What is patina?
The patina is the natural weathering layer that occurs when the metal of a coin reacts with gases in the atmosphere. This chemical reaction causes a discoloration of the mint surface, often in soft, deep tones.
For collectors, there is a natural patina highly desirable, because it has both the authenticity as the old age highlighted by a coin.
How does patina occur?
When a coin is exposed to humidity, temperature differences and gases such as oxygen and sulfur dioxide for years, a thin oxidation layer slowly forms on the surface.
- Silver coins often receive a light brown to purple-blue tint.
- Bronze coins usually develop a blue-green color by the formation of copper oxides and carbonates.
- Copper coins can be a deep brown or green glow show.
The exact color and structure of the patina depend on the surroundings, such as soil conditions, air quality and storage conditions. A coin that has been lying in a wooden box or moist soil for decades develops a different patina than a coin kept in a dry display case.
Why is patina important?
A natural patina has a aesthetic and historical value.
Collectors value patina because:
- The de age and authenticity highlighted by the coin.
- It indicates that the coin has not been cleaned or polished.
- The color and texture of the character strengthen the currency.
A coin with a pristine patina tells, as it were, the story of her journey through time. Cleaning or polishing can irreparably damage this layer, causing both the value as the charm of the currency are lost.
Bronze disease: when patina becomes harmful
Not all discoloration is desirable. Bee bronze coins too strong a reaction with the environment can lead to bronze disease.
Bronze disease can be recognized by a powdery, light green coating that expands across the surface. This damage is the result of a chemical reaction between copper chlorides in the mint and moisture in the air.
When the reaction continues, the corrosion process eats through the metal from the inside, as a result of which the currency is structurally affected. In that case, professional conservation necessary to prevent further damage.
Patina and valuation
At the coin valuation the patina plays an important role. An original, even patina often increases the value of a coin because it indicates authenticity and preservation of the original surface.
Coins with an artificial or damaged patina are usually valued lower. One natural patina is almost impossible to imitate and is therefore an important proof of authenticity and origin.
Patina preservation tips
- Touch coins as little as possible with bare hands; skin oils can affect the patina.
- Avoid cleaning products or cleaning cloths. Even mild cleaning can remove the oxidation layer.
- Store coins in a stable environment, free from humidity and temperature changes.
- Consult a numismatic specialist in case of damage or bronze disease.
