How to clean copper and bronze coins: risk-free methods

Copper and Bronze: Why They Are So Difficult

Copper and bronze coins are a separate category in numismatics that requires a special approach. Unlike silver and gold, copper reacts extremely actively with the environment and forms several different types of plaque, each of which requires a different approach.

First of all, let's deal with the terms. Copper and bronze (an alloy of copper and tin) behave similarly, but there are nuances. Brass (copper with zinc) and kunial (copper with nickel) are also copper-based alloys, but they react somewhat differently.

Types of patina on copper coins

• Dark brown or chocolate patina is a "good" patina, a sign of aging of the coin. It usually does not need to be removed.
• Pink-orange patina is surface oxidation of new copper coins. It is easily removed with weak acids.
• Bright green powdery patina (verridigris) is active corrosion that destroys the metal. Requires immediate intervention, but very careful.
• Dull green stable patina is the so-called "antique green" or malachite patina. On old antique coins, it is considered a value, not a defect.

Methods for cleaning copper and bronze coins

1. Soap solution and soaking

Always start with the mildest option. Warm soapy water (neutral soap without fragrances) + long soaking (from several hours to several days) allows you to soften surface dirt, crusts of soil and unstable plaque. After soaking, the coin is carefully rinsed with water - this is often enough.

2. Citric acid

A weak solution of citric acid (5–10%) copes well with pink oxidation and some types of green plaque. Dissolve 1 teaspoon of citric acid in a glass of warm water. Leave the coin for 5–20 minutes, constantly observing. Then immediately rinse and neutralize in a weak soda solution (to stop the reaction), then rinse again with clean water.

Important: citric acid can change the color of the coin, making the copper more bright pink. This is not always desirable, especially for coins with patina.

3. Acetic acid (vinegar)

Regular table vinegar (9%) or diluted to 5% is another option for copper coins. The principle is the same as with citric acid. Vinegar is somewhat milder, but also requires constant monitoring and mandatory neutralization after use.

4. Ammonia

Ammonia solution (5–10%) effectively cleans copper and bronze from some types of plaque. But be careful: ammonia can give copper a specific bluish tint with prolonged contact.

5. Electrolysis - for heavy soiling

This is the most effective, but also the riskiest method. Electrolysis allows you to remove a thick layer of corrosion and soil crust even from heavily damaged coins. Principle: the coin is connected as a cathode (minus), the anode (plus) is a separate metal object (not silver or copper, stainless steel is better), the electrolyte is a weak salt or soda solution.

With proper connection, bubbles appear on the coin - this is normal. Hold the coin from several minutes to several hours, depending on the degree of contamination. The result is checked every 15–30 minutes. After the procedure - thorough rinsing and neutralization.

Attention: electrolysis removes everything - both dirt and patina. After it, the coin often looks "empty" and needs time to form a new natural patina.

Verrydygris: what it is and what to do about it

Verrydygris (from the French vert-de-gris - "green of Greece") is a bright green or blue-green powdery coating that forms during active corrosion of copper. Chemically, it is a mixture of copper carbonates and acetates.

Unlike the stable green patina of ancient coins, verrydygris is an active and destructive process. It literally "eats" the metal. A coin with verridigris requires urgent intervention, but very careful - aggressive removal can remove the surface of the coin along with the coating.

Safe approach: isolate the coin from others (corrosion can spread), rinse with warm distilled water, carefully (without pressure) remove the loose coating with a soft brush. If the coating is stable and dense, it is best to contact a coin restoration specialist.