If you are a collector of ancient coins, you may have heard of a condition called bronze disease. This is a term used to describe a type of corrosion that affects bronze and other copper-based alloys. Bronze disease can cause serious damage to coins and other artifacts, and may even render them worthless if left untreated. In this blog post, we will explain what bronze disease is, how it forms, how to identify it, and how to prevent and treat it.
Bronze disease is not a disease in the biological sense, but rather a chemical reaction that occurs when copper reacts with chloride ions in the presence of moisture and oxygen. Chloride ions can come from various sources, such as seawater, soil, sweat, or cleaning agents. When copper and chloride form a compound called cuprous chloride (CuCl), it creates a greenish-white crust on the surface of the coin. This crust is unstable and can react further with oxygen and water to form other compounds, such as copper hydroxide (Cu(OH)2) and copper carbonate (CuCO3). These compounds are also known as patina, and they can have different colors depending on the composition and environment.
Patina is not necessarily harmful to coins, and in some cases it can enhance their appearance and value. However, when patina contains cuprous chloride, it can be very destructive. Cuprous chloride is hygroscopic, meaning that it absorbs moisture from the air. This causes it to expand and crack, exposing more metal to corrosion. The process becomes self-perpetuating and can eventually eat away the entire coin. This is why bronze disease is also called active corrosion or malignant patina.
Bronze disease is more common on coins that have been exposed to saltwater or buried in soils that contain chloride salts. It can also affect coins that have been overcleaned and lost their protective patina. Bronze disease is contagious, as the chlorides can spread to other coins if they are stored together.
How to spot Bronze Disease?
Bronze disease can be detected by examining the surface and the edge of the coin. It can be hard to detect in its early stages, as it may look similar to normal patina. However, there are some signs that can help you identify it before it causes irreversible damage. Some of these signs are:
- A powdery or flaky texture on the surface of the coin
- A greenish-white or pale green color that does not match the rest of the patina
- A sour or metallic smell coming from the coin
- A damp or sticky feeling when touching the coin
- A fizzing or bubbling sound when applying water or vinegar to the coin If you suspect that your coin has bronze disease, you should act quickly to stop the corrosion and preserve the coin. The first step is to isolate the coin from other coins and artifacts, as bronze disease can spread through contact or proximity. The second step is to remove the source of chloride from the
How can you prevent bronze disease?
The best way to prevent bronze disease is to store your coins in a dry and stable environment, away from sources of moisture, heat, light, and pollution. You can use silica gel packets or other desiccants to absorb excess moisture in your coin holders or containers.
You should also avoid touching your coins with bare hands, as the oils and acids from your skin can accelerate the corrosion process. You should use gloves or tongs to handle your coins, and store them in acid-free paper envelopes or plastic holders.
How to treat Ancient Coins Affected by bronze Disease?
If you notice any signs of bronze disease on your coins, you should act quickly to stop the reaction and preserve the coin. There are several methods of treating bronze disease, but they all involve removing the chlorides from the metal and applying a protective coating to prevent further damage. Here are some steps you can follow to treat your coins:
- 1. Wash your coins gently with distilled water and a soft brush to remove any loose dirt and corrosion products. Do not use tap water, as it may contain minerals or chemicals that can worsen the condition. Do not use any abrasive materials or chemicals that can damage the coin’s surface or patina.
- 2. Dry your coins thoroughly with a soft cloth or a hair dryer on low heat. Make sure there is no moisture left on the coin, as this can reactivate the bronze disease.
- 3. Soak your coins in a solution of benzotriazole (BTA) and ethanol for several hours or days, depending on the severity of the corrosion. BTA is a chemical that binds with copper and forms a stable complex that prevents further oxidation. You can buy BTA online or from a chemical supplier, but be careful as it is toxic and carcinogenic. Wear gloves and goggles when handling it and follow the instructions on the label. You can also use acetone instead of ethanol as a solvent, but it may dissolve some organic materials on the coin’s surface.
- 4. Rinse your coins with distilled water and dry them again.
- 5. Apply a thin layer of microcrystalline wax or lacquer to your coins to seal them from air and moisture. You can use a brush or a cotton swab to apply the coating evenly and avoid drips or bubbles. Let the coating dry completely before handling or storing your coins.
- 6. Store your coins in a dry and cool place, away from sunlight and heat sources. You can use acid-free paper envelopes, plastic holders, or coin capsules to protect your coins from dust and scratches. Do not store your coins in wooden boxes, cardboard boxes, or PVC bags, as they may contain acids or plasticizers that can harm your coins.
By following these steps, you can treat bronze disease and preserve your ancient coins for years to come. However, you should always consult an expert before attempting any treatment, as some coins may have special features or historical value that require professional care. You should also monitor your coins regularly for any signs of recurrence of bronze disease and repeat the treatment if necessary.
How frequent is Bronze disease on ancient coins?
The answer depends on several factors, such as the composition of the metal, the environmental conditions where the coins were stored or buried, and the cleaning and conservation methods applied to them. In general, bronze disease is more likely to occur on coins that have a high percentage of copper and a low percentage of tin or other alloying elements, as these make the metal more resistant to corrosion. Also, coins that were exposed to humid, acidic, or salty environments are more prone to develop bronze disease than those that were kept in dry and neutral conditions. Finally, coins that were cleaned with harsh chemicals or abrasive methods may have damaged their protective patina and accelerated their corrosion.
The frequency of bronze disease on ancient coins can vary from region to region and from period to period. For example, some studies have found that bronze disease is more common on Roman coins from the late empire than from the early empire, as the quality of the metal declined over time. Similarly, some regions may have had more favorable or unfavorable climatic conditions for the preservation of ancient coins. For instance, coins from Egypt tend to have less bronze disease than coins from Britain, as Egypt has a drier and warmer climate.