BD? Somewhat fixable?

[Aarmale]

I recently bought this coin, and it seems to have lots of BD on it.  Some Bronze Disease seems to be on parts of the obverse as well as some of the reverse.

Is there any possible way to get if off without damaging (or minimally) the coin?

[daverino]

From the photo, your coin seems to have a genuine case of BD rather than just surface corrosion. BD is actually under the patina and it usually means that all you can do is arrest the process. The simplest approach is to dry out the coin thoroughly and carefully(200-250 fahrenheit - no higher) and afterwards keep it in a dessicated environment (silica gel packets). Other than that I would brush off any green stuff that comes away readily and let the rest remain.

You can also try some of the soaking methods that are used to get out chloride ions for a more permanent solution but these might not be necessary.

Regards, Dave

[Aarmale]

Thanks for the response.  This will be my first attempt in stopping BD.  I will try to get the after pictures when I'm done. 
How fast can BD spread?

[daverino]

The rate at which BD spreads is likely to depend on the exposure to moisture/humidity as well as the underlying condition of the coin but it doesn't wreck a coin overnight. After drying it out and removing the loose green stuff I would check back in a week to see if there are new green flakes sprouting. If there are, more treatment might be necessary. If not - good - but you still want to check on it from time to time

[Aarmale]

As promised, pictures.  Is it okay that the coin is still green?  I removed all the powdery green stuff and was left with this.
The coin is currently in a container full of slica gel packets.

[areich]

Maybe you could try Benzotriazol? It is a corrosion inhibitor and might be the best chance you have with this coin.
Of course BTA is a chemical that should be handled with care but it does work.

[daverino]

Hopefully this coin is not a very valuable type since the BD is far advanced. It appears to me that, having scraped off the fluffy green stuff, you see the underlying material of BD which is copper (I) chloride. Normally white in color it is turning green at the surface as air oxidizes it to green copper (II) chloride. So much for the chemistry lesson.

What can you do about it?

If you try scraping it away you will simply get down to the bare metal. In the first instance, keep the coin dry as you are doing since the copper (I) state is stable in the absence of moisture. Then you can try as Areich suggests. I don't know much about benzotriazole except that it has been used as a coating on museum objects to prevent or slow bronze disease. If you have access to it this might be a good place to try it out and let us know how it works. Other sealants such as wax to keep out the air might also be hepful.

You need to ask how much effort you should make to save the coin. I might try a soaking process with silver nitrate to stop the spread of corrosion but it won't undo the damage.

Regards, Dave

[Sosius]

Maybe you could try Benzotriazol? It is a corrosion inhibitor and might be the best chance you have with this coin.
Of course BTA is a chemical that should be handled with care but it does work.

Andreas,

Do you know where to buy Benzotriazol?  I have heard about it for some time, but I've never figured out where to get it.

Thanks,
Cliff

[areich]

I've bought it on Ebay before but right now there's only one offer from Japan. I've bought my last batch from a German chemical supplier.

[daverino]

You might try this link at Fisher Scientific if you are in the U.S.
http://www.fishersci.com/ecomm/servlet/fsproductdetail?productId=5389410&storeId=10652&langId=-1&catalogId=29104&distype=null&fromSearch=1&highlightProductsItemsFlag=Y

The compound is usually listed as benzotriazole and 50 grams sells for $22 although this is in  a high purity grade. It appears to be a non-toxic chemical so ordering it shouldn't be difficult. It likely has to be dissolved in alcohol before application but I am not sure about this.

[Sosius]

Thanks, Andreas and Daverino.