Lets not bother with the warnings about not cleaning a coin or leaving it to the pros, we know this is a given. This coin is very nice but it is, without doubt, in need of first aid. I believe it has bronze disease on the reverse and it has a generally dull and dirty look. Certainly we can help this coin.

I have soaked it in a bit of distilled water for a bit with a pat dry and I have taken a swab to the green, I think its powdery which makes me think bronze disease. Even if it is patina (which it isnt) I wouldn't want that either...

So what would you do with this coin if it were up to you to eliminate the bronze disease and possibly a light, non-intrusive cleaning.

To be honest, cleaning is not near as important as addressing the green, I like the look of the rest of the coin for the most part.

Only respond if you know and have treated a coin like this. I have ideas of my own of course but I am looking for a definitive plan of attack. I want to as little harm to the color and existing toning as possible, just light dirt and disease removal. I think a nice soak in oil might do it good but first I want to address the green.

Hmmm... I've never heard about succesful removal of verdigris.

Maybe you could try and use pure acetone (nail varnish remover). Denpending on how long the verdigris has been on, there might be damage below it. However I think acetone will remove the tone aswel. You could dry applying acetone directly on the verdigris with a cotton bud then rinsing it off.
I don't clean coins but I have heard others swear by it. Don't leave it on for to long. Perhapse 8-10 seconds then rinse off using distiled water.
I too don't like cleaning coins, however if you leave the verdigris on it will eat away and probably do more harm to the coin.

Edited to add acetone might not work but its worth a try unless others no of better ways?

You can remove the green powdery layer with two substances called EDMP or EDTA. These will clean the coin but will also remove the patina eventually. EDTA is often used on excavations when metallic finds arise. With a qtip you could apply EDMP on the are and thus control the exposure of the patina to the substance. I am still working on that "technique".

There is a substance you can use after cleaning the coin, which is called Benzotriazol or BTA for short. This is an inhibitor for any further oxidation in form of bronze disease. Unfortunately it is also a suspected carcinogenic, so only use with mask, glasses and gloves, preferably not in closed spaces.

Acetone will probably do very little, besides remove the visible bronze disease. But if you treat that with BTA after cleaning the visible Bronze disease, you might give the coin some more time, before you have to use BTA again. I think the estimated time for that is around three years.

I have a little coin here that I am trying to cure with EDMP and BTA... BTA mostly, as I want to keep the patina. Still working on it and still the bronze pest comes back. So, if you are really willing to get this coin healthy again buy yourself some patience first of all. Mine is nearly running out.

A Novel Method for Conservation of Copper-Based Artifacts
M. C. Ganorkar, V. Pandit Rao, P. Gayathri, T. A. Sreenivasa Rao
Studies in Conservation, Vol. 33, No. 2 (May, 1988), pp. 97-101
doi:10.2307/1506307
This article consists of 5 page(s).


Abstract
A new method was developed for removing bronze disease and for the conservation of ancient copper and copper-based artifacts. An organic compound, 2-amino-5-mercapto-1,3,4-thiadiazole was found to be a good inhibitor. It removes the bronze disease completely and forms a protective polymeric complex layer on the surface of the metal artifacts, which is resistant to corrosion.

A Novel Method for Conservation of Copper-Based Artifacts
M. C. Ganorkar, V. Pandit Rao, P. Gayathri, T. A. Sreenivasa Rao
Studies in Conservation, Vol. 33, No. 2 (May, 1988), pp. 97-101
doi:10.2307/1506307
This article consists of 5 page(s).


Abstract
A new method was developed for removing bronze disease and for the conservation of ancient copper and copper-based artifacts. An organic compound, 2-amino-5-mercapto-1,3,4-thiadiazole was found to be a good inhibitor. It removes the bronze disease completely and forms a protective polymeric complex layer on the surface of the metal artifacts, which is resistant to corrosion.

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


this might be worth a try

eh i had some aussie pennies that were green i just took my shirt and wiped it and it came off!!!

lol

Blue ribbon coin conditioner and some soft cotton Q-tips will get rid of stuff like that. If you don't like the finish on the coin when you're done, dip it in acetone to take off the gloss.

That is the substance I meant, Drusus. Try to find it already dissolved, because - if prices are there as they are here - ethanol by itself will be quite expensive. I bought my BTA from this German online shop.

And this is their choice for copper coins

My poor byzantine coin has been proving very stubborn. After talking to the person owning this shop, he recommended leaving the coin immersed in the BTA for a day. After 3 weeks small green spots are appearing again. A Jersey penny from 1911 tho, that had the same problem and I treated with EDTMP and afterwards BTA has not had bronze disease since. But it also has no more patina.