How to Clean Late Roman Bronze Coins

You have just received your lot of uncleaned late Roman bronze coins — small, dark, encrusted discs that once passed through the hands of soldiers and citizens in the 3rd and 4th centuries AD. Somewhere beneath that dirt and mineral deposit is a portrait, a legend, and a story. This guide covers the safest methods for revealing them without causing damage.

What You Will Need

• Distilled water (never tap water, which contains minerals and chlorides that can harm the coin)
• Olive oil, regular grade — not extra virgin, which is more acidic
• A soft-bristle toothbrush for gentle scrubbing during and after soaking
• Toothpicks and bamboo skewers for detailed work between letters (they will not scratch the patina)
• Glass or plastic containers (never metal)
• A magnifying glass or desk lamp — essential for seeing fine detail
• Mild dish soap for rinsing
• Renaissance Wax, a microcrystalline wax used in museum conservation for sealing coins after cleaning

Method 1: The Distilled Water Soak

This is the safest method and should always be your starting point. Distilled water gradually loosens encrustations from the coin surface without any chemical risk.

1. Photograph your coins before cleaning, with consistent lighting, so you can compare results later.
2. Place coins in a glass or plastic container filled with distilled water. Do not use a metal container.
3. Soak for at least two weeks, ideally a full month. The water gradually loosens encrustations and breaks them from the surface.
4. Once a week, remove the coins, gently scrub each with a soft toothbrush under running distilled water, then replace with fresh distilled water.
5. After a month, evaluate each coin. If legends or design elements are becoming visible, proceed to gentle mechanical cleaning. If still heavily encrusted, return to soak.
6. Some coins need two or three months. Heavily encrusted pieces may require six months or longer. Patience is the most important tool in your kit.

Method 2: The Olive Oil Soak

For coins that resist distilled water alone, olive oil can help soften stubborn mineral deposits. Use regular olive oil — not extra virgin, which is more acidic.

1. Place coins in a container of regular olive oil and leave for several weeks to several months.
2. Every one to two weeks, remove the coins, scrub with a stiff toothbrush and liquid soap, rinse thoroughly, examine, and return still-dirty coins to fresh oil.
3. For heavily encrusted coins, adding half a teaspoon of lemon juice to the oil can accelerate the process slightly.
4. Olive oil will darken coins over time due to fatty acids and trace sulphur — this is cosmetic, not destructive.
5. When finished, wash thoroughly with dish soap to remove all oil residue.

Method 3: Gentle Mechanical Cleaning

After soaking has softened the deposits, you can carefully remove remaining dirt by hand. Always work under magnification.

1. Start with a toothpick or bamboo skewer. Work around design details, gently dislodging softened deposits.
2. If deposits remain, progress to a blunted sewing needle for finer work.
3. A soft brass brush is useful for sweeping away loosened material.
4. Work in short sessions. Remove deposits one speck at a time to avoid damage.
5. Know when to stop. A partially cleaned coin with intact patina is worth far more than an over-cleaned coin with a scratched surface.

After Cleaning: Preservation

1. Rinse thoroughly with distilled water.
2. Dry completely — air dry or use a low oven at 90°C for 30 minutes.
3. Apply a thin, even coat of Renaissance Wax to seal the surface against moisture and oxygen.
4. Buff gently with a lint-free cloth.
5. Store in a dry environment. Humidity below 39% is ideal for bronze coin storage.

What Never to Do

• Do not use acids. Vinegar, muriatic acid, and hydrochloric acid will strip the patina and eat into the metal permanently.
• Do not use wire brushes or steel wool. These scratch the surface and destroy fine detail.
• Do not use commercial coin dips like Tarn-X. They are designed for modern coins and are far too aggressive for ancient bronze.
• Do not rush. Trying to remove deposits mechanically before soaking has softened them is the most common cause of damage.
• Do not strip the patina. The natural patina — the stable oxidation layer formed over centuries — is both a protective layer and an authenticator. Coins with intact patina are worth significantly more than stripped coins.

Watch for Bronze Disease

Bronze disease appears as bright, fuzzy, powdery green spots — distinct from stable green patina, which is smooth and hard. It is active corrosion caused by chlorides and will spread if untreated. You can test by scratching with a toothpick: bronze disease is soft and powdery. Treatment involves removing the affected material and soaking in a 5% sodium sesquicarbonate solution. Prevention is primarily environmental — store coins below 39% relative humidity with desiccant packs.

Further Reading

• Doug Smith's Guide to Uncleaned Coins — forumancientcoins.com
• The Complete Guide to Uncleaned Ancient Coins — romancoin.info
• Cleaning Ancient Coins — American Numismatic Association (money.org)
• Forum Ancient Coins Identification Help Board — forumancientcoins.com