Cleaning old coins: why collectors do not

Coins · 5 September 2026

The short answer is: do not. Patina is not dirt, it is a thin, stable layer that forms on metal over decades, and every household recipe – baking soda, vinegar, cola, toothpaste, a wire brush – removes it along with a layer of the coin itself. The result is a coin that looks bright and reads as damaged to anyone who knows what to look for, permanently.

What patina actually is

Patina is a stable oxide layer that forms slowly on the surface of metal. On silver it darkens evenly to a warm grey or near-black; on copper and brass it goes brown and then green; on copper-nickel it dulls without changing colour much. Two things make it valuable rather than ugly: it protects the metal underneath, and it accumulates in the recesses of the design, which is exactly what makes relief readable. Strip it and the coin loses both the protection and the depth.

What each household method actually does

• Baking soda. A mild abrasive. It leaves a field of fine scratches that catch the light at an angle – invisible head-on, obvious when the coin is tilted.
• Vinegar and lemon. Acid. It eats the surface unevenly, leaving a pink or orange cast on copper that never returns to normal.
• Cola. Also acid, with sugar. Same etching, plus a sticky residue that continues to react afterwards.
• Toothpaste. The most abrasive of the lot, designed to polish enamel. On a coin it rounds off the fine detail of lettering.
• Wire brush or scouring pad. Immediate, visible parallel scratches. Nothing reverses this.
• Ultrasonic cleaner. Marketed as gentle, but it drives loose particles across the surface and can lift already-weak patina in patches, which looks worse than uniform darkness.

Why the damage is permanent and visible

A cleaned coin does not simply look newer. Under raking light the surface shows a different texture from an untouched one – too uniform, too bright in the fields, with softened edges on the lettering. Recesses that should hold darker tone are the same colour as the raised areas, which flattens the relief. And because die varieties are read exactly in those fine details – the lettering, the star, the awns of the arms – a scrubbed coin also becomes harder to attribute. The variety guide for the circulation series covers what those details look like on an intact coin: Soviet coins 1961–1991.

The narrow list of what is safe

1. Loose dirt. Rinsing in plain distilled water and letting the coin air-dry on a soft cloth removes dust and nothing else.
2. Never rub while wet. Grit suspended in water becomes an abrasive the moment you apply pressure.
3. Pat, do not wipe. Blot with a soft cloth; wiping drags particles across the field.
4. Active corrosion is the exception. Bright green powdery spots that spread are a conservation problem, not a cleaning one – that is a job for a conservator, not a kitchen.

What to do instead

• Photograph the coin as found, before doing anything. Condition is evidence.
• Store it so it stops getting worse: individual capsules or inert sleeves, no PVC, dry and dark.
• Keep coins from touching each other – most surface damage in a collection happens in a shared bag or jar.
• Handle by the edge only. Skin oils on the field turn into fingerprints that etch in over time.
• If you are judging value, read the surface as it is: condition matters more than the subject on almost every common coin, as described in the guide to what makes a Soviet coin valuable.

What the archive shows

The open archive holds 855 Soviet coins, 523 of them dated, from the silver small change of the 1920s to the last issues of 1991. Browsing them side by side is the fastest way to learn what an untouched surface looks like at each age: the even warm darkening on early silver, the dulled but unscratched fields on copper-nickel of the 1960s–1980s. That comparison is worth more than any recipe.

Open the sections: Soviet coins and banknotes, badges.