Cleaning and preserving finds: what NOT to do

The single biggest cause of avoidable value destruction on detector finds is over-cleaning. Decades of evolved auction wisdom and archaeological best practice agree on one principle: when in doubt, don’t. Here’s what you can safely do, what you should never do, and how to store what you keep so it’s still good in ten years.

The cardinal rule

Patina is information — both archaeological and economic. A stable, dark, even patina on a hammered silver coin (or a green-brown patina on a copper-alloy artefact) tells you the burial context, the chemistry of the soil, and the time the piece spent underground. Stripping it removes information that no amount of careful re-imaging can restore. It also typically halves the market value.

What you can safely do

The approved cleaning protocol for a fresh detector find is short:

1. Rinse with cold water while gently rubbing with a fingertip to remove field mud. Hold the coin by the edge.
2. Pat dry with a soft cloth or kitchen roll. Air-dry on a clean surface for a few minutes.
3. For stubborn caked mud: a soft toothbrush (not a new one — one that’s been broken in for a few months and is no longer stiff) used gently with water can lift mud without damaging patina. Watch the bristle direction.
4. Stop. That is the entire approved protocol.

What you must never do

• Brass brush, wire wool, or any abrasive. Will permanently scratch the surface and remove patina along high points first — exactly where the detail you need lives.
• Lemon juice, vinegar, ketchup or any acid. Strips silver oxide and copper carbonate alike. The coin will come out shiny but with the surface stripped, often pitted.
• Tin foil with salt water (the “electrolytic” trick). The chemistry works — it does strip tarnish — but it also strips diagnostic patina, and the uneven action often leaves pits where the foil contact pattern accelerated reduction.
• Ultrasonic cleaners. Can shatter embrittled ancient silver and dislodge surface detail. Originally designed for modern jewellery, not 1,500-year-old metal.
• Silver polish or metal polish. Removes the patina layer. Don’t.
• Hot water or any heat treatment. Can crack a fragile silver coin; can also accelerate corrosion of a poorly patinated piece.

Specific find types

Hammered silver

Rinse, pat dry, leave alone. A dark, even tone is desirable. Hammered silver with stable patina sells at full Spink price; the same coin stripped to bright sells at half. Don’t.

Roman bronze

Green-brown patina is good. Bronze disease— a powdery, light-green corrosion that’s actively eating the coin — is bad. Bronze disease (cuprous chloride) is active chloride corrosion; if you see fresh light-green powder on a Roman coin, dry it carefully, store it desiccated, and consult a conservator. Treatment is specialist work involving benzotriazole and is not a kitchen-table job.

Roman silver

Often more fragile than hammered medieval silver. Rinse only, air-dry, no scrubbing. Roman denarii can be embrittled enough that even gentle handling cracks them.

Lead and pewter

Particularly fragile. Lead corrosion (a white powder) is active and can spread within a collection if not isolated. Store lead artefacts in dry conditions, away from anything containing acids (cardboard, wood, paint, vinegar).

Gold

Gold doesn’t corrode and almost never needs cleaning. Rinse carefully and leave alone. Avoid any abrasive cleaning — gold is soft and will scratch easily.

Iron

Iron is the most chemically active common detector find and the hardest to conserve. Even a stable find will continue corroding if not dried thoroughly. Significant iron finds (sword blades, spearheads, tool fragments) need professional conservation; small bits can be air-dried and stored desiccated.

Storage

Three options, all suitable depending on your budget and the importance of the find:

• Inert plastic flips (2 × 2 inch): cheapest and adequate for everyday material. Buy “PVC-free” or polypropylene flips — PVC out-gases hydrochloric acid over time, which corrodes silver and copper. Avoid soft-PVC sleeves.
• Hard-plastic coin capsules or Lighthouse trays: Lighthouse cabinets and inert trays are the auction-house standard. Modular, well-padded, archivally stable.
• Acid-free archive boxes: for paper records, context cards, photographs, and as overflow storage. Acid-free tissue around finds prevents reactive contact.

Recording and reference numbers

For every find of historical interest, keep:

• Find-spot: GPS coordinates or grid reference to the nearest 10 m. The single most valuable piece of data attached to any detector find.
• Date found.
• Attribution and any reasoning behind it.
• PAS reference number if recorded.
• Photographs of obverse, reverse, edge profile, and any close-up of diagnostic detail. See our photography guide for the setup.
• Measurements: diameter, weight, thickness.

When to consult a conservator

Most ordinary detector finds don’t need professional conservation. The cases that genuinely warrant it:

• Bronze disease that’s actively eating the coin.
• Iron artefacts of significant size (sword fragments, tools).
• Anything with organic remains attached (textile, leather, wood, bone).
• Anything that qualifies as Treasure under the 1996 Act — the FLO will arrange conservation as part of the formal process.
• Particularly important / rare pieces where the long-term value justifies professional intervention.

Your local FLO can refer you to museum conservators or, for Treasure cases, will arrange it as part of the process. Don’t pay for unsolicited “cleaning services” advertised online — many of them are aggressive cleaners by another name.