Methods for cleaning ancient bronze coins

Cleaning ancient bronze coins is a delicate process that requires careful consideration to preserve their historical value and aesthetic appeal. Whether you’re a seasoned collector or a newcomer to numismatics, understanding the proper techniques for cleaning ancient bronze coins can significantly enhance your collection’s appearance without damaging the coins. Here’s a detailed guide on how to approach this task.

Why clean ancient bronze coins?

Over time, bronze coins can accumulate dirt, grime, and a green or brown encrustation known as bronze disease, which is actually chloride corrosion that can progressively damage the coin if not treated. Cleaning these coins not only helps in preserving them but also makes the inscriptions and imagery more visible and appealing.

As a rule, bronze should be cleaned mechanically (i.e.. with brushes, and toothpicks), and silver should be cleaned chemically (i.e., with ammonia). Some Roman bronze coins have a thin silver plate.  These coins are “silvered” coins.  Silvere coins are cleaned like bronze but must be treated gently to preserve the silvering 

Pre-cleaning assessment

Before attempting any cleaning, thoroughly examine each coin under good lighting and possibly a magnifying glass or microscope to assess its condition, noting any areas of significant corrosion or wear. Understand that cleaning can sometimes reduce a coin’s numismatic value, especially if done improperly. For high-value coins, consider consulting a professional before proceeding.

Mechanical cleaning

Tools Needed : Soft brushes, wooden picks, and dental tools.

Method : Gently remove loose dirt without scratching the coin’s surface. Carefully use the tools to pick off encrustations.

Caution : Avoid using metal picks or sharp instruments that can leave scratches.

Soaking

Distilled water, olive oil, or a solution of distilled water and mild soap.
Process: Soak the coins in a container filled with your chosen solution. Change the liquid daily and lightly brush the coins with a soft toothbrush to remove loosened dirt and corrosion. This can take anywhere from a few days to several months, depending on the coin’s condition.

Electrolytic cleaning

This method requires more preparation, involving a sacrificial anode, an electrolyte solution, and a direct current source. Electrolytic cleaning is more invasive and should be used only as a last resort when other methods fail because it can alter the coin’s surface permanently.

Post-cleaning care

Drying : Allow the coins to air dry on a soft cloth or a paper towel. Avoid using heat or blow dryers, which can damage the metal.

Storage : Store cleaned coins in a dry, cool environment. Use acid-free materials and consider appropriate holders like coin flips made from inert materials to protect the coins from environmental factors and handling.

Conclusion

Cleaning ancient bronze coins is an art that requires patience and precision. By choosing the right method based on the coin’s condition and value, collectors can preserve or even enhance the beauty and longevity of these historical treasures. Remember, when in doubt, particularly with highly valuable or rare coins, consult with or hire a professional conservator to avoid potential damage.