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The safest way to clean a coin dug from the ground is usually not to clean it immediately. Photograph and record it first, keep all soil, crusts, and corrosion in place, and have a conservator assess anything fragile, salt-contaminated, actively corroding, or covered in concretions. Rubbing, polishing, acidic “coin dips,” and abrasive metal cleaners can permanently remove the original surface and details.
A limited wash is appropriate only for a coin that is stable enough for routine handling and has ordinary surface dirt or handling residue rather than corrosion. The Canadian Conservation Institute (CCI) describes lukewarm distilled water, diluted mild soap, a distilled-water rinse, and air-drying on paper towel for that narrow situation. That museum guidance is not a universal treatment for archaeological finds: a dug coin may contain salts, have a fragile surface, or carry an old coating that water or solvents could damage.
Preserve the find before you try to clean it
A coin from the ground is evidence as well as an object. Soil, deposits, color changes, and the way the surface breaks can reveal how it was stored, what alloy is present, and whether corrosion is still active. Removing that material before it is documented can destroy information and reduce the coin’s historical and monetary value.
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- Document its condition. Photograph both faces, the edge, and any deposits before moving or washing anything. Record where and when it was found, whether it was wet or dry, and whether loose material falls away. Keep the original images with the coin’s notes.
- Do not pick, scrape, brush, or polish. A hard crust may be protecting a more fragile metal beneath it. Even apparently soft dirt can carry abrasive grains.
- Keep finds separate. Do not let coins touch one another or other metals. Separate packaging prevents a corroding object from transferring moisture or salts to a stable one.
- Check local obligations before altering an archaeological find. Reporting and ownership rules vary by jurisdiction, and the available CCI guidance does not determine the law where you live.
If the coin is wet, heavily encrusted, or suspected of containing soluble salts, do not improvise a drying treatment. Move it only as needed to prevent physical loss and ask a conservator for context-specific instructions.
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Dirty, tarnished, and corroded are different conditions
“Dirt” is loose soil or handling residue sitting on the surface. Tarnish is a relatively stable color change or thin reaction layer. Active corrosion is a continuing chemical reaction that can produce powder, blisters, pits, flaking, or expanding crusts. The same coin can have all three at once.
| What you see | What it may mean | Safest first action |
|---|---|---|
| Loose, dry soil with an intact-looking surface | Surface dirt, although hidden corrosion is still possible | Document it; do not rub. Ask whether a conservator considers a limited wash appropriate. |
| Even darkening or color change that is hard and firmly attached | Possibly stable tarnish or a stable corrosion layer | Leave it in place unless a conservator recommends treatment. |
| Green or reddish powder, blisters, pits, lifting flakes, or a spreading crust | Possible active corrosion, often involving copper or a copper-bearing alloy | Stop cleaning and seek conservation advice. |
| Hard soil/concretions covering much of the coin | Deposits may be mechanically bonded to a fragile surface | Do not chip or soak it on your own; obtain an assessment. |
| Salt-contaminated archaeological metal or a coin recovered wet | Soluble salts can continue reacting as moisture conditions change | Do not allow it to dry before a conservator gives treatment instructions. |
CCI’s direct rule is: “In general, removing corrosion from the surface of a coin should be left to a conservator.” That applies even when the coin appears valuable, because an aggressive cleaning can remove design details that no later treatment can restore.
Why the apparent metal does not identify the treatment
Gold coins
Gold is often thought to be chemically inert, but a gold coin may be an alloy rather than pure gold. CCI notes that gold can be alloyed with copper. Copper in the alloy can form green or reddish corrosion, so a colored deposit is not proof that the coin is silver or copper, nor is it proof that the deposit is harmless. A thin, even change in color may be stable; powder, pits, or lifting material require assessment.
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Silver commonly develops a dark tarnish. Color alone cannot tell you whether that layer is stable or whether a contaminant is driving further corrosion. Airborne pollutants can tarnish silver even when a general storage environment appears acceptable. Avoid polishing because removing a dark surface can also remove the coin’s original texture and remaining detail.
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Copper and copper-alloy coins
Copper can develop green, blue-green, or reddish products. Some are stable; others are active and powdery. A green mark by itself is therefore not a treatment diagnosis. Salt-contaminated copper alloys are particularly sensitive to humidity: CCI’s archaeological-collections guidance reports moderate corrosion risk above 42% relative humidity and very high risk above 68%, citing Rimmer and colleagues (2013). Those thresholds apply to salt-contaminated copper alloys, not to every copper coin or every storage situation.
