Colourful Coins...

You will need:

Saucer
Paper towel
Vinegar
A few pennies (or other foreign coins with a copper coating)

What to do:

Fold the paper towel in half a couple of times and put into the saucer. Pour on enough vinegar to cover the paper towel.
Place the coins on the wet paper towel and leave for a few hours.
What has happened to the coins? Pick them up and compare the sides exposed to the air with the sides next to the paper towel.
Wash the coins and your hands carefully when you have finished the experiment.

What’s going on?

The coins go a blue-green colour because the vinegar (aka acetic acid) reacts with the copper to make copper acetate.

Copper acetate has been used since Egyptian times as a green pigment called verdigris. It is the most common green pigment found in medieval manuscripts, probably because it is so easy to make. The scribes or artists simply had to place some copper strips in vinegar and then scrape off the green powder when it had formed.

You may see less verdigris on the bottoms of the coins. This could be for two reasons. One is because the reaction needs an oxidising agent. In this situation the oxidising agent is oxygen from the air. The bottoms of the coins are right next to the wet paper towel and don’t have as much oxygen around them, so more of the copper will stay as it is.


And more?

Most copper coins go dull after a while as the copper becomes oxidised by the oxygen in air. Leave dull copper coins soaking in tomato ketchup. The oxidised copper reacts and the product goes into the tomato ketchup solution, so you are left with unreacted shiny copper.

You may get a surprise when you see the real colour of the copper coins! To be certain this colour did not come from the tomato ketchup, try using brown sauce instead – do the coins go the same colour as they did in the ketchup?

Note:

Copper acetate can be an irritant and is harmful if inhaled or swallowed, so make sure you wash your hands (and the coins) very well.

Also, technically it is illegal to ‘deface coins of the realm’ – and that’s why we suggested using foreign coins. (We don’t want anyone arrested for trying out our experiments.)