Why coins turn green, and what to do about it

Coins · 6 September 2026

Green appears on copper, brass and bronze because copper reacts with moisture, air and whatever else is around. There are two very different results. A smooth, even, hard green is patina: stable, protective, and part of what makes an old coin look its age. A bright, powdery, uneven green is active corrosion: it spreads, it feeds on the metal, and it does not stop by itself. Everything practical follows from telling those two apart.

Two kinds of green

• Stable patina. Smooth, matt, evenly distributed, hard to the touch and firmly attached. It follows the surface of the coin rather than sitting on top of it, and it usually took decades to form. Leave it alone – it protects the metal beneath and it is part of the coin's history.
• Active corrosion. Bright, light green, powdery or crusty, often in isolated spots that look raised. Scrape gently at the edge of such a spot with a wooden toothpick and it crumbles. Under it the metal is pitted. This kind spreads, especially in damp conditions.

Why it happens

1. Moisture. The main driver. Damp air, condensation on cold metal, storage in cellars, attics or unheated rooms.
2. Contact with skin. Fingerprints leave salts and oils that hold moisture against the surface and start localised spots.
3. The wrong storage material. Soft PVC sleeves release plasticiser that reacts with copper, producing a characteristic green haze across the whole surface.
4. Contact with other materials. Cardboard with high acidity, adhesive residues, rubber bands and paper coin envelopes containing sulphur all accelerate it.
5. Salt. Coins found in soil or handled near the sea carry chloride, which drives the aggressive form that keeps returning.

What to do about the active kind

The goal is to stop it, not to remove it. Removal means abrasion or chemistry, and both damage the coin permanently – the argument set out in the guide to cleaning old coins.

• Isolate the coin. Take it out of the shared box or album so the problem does not spread by contact.
• Dry the environment. Move the collection into a heated, dry room and add silica gel in a closed box, replacing it when saturated.
• Remove the cause. Discard soft plastic sleeves, rubber bands and acidic card; replace with inert materials, as in the guide to storing coins and metal.
• Photograph and monitor. Record the spot and check in a few months. Stable means it stopped; growth means the environment is still wrong.
• Serious cases go to a conservator. Aggressive chloride corrosion has proper treatments, and none of them are kitchen chemistry.

What not to do

• Vinegar, lemon, cola: acid strips the green and the surface with it, leaving a pink, etched coin that reads as damaged forever.
• Baking soda, toothpaste, brushes: abrasives that scratch the field and round off lettering.
• Lacquer or wax "to seal it": irreversible, and it traps any active corrosion underneath where it continues quietly.
• Storing a corroding coin back with the others and hoping.

Which coins are affected

Anything containing copper, which is most circulating coinage: bronze and brass small change, and copper-nickel alloys too, though those dull rather than going bright green. Silver behaves differently – it darkens evenly instead, and that darkening is usually desirable. The three eras of Soviet silver are covered in the guide to silver coins.

The open archive holds 855 Soviet coins, 523 of them dated, from the copper and silver small change of the 1920s to the last issues of 1991 – enough to see how an untouched copper surface should look at each age before deciding anything is wrong with your own.

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