Bronze Disease

One my favorite coins in my collection is my Marcus Aurelius silver denarius, minted around A.D. 176-177. A week or so ago, I pulled it out and noticed that there were some real problems with it.

It appears to have "bronze disease," which is a serious affliction for bronze antiquities. This is sort of unusual for a denarius; I'm still sleuthing, but I think it's rare for a Roman silver coin from this time to develop bronze disease. Later in the Empire it's more likely because the silver is more debased. Apparently, at least for this coin, there is enough bronze mixed in with this silver for it to be a problem.

Bronze disease occurs when copper chlorides form in the presence of salt and moisture. It's kind of like a cancerous patina; the formation of new copper chlorides becomes a self-sustaining reaction and the metal starts to eat itself. The chlorides form from the bronze, and slowly (or sometimes, alarmingly quickly) spreads. The bronze turns to bright greenish or brownish powder.

I don't know how far it could spread in a silver denarius of Marcus Aurelius. I guess it's possible that, if the silver content is high enough and the coin is kept dry, it could burn itself out. However, I certainly don't want to test it to see, so I researched the problem and decided to treat it with a solution of sodium sesquicarbonate.

When I first heard of sodium sesquicarbonate, I thought, "Well, that's just one more weird chemical that I don't have at my house and that I probably don't want to fool with." Then I read a recipe for it and realized that I actually had the two ingredients: baking soda and soda ash. I just so happened to have soda ash because I had bought some to use to prepare fabric for tie dying. It's also used for treating swimming pools. Soda ash, therefore, is actually easy to find and pretty cheap.

So I mixed a solution of 5% sodium sesquicarbonate in water (I actually made it a little more diluted, more like 3 or 4%).

I used a soft paint brush to brush off the green corrosion, then used a straight pin to dust it out of the holes and crevices. It came off very easily, which is a sure sign that it really is bronze disease and not patina, though I had no doubts. Then I put it in the sodium sesquicarbonate, which is a very hard word for me to type, by the way, and I'll let it soak there for a few days.

Next, I will take it out, rinse it in distilled water, and then dry it for an hour at a very low temperature in the oven.

With luck, bronze disease won't be back. But if it is, at least I know what to do.

(photos: all the green that you can see on the coin is bad stuff)