Use this decision path before any wash
Choose the least intervention that answers your immediate need. The following path keeps routine surface cleaning separate from corrosion treatment.
- Is there powder, flaking, deep pitting, a blister, a spreading green deposit, or a hard concretion? If yes, do not clean it at home; arrange a conservator’s assessment.
- Was it recovered wet, from a saline environment, or with visible salt deposits? Keep it from drying and obtain instructions before changing its moisture condition.
- Is the surface friable, plated, coated, or otherwise uncertain? Treat it as fragile and do not use water, solvents, or mechanical tools until assessed.
- Is the coin stable, with only loose soil or handling residue and no signs of corrosion? A conservator may approve the limited wash described below. If you cannot confidently make that distinction, stop at documentation.
| Condition | Home action | Professional action |
|---|---|---|
| Stable surface; loose dirt only | Only a conservator-approved limited wash; no scrubbing | Confirm suitability and document before/after condition |
| Stable tarnish or firmly attached color change | Leave in place | Assess whether it is stable and historically significant |
| Active corrosion, powder, flaking, or pits | No cleaning or polishing | Conservation treatment and stabilization |
| Concretions or uncertain plating/coating | No scraping, soaking, or solvent | Controlled examination and removal, if justified |
| Salt contamination or wet archaeological metal | Do not dry | Conservator-directed desalination and drying |
The limited wash for a demonstrably stable coin
CCI’s coin-care note describes a narrow cleaning procedure for coins suitable for routine handling. It is intended for surface dirt and handling residues, not for removing corrosion from an excavated coin. Do not proceed merely because the coin is made of gold, silver, or copper; condition is the deciding factor.
- Prepare a protected work area. Use a clean, padded surface and keep the coin away from sinks, drains, jewelry, and other metals.
- Use lukewarm distilled water. Tap-water minerals can remain on a surface. Lukewarm means comfortably warm, not hot; avoid thermal shock.
- Add diluted mild soap. Use only a mild formulation and the dilution recommended by the conservation guidance you are following. Do not add polish, acid, ammonia, or a metal-cleaning compound.
- Let the water do the work. Handle the coin gently. Do not scrub with fingers, cloth, cotton swabs, brushes, or tools. If soil does not release without force, stop.
- Rinse with distilled water. Rinsing removes soap residue. Do not substitute a chemical rinse.
- Air-dry on a clean paper towel. Set the coin down without rubbing it. Do not burnish the surface or use heat to speed drying.
The same CCI note mentions soaking in acetone to degrease and dewater. Acetone is a solvent, not a general-purpose final step for dug coins. A find may have a fragile surface, an old coating, adhesive residue, or an unknown treatment that reacts to solvents. Use acetone only when a qualified conservator has established that it is appropriate, with suitable ventilation, fire precautions, and waste handling.
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Products and techniques to avoid
CCI states: “Coin dips and metal cleaners (cloths, wadding, and liquids or pastes) should not be used to clean coins.” Coin dips may contain acids; if they are not completely rinsed, the acid can continue degrading the metal. Many metal cleaners are abrasive and can scratch a coin. The visual result may be brighter while the original surface is permanently thinner and less authentic.
- No polishing cloths or abrasive pastes: they can flatten lettering, rims, and microscopic surface structure.
- No acidic or commercial coin dips: their chemistry is unsuitable for an unknown alloy, corrosion product, or coating.
- No scraping or picking: tools can gouge the coin and can remove a protective corrosion layer along with the deposit.
- No household metal polish: a product designed for modern utensils is not a controlled treatment for archaeological metal.
- No “brightening” test on one side: an irreversible experiment is still irreversible if the result looks worse.
Salt-contaminated coins and concretions need a different response
Soluble salts can remain inside corrosion layers and pores. When humidity changes, they can dissolve and recrystallize, driving further reaction and mechanical damage. That is why a salt-contaminated archaeological coin should not simply be dried on a windowsill or placed in a warm room.
