Should I clean an old coin, medal or stamp?

This is the one piece of advice that experienced collectors give most consistently and that newcomers most consistently ignore, because a dirty coin looks like it needs cleaning. What looks like dirt is usually toning or patina — a stable oxide layer formed over decades — and it is part of the object, not something on top of it.

Step by step

1. Understand what patina is.The surface of an old coin or medal has oxidised slowly into a stable layer that protects the metal underneath. Collectors value it because it demonstrates that the surface is original and undisturbed. Removing it exposes raw metal that then tones again, unevenly and quickly.
2. Understand what cleaning does mechanically.Any abrasive, including a soft cloth, leaves fine parallel scratches across the fields of a coin. Chemical dips strip the surface layer and leave a flat, lifeless brightness. Both are visible under magnification and both are permanent.
3. Accept the value consequence.A cleaned coin is described as cleaned in every catalogue and by every grading service, and it typically sells for a small fraction of an uncleaned equivalent. Grading services will not assign a numerical grade to an obviously cleaned coin.
4. Apply the same rule to medals.Campaign and service medals are valued for original condition including the ribbon and the toning. Polishing a medal removes detail from the high points and is regarded by collectors as damage.
5. Never wash, soak or trim stamps.Water removes gum, which is a large part of the value of an unused stamp. Trimming perforations, removing hinges or attempting to lift a stamp from paper all destroy value. Leave a stamp on its original cover if it is on one.
6. Know the narrow exceptions.Loose surface dirt or soil on a modern circulation coin of no collector value can be rinsed in plain water and patted dry. A metal-detected find should be shown to a specialist before any cleaning. Anything else, do nothing.
7. Deal with active corrosion differently.Bronze disease — a light green powdery growth that spreads — and active verdigris are deterioration rather than patina, and they will destroy the object if untreated. That is a conservation problem, and the correct step is advice from a museum conservator or a specialist dealer, not a household remedy.
8. Handle and store to prevent the problem.By the edges, with clean dry hands, in inert holders. Store dry and stable and away from PVC, which is the other quiet destroyer of coin and stamp collections.
9. Ask before you act, and ask a dealer not a forum.A reputable dealer, an auction house specialist or a collectors society will look at something and tell you honestly. That conversation is free and it is the difference between a valuable item and a ruined one.

Tips

• The instinct that a coin looks better cleaned is exactly backwards from how the market values it. Bright and shiny reads as damaged to anyone who knows the field.
• Never use metal polish, vinegar, lemon juice, bicarbonate, tomato ketchup or an ultrasonic cleaner on anything collectable. All of them are recommended online and all of them cause the damage described here.
• If you inherit a collection, do nothing at all to it until someone knowledgeable has seen it. More value is lost to well-meaning tidying than to any other cause.

Common mistakes

• Polishing a coin or a medal to make it look better — It leaves permanent hairline scratches and removes original surface, and it is instantly recognisable. It is the single most common way an inherited collection loses most of its value.
• Using a chemical dip — It strips the toning chemically, leaving an unnaturally flat brightness that ages back unevenly and blotchily. Grading services detect it and refuse to grade the coin normally.
• Washing stamps or removing hinges — Gum, hinges and the paper a stamp is attached to are part of what is being valued. Water and solvents destroy gum irreversibly.
• Storing in PVC — PVC breakdown produces a green sticky residue on coins and lifts ink from stamps. Inert polypropylene, polyethylene, Mylar-type polyester and paper envelopes made for the purpose are the safe materials.

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