we could try and make a bit more malachite.  We'd seen some malachite on a recent visit to the Natural History Museum, and admired its lovely green/blue colour.

The two pence coin before we added salt and vinegar

We put a previously cleaned two pence coin on some kitchen roll, and sprinkled it with table salt (sodium chloride).  The small boy carefully squirted some undiluted spirit vinegar (acetic acid), rather than watered down vinegar we used in cleaning coins, onto the top of the coin.  We then watched as the salt crystals disappeared (dissolved) in the vinegar.  We left the salt and vinegar covered coin on one side whilst we cleaned the other coins, and - because of the general chaos of life with a toddler and a baby - we forgot about it until the next morning.

Malachite formed on the coin

I spotted the coin on the side about 16 hours later, sporting a brilliant bluey-green covering, and took it to show the small boy.  He seemed moderately impressed that the coin had changed colour, and he couldn't see the Queen anymore.  I told him that the chemical reaction had made malachite which was a different colour than the original coin, but we didn't discuss any more science (there's more on my previous blog post about the actual chemical reaction if you're interested).

All in all, an interesting addition to cleaning coins, and personally I think it's quite impressive that you can make something such a different colour with a couple of kitchen ingredients, but it was not a massive success as an activity to occupy a toddler.