CCI’s archaeological-collections guidance specifically says metals covered in concretions or contaminated with salts should not be allowed to dry before conservation treatment. The immediate procedure depends on whether the object is still wet, the surrounding soil, and the type of metal. Keep the find supported, prevent loss of loose fragments, and contact a conservator for instructions rather than choosing a home desalination bath.
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Do not infer safety from a single humidity reading. The CCI thresholds of more than 42% relative humidity for moderate risk and more than 68% for very high risk concern salt-contaminated copper alloys. They are not universal targets for all coins, and they do not replace a condition assessment.
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Store the coin according to its condition
Storage is part of treatment. A clean-looking coin can deteriorate in a poor holder, while a fragile coin can be damaged by a holder that is perfectly adequate for a modern mint-state piece.
Choose an inert enclosure
CCI recommends inert plastic containers such as polyethylene or polystyrene for coins. Rehouse coins that are in PVC holders: PVC can release chemicals that damage metal surfaces over time. Use a container that supports the coin without forcing it into a tight pocket or allowing it to rattle.
Mylar holders and flips are not automatically safe for every archaeological coin. CCI cautions that some archaeological coins are too fragile for them. If a coin has flaking, powder, or a weak surface, a conservator should choose the support and the amount of contact.
Control contact and surroundings
- Keep the coin from direct contact with dissimilar metals, which can contribute to corrosion.
- Use clean, dry storage and avoid environments where moisture, salts, or pollutants accumulate.
- Do not seal a damp or salt-contaminated coin in an airtight container without professional instructions; trapping an unsuitable moisture condition can preserve the problem rather than solve it.
- Inspect the coin periodically for new powder, color spread, lifting, or condensation. Document changes instead of wiping them away.
For broad collection storage, CCI’s metal-storage note says that no corrosion should occur on most stable metal objects in a mixed collection at or below 55% relative humidity, while warning that some airborne pollutants can still tarnish silver or corrode lead. This is a general statement for stable metal collections, not a guarantee for a salt-contaminated archaeological coin.
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What to do in common situations
“It has a green spot, so it must be bronze.”
Not necessarily. Copper can be present in gold and silver alloys, and it can produce green or reddish corrosion. Leave the spot in place and have the composition and activity assessed.
“The black color looks ugly.”
Blackening may be stable tarnish rather than removable dirt. Polishing can erase original surface material. Keep the color unless a conservator determines that treatment is justified.
“A crust hides the date and design.”
Do not chip at the crust. The deposit may be bonded to the design or protecting a fragile layer. Mechanical removal is conservation work, not routine cleaning.
“I already polished it.”
Stop further treatment, save photographs of the current appearance, and record what product or tool was used. Do not try to correct scratches with more polishing. A conservator can assess what original surface remains.
“The coin is wet and covered in soil.”
Do not dry it just to make it easier to handle. Salt-contaminated or concreted archaeological metal should remain under conservator guidance until an appropriate treatment plan is established.
When to contact a conservator
Seek professional assessment before cleaning when any of the following applies:
- green, reddish, or other powdery deposits;
- flaking, blisters, deep pits, cracks, or a surface that feels friable;
- hard concretions that obscure the design;
- recovery from seawater, brackish water, a salty soil, or another environment likely to contain salts;
- an unknown plating, lacquer, wax, adhesive, or previous cleaning;
- uncertainty about whether the coin is gold, silver, copper, or an alloy;
- historical, archaeological, or monetary importance that makes an irreversible mistake unacceptable.
When you contact a conservator, provide the photographs, find context, current moisture condition, and a complete account of anything already applied. That information helps the conservator distinguish removable soil from corrosion and select a holder and treatment that match the coin’s fragility.
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A practical preservation checklist
- Photograph the coin before touching the surface.
- Record its find context and current condition.
- Handle it by the edge over a padded, clean surface.
- Do not scrape, brush, polish, acid-dip, or use metal-cleaning products.
- Keep wet, salty, concreted, or fragile finds from drying until advised otherwise.
- Use a limited distilled-water and mild-soap wash only when the coin is clearly stable and the procedure has been approved as suitable.
- Rinse with distilled water and air-dry on paper towel; never rub it dry.
- Store in an inert polyethylene or polystyrene container selected for the coin’s condition.
- Remove PVC storage and avoid direct contact with dissimilar metals.
- Monitor for new powder, flaking, or spreading corrosion and obtain professional help if any appears.
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