Help! My Coins Have A Fungus!!
The Dreaded Bronze Disease.
What is Bronze Disease?
Treating It
Preservation and Storage
Cautions
So you say your ancient bronzes have a fungus and you don't know what to do about it?
What looks like a 'fungus' is known as bronze disease. It is not a fungus or has any biological component at all. Although, for years people thought that bronze disease was caused by bacteria because heat seemed to cause a remission in the symptoms.
Today, we know better...
It is a reaction of cuprous chloride, which may be present in any ancient bronze coin, and water. The water required to start the reaction is not much, a normal humid day in many parts of the USA is considered quite enough. Once the reaction starts, cuprous chloride turns to cupric chloride and hydrochloric acid. And, of course, hydrochloric acid on anything is not a good idea.
Once started, a chain reaction can occur with the hydrochloric acid eating into the coin and possibly exposing more cuprous chloride. Which in turn reacts with humidity to form more of the cupric chloride/hydrochloric acid pair, and so on. If not properly removed, it will leave your coin looking like it was attacked by a bynch of mutant bronze-eating moths.
From a distance, bronze disease looks like a bright green or blue-green, fuzzy or powdery patch on a bronze coin. While the exact color may vary, the fuzziness is the key to determining if your coin has bronze disease. If it is fuzzy, it is bronze disease. If it is hard, then it may be safely ignored.
It should be noted that bronze disease will not spread to other coins in your collection, provided
the infected coin is in direct contact with others.
The Augustan As (above) shows the early stages of bronze disease. It lightly coats areas of the
obverse, and has much smaller patches on the reverse. At this point the bronze disease is at the surface and has not eaten very far into the coin. You may notice that in the early stages you can wipe the fuzz off with your fingers, but doing so will not help you because the cause of the problem is below the surface of the coin.
This poor Hadrian As has a rather advanced case of bronze disease. Notice how it is slowly spreading across the obverse and the reverse has a fair amount also. If I were to remove the bronze disease, you would see severe pitting in the areas where it existed. The red boxes show you where the close-up
images are so you can get a better idea of what it looks like up close.
This Domitan As has a minor case of bronze disease and may be typical of what you will normally see.
The bronze disease is not very visible when you look at the coin from a normal distance. In close-up
you can see where is is hiding. Most of it, particularly on the reverse, is scattered and confined to small pits or cracks in the flan surface. The cracks are very small and would resist mechanical cleaning attempts.
Treatment:
The first thing to consider is that treating bronze disease can be a long process. There is no quick solution to its removal, at least none that leave the coin relatively intact. So, if you want to do a proper job of it and make sure it does not come back again, be prepared to do a lot of waiting.
So settle back and be patient.
Heat:
Heat will temporarily neautralize bronze disease by removing the water that is needed for the reaction. Simply bake your coin in the oven at 250 degrees for 30 minutes to an hour. This is not a cure since as soon as the coin cools down, the moisture in the air may start the reaction again. At best, heating a coin will temporarily stop or slow the advance of the disease.
Note: Heat may change the patina color. I have noticed that heated coins tend to be darker after
a treatment. You may find this to be a good thing. but you should now ahead of time that it may happen.
Mild Cases:
For mild cases, you may want to try a water soak in distilled water. This may seem to be counter-intuitive since moisture is needed to start the reaction in the first place, but it does work. (Do NOT use tap water. The chemicals in tap water could actually increase the bronze disease.) Over time, distilled water will attract the chloride ions (bronze disease) and gradually
reverse the effects. As the water becomes saturated with chloride ions, the effectiveness will be reduced. So be prepared to change the water periodically.
There is no hard and fast rule to how often to change the water and how long the process will take.
A rule of thumb would be to change the water every few days to a week. You will want to periodically
check the condition of the coin by drying it off and giving it a good examination. Repeat as needed.
Tough Cases:
Sodium Sesquicarbonate (Thanks to Charlie Karukstis for the details.)
Place the coins in a glass container and fill with a 5% solution of
sodium sesquicarbonate. Let them soak for about fourteen days, replace the
solution, and soak for another fourteen days. Then, place the coin
in distilled (or deionized, it truly doesn't matter) water for about a
week. Stabilize with Renaissance Wax or Incralac.
(1) Don't have sodium sesquicarbonate lying around? You can make it
with equal molar amounts of sodium carbonate (Na2CO3 - also called soda
ash) and sodium bicarbonate (NaHCO3 - yes, you know what that is). For
example, a 5% solution would then be 10.6g of carbonate and 8.4g of bicarbonate
in 100ml of water.
(2) a 5% solution WILL REMOVE any "patina" on the coin! If there is
an exceptionally aesthetic "patina" to be preserved, try a 1% or 2%
solution. Be warned, though, that it will take three times as long, and has a
higher risk of being ineffective.
Benzotriazole (Thanks to Hannes Mayer)
Dissolve benzotriazole in ETHANOL (6-7% solution) and soak the coin from 1 hour to 2 days.
Water may be used and has the advantage that it doesn't roughen the patina as ethanol does.
But, it has been reported that it does not dissolve well and easily. See The Captains page
at
http://www.captain.at/roman/index.php?p=disease&f=1 for more information.
Warning: Benzotrialzole is suspect of being a carcinogen.
Preservation:
The jury is sharply divided on whether it is a good idea to seal a coin after a treatment for bronze disease. The problem is, if the removal was not complete, bronze disease may re-occur at any time
from days to years later. And if the coin is sealed it may make it that much more difficult to treat the next time.
If you intend to seal your coin, try using a product like Renaissance Wax which is a microcrystalline wax used by the British Museum for cleaning and conservation. There are other products, such as Incralac that may be used, but they are more difficult.
Cautions:
When using chemicals, always follow the manufacturers instructions for use and in mixing solutions. Many of the chemicals you will use are harmless, but for safety sake always read and follow instructions.
Follow the usual precautions of using gloves, not breathing vapors and keep the liquids away from your eyes.
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