Patina on coins – what is it and how does it affect their value?

Patina (often referred to as "toning" on silver) is a natural, thin layer of chemical compounds that forms on the surface of a coin as a result of the metal reacting with its environment. For collectors, it is not just a visual effect: a well-maintained, natural patina is often a sign of authenticity and "original surfaces," and is sometimes rewarded on the market — especially when it is attractive and even. PCGS's professional grading standards explicitly state that collectors often pay premiums for naturally attractive toning, while penalizing coins that have been cleaned or artificially patinated, known as "details grade."

Table of contents

• What exactly is patina (and what is it made of)?
• How does patina affect value?
• Is it worth cleaning coins with patina?
• What factors determine a "good" patina and valuation?
• How to store coins to reduce corrosion?
• The most common myths—in a nutshell
• Summary

What exactly is patina (and what is it made of)?

• Silver most often forms silver sulfide (Ag₂S) on its surface, which is responsible for its gray-black or iridescent hues. This layer grows faster in the presence of sulfur compounds, moisture, and air pollution.
• Copper and bronze produce, among other things, cuprite (Cu₂O) and copper carbonates (e.g., malachite), which can form a protective layer that slows down further corrosion—although not always (see "bronze disease").
• Gold is a precious metal and hardly tarnishes on its own; slight discoloration usually occurs due to impurities (e.g., copper in a 90% Au alloy), hence the famous "copper spots."

Note: not every color change is desirable. Bronze disease (active chloride corrosion on copper alloys) manifests itself as a powdery, light green coating and requires conservation intervention.

How does patina affect value?

• A natural, even, and attractive patina usually increases a coin's appeal (known as "eye appeal"), and the market often rewards such specimens. PCGS standards explicitly emphasize this.
• Artificial patina / "questionable color" or traces of improper cleaning result in a lower grade (details) and value. Large grading companies (PCGS/NGC) have been pointing this out for years.
• If the patina is unstable (e.g., progressive blackening on silver, active green efflorescence on bronze), it may even threaten the surface and require consultation with a conservator.

Is it worth cleaning coins with patina?

In short: in the vast majority of cases, no. Cleaning coins yourself (mechanically, with pastes, dips, soda, etc.) damages the original surface, leaves micro-scratches or reagent residues, and reduces the coin's collector's value. This is why professional grading companies treat "improperly cleaned" coins so harshly. If anything, professional conservation (NCS at NGC or PCGS Restoration) is considered, which removes harmful deposits and stabilizes the surface without interfering with the metal.

What factors determine a "good" patina and valuation?

• Authenticity of the process – naturally formed patina > artificially accelerated.
• Uniformity and aesthetics – delicate, harmonious color transitions enhance the eye appeal.
• Stability – no active corrosion, no chemical stains, no progressive blackening.
• Metal and alloy – silver and copper react differently; gold hardly tones at all (exception: areas richer in copper produce "copper spots").

How to store coins to reduce corrosion?

Environment

• We strive for stable conditions: approx. 18–23°C and relative humidity ~45–55% — this range is recommended, among others, by the Central Bank of Lithuania for collector coins; for metals alone, museums often maintain low humidity (35–50%) and stable parameters. Use moisture absorbers (gels) and avoid sudden fluctuations.

Materials and packaging

• Use caps, acid-free holders, or hard, neutral capsules.
• Avoid PVC (soft vinyl flip charts) and materials containing sulfur (some papers, cardboard, wood) — they accelerate tarnishing and leave aggressive residues.

Service

• Always hold coins by their edges, preferably with gloves on.
• Store in a dry, cool, shaded place with good ventilation and without fluctuations in parameters.

If you notice a powdery, light green coating on bronze/copper (bronze disease) or unstable, progressive discoloration, consider professional conservation (NCS/PCGS Restoration) instead of home remedies.

The most common myths—in a nutshell

• "Patina always increases value." Not always. Natural, aesthetic, and stable patina is desirable; artificial or ugly patina lowers the grade and price.
• "Silver rusts." It does not rust (rust is iron compounds). Silver sulfides (Ag₂S) form, creating toning.
• "Gold tarnishes just like silver." Pure gold practically does not; visible "spots" are usually reactions of copper admixtures ("copper spots").
• "Gentle cleaning with soda/dip won't hurt." It may remove a layer of metal or leave marks—you risk damaging the details grade. Leave it to the conservators.

Summary

Natural patina is usually an asset – it adds character and can be rewarded, as long as it is authentic and stable. The worst enemies of coins are aggressive cleaning and poor storage conditions (moisture, PVC, sulfur). Ensure a stable environment, use neutral materials and, if necessary, seek professional conservation services instead of resorting to "home remedies."