Advice about Bronze Disease Treatment Problem

[Anthony P]

Dear All,
Can anyone help or advise? I have a nice sestertius of Nerva. A year ago I noticed bright green powdery spotting on the obverse. I took this to be bronze disease (see photo of coin 'Before'). When I looked more closely it also appeared around the profile of the face. There was also a strange smear of mark to the righ of the face near the legend which I thought might be caused by previous cleaning. The disease seemed to spread quickly and so I tried to arrest it. I wish now that I had simply 'baked' the coin on a low heat in the oven but decided, instead, to stabilize the coin by soaking it in distilled water which is recommended on several good websites. After only two days, however, large green patches have developed across the coin, including the area that appeared to be streaked (see photo of coin 'After'). I am at a loss to know what is happening or why things have turned out like this. Have I ruined the coin? it looks like it. I thought distilled water was relatively inert? Or has it drawn out some latent or dorment bronze disease that was dormant?

Can anyone advise about i) what process is taking place? Is this normal when soaking in distilled water? But also, more importantly, ii) how I should proceed: should I stop the distilled water soak now and cut my losses? Or should I continue with the distilled water over a longer period? I just don't know what to do for the best and am out of my depth.

If any one has any suggestions I would be very grateful to hear them.

Anthony P.

[PMah]

There are two excellent articles on "Bronze Disease" on the Numiswiki section of this website.  I dare not summarize them, but I suggest comparing what you did to what is recommended in those articles.  It is a topic than engenders a lot of "home remedies" in open-ended posts, but, as this forum can say, the page on the wiki is constantly being referred to as the core information.

[Jay GT4]

I agree with looking at the numiswiki pages,  but you did nothing wrong.  The distilled water just "pulled" the bronze disease to the surface.  It was probably treated in the past and then covered over with some kind of fake patina.  It's a nice looking coin and it can be saved, but patience is key.

[Anthony P]

Thank you to both of you for taking the time to read my post and make suggestions. I will read the articles. Thank you, too, for reassuring me that I have done nothing wrong - that was my biggest worry. I always wondered about the patina and if it might be fake. OK - patience, as advised! Thank you!

[Jose N]

I have neglected my roman copper and bronze for many years and been lucky with them. only minor corrosion in a Commodus sestertius. But I feel for people dealing with bronze disease in ancients. Along with fakes it is one of my fears when looking at expensive ancient copper.

On medieval coins I have had good results with the method described in the wiki. Only difference is I never boiled the sodium sesquicarbonate bath. Sometimes it took multiple baths over weeks but there was no hurry.

With that level of oxidation in your coin expect a brown oxide layer to form while in the bath. Remove it after fishing out the coin. It comes off just with a gentle scrubbing. It feels like a kind of slime.

[Severus_Alexander]

Completely agree with the advice provided by others on this thread.   If you collect long enough and collect ancient bronzes, you'll more than likely encounter bronze disease.  I had a few coins that I found with a few spots and following the steps and was able to successful treat them.   For costly or special coins you can also have different professionals do the treatments if you prefer not to try them.   Don't be too hard from yourself and it can help you to learn to evaluate how the coins are stored and if necessary make change to their conditions.   For example, if you store in high humidity situation, addressing the humidity issues can help.