The bronze disease on ancient bronze coins is a well known problem.
Many techniques were discussed, but only one solves the problem permanently.

What does it look like ?

Bronze disease looks like light-green powdery stuff on the surface, which sometimes simply falls off or is easy to loosen with a pin or to brush it off. If you remove it, it will grow again in a few weeks.

What does it cause ?

If the coin was exposed to soil which contains mineralic salts (chlorides), the salts attached became active when the coin was excavated. The humidity of the air is assimilated by the salts and this reacts with the bronze. The deadly thing is, that it fully 'eats' the coin if you don't stop it.

How to stop it ?

The method with sodium sesquicarbonate works very good, but the patina is damaged/removed. Another 'non-working' method is to heat the coin (a temporary solution -> the water is removed, which is assimilated again after the heating).

BENZOTRIAZOLE (BTA):

This is a professional way to stop it, used by restorators and museum preservers.
Dissolve the (solid) Benzotriazole in Ethanole. Ethanole roughens the patina slightly after some hours of soaking, but that effect is very minor.

The preservers of the roman Museum of Carnuntum in Austria use the Benzotriazole since 1987 with much success. They use a 6-7% solution and soak the affected coin about 1 hour - 2 days.

Prepare the coin before the chemical process ?

There is no need to remove the light-green powdery stuff. But it is better to remove most of the disease with a pin or a plastic stick or something similar. The solution penetrates better into the affected areas.

How to use the BENZOTRIAZOLE - ETHANOLE solution?

If your coin is suffering from minor bronze disease, soak it a few hours.
If your coin is suffering major bronze disease, soak it in the solution for at least 5 hours. There is no formula nor do I have long-term experience.

What to do afterwards ?

Rinse the coin in water and wipe it off with a rag to remove excessed benzotriazole. If you discover new bronze disease after some time simply repeat the benzotriazole process.

Chemical details:

BENZOTRIAZOLE = C₆H₅N₃
The benzotriazole does not remove the chlorides from the coin. It forms a insoluble, complex compund with cupric ions. This barrier prevents the chlorides to become active and cause bronze disease.

Benzotriazole is solid. Only use benzotriazole in the cleanest form. This is expensive, but has the best results.

Store the benzotriazole - ethanole solution in a light-tight vessel. Otherwise it will reduce.

Benzotriazole is available from the following sources:
artcraftchemicals.com
Should be available from major, local chemistry stores or major photo chemistry stores too.

LINKS:
http://www.bitsofhistory.com/info/bronze_disease.html

http://www.witdet.co.uk/sass/pages/conservation.html http://nautarch.tamu.edu/class/anth605/File12.htm http://www.digbible.org/restoration.html

ATTENTION !!! Benzotriazole is suspected of being a carcinogen. Don't inhale it. Wear gloves and eye-protection. Only use it in an AIRED room. Wash your hands carefully. Wipe off your coins carefully to remove all excessed benzotriazole.

There is no guarantee that this method presented in this page will work on every coin nor am I responsible for any sort of damages.

last updated: 18:43 GMT 23/SEP/2001