Storing and conserving ancient coins
Inert holders, stable humidity, bronze disease, and the strict limits of what a private collector should ever do to a coin's surface.
Key facts
- Target humidity
- Below 40% RH
- Materials to avoid
- PVC, untreated wood, rubber bands
- Active threat
- Bronze disease (bright green powder)
- Golden rule
- Stabilise, never beautify
Environment does most of the work
Almost all avoidable damage to a coin collection comes from three sources: moisture, reactive plastics, and handling. Control those and the collection will outlive you in the condition you bought it.
- Keep relative humidity below 40%; use silica gel in a sealed cabinet and replace or regenerate it on a schedule.
- Choose a stable, cool location away from exterior walls, lofts, cellars and bathrooms.
- Never store coins in PVC flips — plasticiser migrates and leaves a green, sticky film that etches the surface.
- Use inert polyester (Mylar), polyethylene or polypropylene holders, or traditional coin trays lined with acid-free card.
- Avoid oak and other tannic woods in cabinets; tannin vapours corrode silver and bronze.
- Handle by the edge, over a padded surface, with clean dry hands or nitrile gloves.
Holder materials at a glance
| Material | Verdict | Notes |
|---|---|---|
| Mylar / polyester | Safe | Rigid, clear, chemically inert |
| Polyethylene / polypropylene | Safe | Slightly hazy but stable |
| PVC (soft, flexible flips) | Avoid | Causes green PVC residue |
| Paper envelopes | Conditional | Only if acid- and sulphur-free |
| Cardboard 2×2 with staples | Conditional | Staples scratch; crimp with care |
| Sealed slabs | Safe | Good for modern issues and transport |
Bronze disease: the one emergency
Bright, powdery, pale-green spots on a copper-alloy coin are not patina. They are active chloride corrosion, and they spread — to the rest of the coin and to neighbouring coins in the same tray. Isolate an affected coin immediately.
- 1Quarantine the coin in a separate, dry container away from the collection.
- 2Photograph the affected areas so progression can be judged later.
- 3Reduce humidity hard; dryness alone halts the reaction.
- 4For anything of value, consult a professional conservator rather than attempting chemical treatment.
- 5After stabilisation, keep the coin in a monitored, desiccated holder and re-inspect quarterly.
Banknotes and documents
- Store flat in archival, acid-free polyester sleeves — never fold to fit a smaller album.
- Keep out of daylight; inks on Qajar and early Pahlavi notes fade permanently.
- Do not repair tears with adhesive tape; conservators can reverse almost anything except tape.
- Interleave with acid-free tissue if sleeves are unavailable, and keep away from cardboard boxes that off-gas.
Frequently asked questions
Should I clean an ancient coin I just bought?
No. Cleaning is the single most common way collectors destroy value. Stabilise the environment, remove loose dirt with a soft brush at most, and leave chemical or mechanical work to a conservator.
How do I know if a coin has PVC damage?
Look for a green, slightly tacky or oily film, often concentrated in the recesses of the design. It is distinct from the hard, matte green of natural patina and from the bright powder of bronze disease.
Is a bank vault the right place for a collection?
Vaults are excellent for security but often poor for humidity. If you use one, seal coins in a desiccated container with a humidity indicator and check it whenever you visit.
Related guides
Cast copies, transfer dies, tourist souvenirs and altered mint marks — the diagnostic tests that separate a genuine ancient Iranian coin from a modern forgery.
How to build a coherent collection of Persian and Iranian coinage — choosing a theme, setting a budget, sourcing responsibly, and recording provenance from the first purchase.
From the Imperial Bank of Persia to Bank Markazi: the issuing authorities, the 1979 overprints, replacement notes, and how to build a complete run.