Bronze disease appears as a fluffy blue/green powdery growth on bronze or copper alloy based coins. It is relatively rare, but can be very destructive. Generally, if you cannot rub off the powder, it probably is not bronze disease.

Bronze disease only occurs when coins are kept in conditions with relatively high humidity. Cuprous chloride combines with oxygen and water in the air to form hydrochloric acid which attacks the metal.

If you suspect you have a coin, or coins with bronze disease, act quickly, as it can be very destructive. The following should not harm unaffected coins, so if in doubt, and treat coins as affected.

Firstly, remove as much of the powder as you can, but as carefully as you can. Use your fingers, finger nails or soft wooden picks.

Soak a small number of coins together in distilled water, changing the water every day for at least two weeks. The chlorides are slowly removed into the water, so regular water changes are very important.

Bake the coins in the oven for a couple of hours to remove all moisture. If you are unhappy about doing this, dry the coins on a sunny window ledge, but ensure that they are completely dried!

If you still see any powdery green, repeat as above. If not, you can further protect the coin using Renaissance wax.

One final point, at some point the coin has been exposed to a humid environment, and if this was while you owned the coin, you will need to ensure that this and other coins are not similarly exposed in the future.