This month I would like to review an often misidentified corrosion product commonly referred to as “bronze disease.”
Regular readers will remember that “bronze disease” is a corrosion product and not a disease.
Thankfully, it does not spread from one coin to another, but occurs due to exposure to a salty environment. In this case, the salt is a chloride.
Bronze disease is most commonly found on copper alloy objects (i.e., either brass or bronze) that have been buried in soil that contains chlorides.
Therefore, collectors of ancients are most likely to encounter bronze disease. Bronze disease is a pale green powdery corrosion product that occurs in localized areas, looking somewhat like green measles. It is an insidious and unstable corrosion product.
Corrosion, particularly in a burial environment, is a complicated electrochemical reaction.
In a corrosive burial environment — where chlorides are present — copper-alloy coins will develop a number of corrosion products that appear as a hard crust, or patina, on the outside of the coin.
The crust is made up of layers of different corrosion products. Many of the copper-alloy corrosion products are stable and cause the coin no harm. Bronze disease is the exception.
Bronze disease is basic cupric chloride or paratacamite. Bronze disease results when nantokite, a waxy gray-colored copper chloride corrosion product and the deepest of all the corrosion layers lying closest to the original surface of the coin, is exposed to oxygen and moisture.
The chemical reaction is as follows:
Nantokite is stable provided that oxygen and water are absent. Once excavated, a buried coin can be exposed to sufficient oxygen and moisture for this reaction to take place.
However, some coins do not exhibit this corrosion product for many years because the patina can act as a hard protective crust keeping moisture and oxygen away from the nantokite layer.
If the patina’s seal is compromised through mishandling, cleaning or rapid temperature fluctuations, moisture or oxygen can access the nantokite and cause the conversion reaction.
When nantokite is converted to bronze disease, it exerts physical pressure on all of the corrosion layers above it, which eventually results in it breaking through the patina.
Unlike many of the copper corrosion products, bronze disease is not stable, and if left unchecked will result in total destruction of a coin.
If you have a coin with bronze disease, do not despair. It can be stabilized.
Since moisture and oxygen are necessary for the production of bronze disease, it stands to reason that excluding one or both would halt the reaction.
Creating and maintaining an oxygen-free environment is practically impossible. This leaves eliminating moisture.
Moisture can be eliminated by storing a coin at as low a relative humidity (RH) as possible through the use of silica gel. Silica gel is most effective when used in a microclimate. A microclimate can be easily made at home using a small, see-through resealable plastic container such as those made by Tupperware or Rubbermaid.
To create a microclimate, choose the container to suit the size of your collection or coin. Line the bottom of the container with a silica gel sachet.
Place your coins in holders in the chamber then close and properly secure the lid. Once created, a microclimate requires a commitment on your part to maintain it.
If the gel you are using is not the indicating variety, you will need to place a small hygrometer in the container to monitor the RH.
If the RH begins to climb, remove the gel and replace it with dry gel.
Editor's note: This version of the column was revised March 27 to correct an error regarding nantokite appearing in an earlier version.
Susan L. Maltby, Toronto, is a private conservation consultant, with an interest in numismatic preservation.