Can a silver denarius get bronze disease?

I pulled out my Marcus Aurelius denarius a few days ago and was distressed to notice that it had a problem.

When I first got it, I noticed that there was a tiny spot of green at one edge. I figured it was a small bit of patina on an impurity, and it didn’t bother me. It seemed to stay like that for years.

But now that spot is larger, and the green is fuzzy! Even worse, it’s on a couple of other places around the edges of the coin. It reminds me of bronze disease, which I’ve seen photos of, but haven’t had in my collection--thank goodness.

Can a silver denarius get bronze disease? Is there a silver equivalent? Can I treat it with the sodium sequicarbonate solution that I’ve seen mentioned on few sites for treating bronze disease?

This is my favorite coin in my collection, any advice would be really appreciated.

 

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Sounds like you might have a fouree with a previously unexposed copper core. I wouldn't think that sodium sesquicarbinate would do anything to harm the silver layer, but you might want someone with more experience with this sort of thing to confirm.

 

Ive got some that have copper exposed. I was told one could use pure lemon juice but it may leave pits, so I never bothered.

 

Do you have any photos?

If the coin has impurities it can.
I had a double denarius of Gordian III get bronze disease, and had to treat it.

 

I'll see if I can get a decent phot of it tonight.

 

Remember that later denarii are less that half silver. I have seen the disease on Severan but not on Aurelius though I can't say it would not happen.

 

I agree they can get bronze disease, and it can be a severe problem. The main problem is if you treat it is can treat underneath the silver layer potentially and dislodge the entire surface. If the coin is a fourree this is more of a concern, and I have not seen this problem on a MA, but have seen it on a Gordian III coin that I do not believe was a fourree.

Myself, I would not treat it in anything liquid, and would be more tempted to bake out the moisture on a silver coin. This shouldn't darken silver like it could copper. However, please do not take anyones advise here and have a professional look it over.

 

Here’s a photo of the coin; I’m sorry it’s a little blurry.

In the photo’s orientation, you can see the green in the 8 and 9 o’clock positions. There is also some in the 12 o’clock position, but it’s mostly hidden by the glare. All of it is in the rim nicks. None is visible on the reverse.

I wonder if it’s possible to spot-treat it with a fine bristle brush—a solution of baking soda and soda ash applied repeatedly to just the affected areas?

 

Terminologies can be a problem. 'Bronze Disease' is a form of green corrosion that is soft and powdery but, most significantly, produces hydrocloric acid by products that greatly speed the corrosion. Unlike Patina that stays on the surface and protects the coin from further deterioration, bronze disease penetrates into the coin and eats away sections rather like termites eat wood. Bronze disease requires moisture to work so it is more a problem in humid climates and some people slow it by baking the coin in a warm oven long enought to drive water out of recesses. In general, bronze disease is a lighter green than other patina forms but the significant feature is it being powdery. It is not hard to brush it away but it returns in a short time due to the remaining traces of chemicals and moisture in the air. This site has about as much as I have seen on the matter which is less than fully understood:
http://proteus.brown.edu/greekpast/4867

I do not currently own a coin with active disease but I'll attach a coin treated well over 20 years ago that has remained free since. In effect, it is a one sided coin now but considerably better than it would have been if left as it was. A coin has to be very special before I will consider buying it with even a small spot of the active problem. It is not always obvious what lurks below the surface. I have never heard the term 'verdigris' appled to this prefering that to mean any generic green rather than this specific chloride. Does the situation described on that link match what you call verdigris?

 

The first thing to do is to remove the coin from that holder and pick gently at the green with a straight pin. If you find the green is hard and does not move with a very little prodding, it is not disease and you should do nothing. If the whole mass disintergrates into a powder, it is most likely something to be addressed. Looking at the photo, I am tending to suspect it is not a problem but good photos can be deceiving and this is a long way from being a good photo. (Tough love time: If you don't want a photo bad enough not to take the coin out of a cardboard flip, you don't want the photo bad enough to take the picture.)

 

Oops! I lied. I checked a coin that was treated about ten years ago and discovered the disease is back. The coin as it existed this morning is shown at the top with the right inset just being an enlarged section showing the green. At the bottom left is the pin point and results of touching the powder, Notice that it scattered and a bit even adhered to the pin. Now I have to get the chemicals and go through that again (hopefully with better results).

BTW: Some are wondering why anyone would want to preserve such an ugly denarius. It is your right to hold that opinion and I will discuss it with you only after you find any photo of this exact coin (both sides used together) or even a listing in a catalog. Think of it as a research challenge.

Julia Domna, Alexandria mint denarius, IVLIADO MNAAVG / VENVS FELIX

 

The green stuff is powdery; I was able to make it crumble by simply touching it through the plastic.


Also, I’m sorry, Doug (Smith), you are right: it is a lame photo. However, I had said I would try to get a photo up as soon as I could, so despite the fact that it was a cloudy, gray day (too dark to get a good photo) I took it outside to get a quick photo. Since now I’m feeling paranoid about it, I didn’t want to take it out of its holder. This wasn’t a very conscious decision and if I had really been thinking about it I’d have taken it out to get a better shot.

I’ll do better next time, I promise!

 

Silver coins are touchy. I would be very careful using metal on the coin to scrape it off. Most metal will be harder than the silver. There used to be coin cleaning companies selling silver brushes and silver pointers for use on silver coins, (point being the silver will scratch less than brass would, and not leave brass colored markings on your silver coin). Short of finding those, maybe find and AC supply place and buy a stick of silver solder and cut it down for use to scrape this off. I keep a stick of silver nearby and sharpen it on a stone before use.

I still think 200 degrees in the oven would help stop it by drying it out after you scrape it off. I never recommend this for copper as it can discolor the coins, but silver shouldn't.

 

Below are two ancient coins (both orichalcum dupondii of Nero). The first shows what I call toning. The brass is darker yellow but the bright metal is only a few molecules below the surface (except for a few places with extra dirt) . The second has 'green patina' which is considerably thicker and hard. This one has been smoothed in cleaning. Some 'ancient guys' (especially dealers) use incorrect terms including 'patina' and would call the yellow one 'Tiber Patina' and a coin caked with hard dirt 'Sand Patina' but, technically, ancient patinas are like the green one here. These uses are rather like the metal 'German Silver' which has no silver in it at all but sounds good when selling.
http://www.pbase.com/dougsmit/image/115368667.jpg
http://www.pbase.com/dougsmit/image/111658120.jpg
I am unclear on how old a bronze coin has to be before it can have a natural green (or blue or red) patina. Perhaps metal detectorists know that one and can provide an image of a modern coin with patina. My guess would be about 1000 years but that is just a guess.

 

Here are better photos.

You can even see the green forming on the face of the reverse. Pretty much all of the crevices on the rim of the coin have it.

This seems really weird to me.

 

I just reviewed my older photos of this coin and realized that there was more "green" in evidence when I first bought it than I remembered. It has, however, become worse--a little. So I think this weekend I'll spend some time with a small brush and a fine point tool and get the crud off; soak it in distilled water for a couple of days; and then bake it to get the moisture out.

Then I'll make sure I have some silicate gel wherever I end up storing it.

 

After treating bronze disease, wouldn't it make sense to dry the coin completely and put it into an airtight container?