Numismatic and History Discussions > Coin Photography, Conservation and Storage

Seeking advice on bronze disease

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Robert L1:

Hello- first time poster here,

I purchased a nice bronze coin of Carthage recently and received it on the 3rd (I've noted dates to give an idea of when the following images were taken).

I kept the coin in a flip and didn't pay much attention until the 7th, when I noticed bright green patches on the top and bottom edges of the coin (unfortunately, I did not get images of the initial bronze disease). The green was very powdery and came off easily and I recognized it as bronze disease.

While I was figuring out how to treat it, I placed the coin in distilled water. On the 9th/10th, I took the next set of images after taking it out of the distilled water. I noticed several red/orange "rust" colored spots, which seemed to be spreading with each passing day. I got baking soda & washing soda involved as is commonly discussed, and started soaking for 24-36 hours twice. I dried the coin in an oven at a lot temperature and sealed it in an airtight container and observed for a few days, as I did not want to seal it with wax only to have the bronze disease break out again.

Unfortunately, on the 16th I noticed the green spots again (last image)  and resumed the baking soda & washing soda treatment. At this point I'm not hopeful that I'll save the coin and I'm trying to use this experience to learn about treating bronze disease in the future.


My biggest questions are:

What is the orange "rust" that gradually appeared across the coin? Is that part of the bronze disease spreading?

What should I do at this point? Should I write this one off, or could it benefit from more soaking & drying with the baking soda & washing soda?

I'd appreciate any input or advice

Thanks,

-Rob

SC:
Welcome Robert,

Here is what I think is happening. 

Your coin has bronze disease and has had for some time. The powdery green stuff is active bronze disease - it is a form of cuprous chloride that will react with oxygen and moisture in the air to create hydrochloric acid which will transform more of the copper in the bronze into cuprous chloride.  Thus the cycle will continue until arrested - especially when the moisture is eliminated.

The red-brown bumpy stuff is is a form of cuprite left over once the bronze disease process ceases.  It is common for coins with active bronze disease to have these red-brown spots in patches or blisters. 

However, I don't think the red-brown is spreading.  It looks more like both more cuprite patches and much normal almost-bare copper is being revealed under a disappearing patina.  As you are using nothing but DW I suspect that the black patina must be artificial. It is quite rare for a patina to disappear from only DW - though some rare thin and brittle ones will.

I would send some time reading past threads about BD treatment.  I would then start with the simplest stuff like removing as much of the powdery green as you can (preferably all of it) with toothpicks and cut-down toothbrushes.  I would then try the baking approach to lock out the moisture and either leave it out - i.e. in a coin tray - or put it in a new clean mylar (i.e. archival safe) flip.

SC

Ron C2:
Bronze disease does not progress that quickly.  A coin does not go from patinated to bright copper color in a week just from having BD. 

I'm not sure if there are other reactive ingredients in whatever type of washing soda you used or not, but I think it very likely the coin had artificial patina on it, and had previously been chemically cleaned.

Interestingly, a common (and frowned upon) way of cleaning very soiled bronze coins was to soak them in a lye bath.  This turns coins the exact color in your last photo.  That said, lye would have neutralized and acidity on the coin as it's a strong base chemical. 

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