Verdigris and PVC: Saving My Worse example of the new Bay Authenticity Guarantee- what could possibly go wrong from Environmental Damage

Improper storage is the enemy of numismatics. Let’s identify the signs of environmental damage specific to this metal type.

I’ve examined thousands of copper-alloy coins over the years. Roman sestertii that survived two millennia in the ground. Modern commemorative bronzes ruined in less than two decades inside a plastic flip. Coins that should have been in perfect mint condition. Gone.

In my experience grading and conserving, the most heartbreaking losses are not from burial. Not from war. Not from the harsh cleaning of the 19th century. They are from improper storage in the last 50 years. In drawers. In attics. In soft vinyl holders collectors trusted.

That is why a recent forum thread titled “My Worse example of the new Bay Authenticity Guarantee- what could possibly go wrong???” stopped me cold. The original poster documented a terrible “coin” in a fake PCGS slab. A genuine coin for the cert number. A seller with a trail of feedback. And then — after the authenticity program tripped above the $500.00 threshold — another identical listing posted right after the first order.

Other members joked, “ohhh boy.” They debated whether PCGS would ship a fake back to the seller or confiscate it. They noted the seller would simply return under another username at $499 OBO. I read that thread differently.

As a numismatic conservator, I saw a perfect storm for environmental damage. A coin sealed in a counterfeit holder, of unknown plastic composition, stored for an unknown length of time, handled by a seller who clearly does not understand conservation. That coin is not just inauthentic. It is chemically compromised.

Even if it were genuine, after years in the wrong plastic, in the wrong humidity, it could be actively dying. This article is my conservator’s breakdown of exactly what kills bronze and copper coins. How to spot it before it spreads. And what proper chemical conservation can — and cannot — save.

Bronze Disease: The Cancer of Copper Alloys

If you collect ancient bronzes, medieval coppers, or even early American large cents, you must learn to recognize bronze disease. I do not use that term lightly. In my lab, I treat it as an active, contagious corrosion process. Not a static patina.

Bronze disease is caused by chlorides. Salts from burial soil. From the sea. From fingerprints. From PVC plasticizers. They get trapped in the metal’s pores.

When moisture reaches those chlorides, they form cuprous chloride. That reacts with water and oxygen to produce hydrochloric acid and powdery cupric chloride. The acid attacks fresh metal. It creates more cuprous chloride. The cycle is self-perpetuating until the coin is consumed.

How It Starts and How It Spreads

I’ve examined newly excavated Roman coins from Britain and the Mediterranean that looked stable in the field. Only to erupt in my cabinet three months later. The trigger is almost always a change in relative humidity above 35-40%.

A coin stable in dry soil begins to sweat in a humid home. A coin stored in a damp basement collection will bloom overnight. Provenance does not protect it. Once chlorides are inside, the environment decides.

Crucially, bronze disease is contagious in a storage box. The powdery spores — microscopic crystals — can transfer to neighboring coins via a shared flip, a shared tray, or even airborne dust. I have quarantined entire trays of British bronze tokens because one coin in the center was actively powdering.

What It Looks Like Under Magnification

Do not confuse bronze disease with a normal green patina. A stable patina is hard. Adherent. Integrated. Bronze disease is:

• Bright, powdery, pale green or sky-blue spots, often in pits, in lettering, or around the rim — like tiny piles of dust
• Small, deep pits that appear to be eating into the surface, sometimes with a dark green or black center
• Crusts that flake or lift when lightly touched with a soft brush, revealing fresh, pitted metal underneath
• A musty, slightly sour smell when you open a holder containing an active coin — a sign I trust as much as visual inspection

Conservator’s takeaway: If you see powdery light green, isolate the coin immediately. Separate, dry, acid-free holder. Silica gel. Do not put it back in a shared album. Do not brush it dry — you will spread the chlorides.

Photograph it. Note the date. Monitor it. Active bronze disease will grow visibly in weeks. It destroys eye appeal first. Then detail. Then the coin itself.

PVC Plasticizer Damage: The Modern Silent Killer

While bronze disease is ancient in origin, PVC damage is entirely modern. In my experience, it is now the number one cause of preventable loss for 20th and 21st century copper coins, tokens, and medals.

For decades, collectors stored coins in soft vinyl flips, vinyl pages, and even in the flexible plastic inserts of early third-party slabs. Those plastics contain plasticizers. Chemicals added to make vinyl soft and pliable.

Over time, especially in heat, those plasticizers migrate out. They react with moisture and with the coin’s surface to form hydrochloric acid and green copper salts. The numismatic value drops fast. Even when the strike remains sharp.

Why Flips and Fake Slabs Are Especially Dangerous

The forum example is instructive here. A counterfeit slab is not just a fake label. It is an unknown polymer shell. Likely heat-sealed with no archival stability, no inert gasket, and no moisture barrier.

I have examined counterfeit holders where the plastic itself had yellowed, become brittle, and off-gassed. A genuine coin sealed in that environment for five years will often emerge with a hazy, green film that no amount of gentle wiping will remove. Because the damage is chemical, not surface dirt.

Genuine, modern archival holders — Mylar, polyethylene, polypropylene — do not contain plasticizers. Soft vinyl does. If your coin is in a flip that smells like a shower curtain, feels tacky, or has turned stiff and yellow, remove it. Today.

The Visual Signature of PVC Damage

PVC damage on bronze and copper is deceptive. It mimics natural toning at first. It kills original luster slowly.

• A green, slimy or greasy film that appears on the surface, often starting where the coin touched the plastic
• Spots that are a vivid, almost neon green — brighter than any natural patina I have ever seen from burial
• Hairline corrosion following the contact points of the flip’s weld, or a perfect outline of the coin’s edge where plasticizer pooled
• Corrosion that continues even after removal from the flip, because the chlorides have already impregnated the surface

In my experience grading, PVC-damaged coins are routinely rejected or downgraded. Not because the design is lost. Because the surface is actively unstable.

Once PVC has reacted, you have essentially seeded the coin with the same chlorides that cause bronze disease. The two problems are chemically linked. A rare variety in PVC is no longer rare in value. It is a liability.

Actionable advice for buyers: When you receive a coin, smell the holder before you open it. Look at the plastic under bright light. If the holder is soft vinyl, cloudy, or discolored, assume the coin has been exposed. Budget for immediate conservation and re-housing in inert material. Check the coin again in 30 days for new growth.

Oxidation vs. Patina: Learning to Read the Surface

Not all color change is damage. Collectors often panic at a dark brown or black surface on a bronze coin. They fear oxidation. When in fact they are looking at a desirable, stable patina that adds value.

As a conservator, my job is to distinguish between protective oxidation and destructive oxidation. It is a line I walk every day in the lab.

Stable patina forms slowly over decades or centuries. On copper alloys, it is typically cuprous oxide (reddish-brown) and tenorite (black). It forms a hard, thin layer that actually protects the metal beneath. It is adherent. Even. It does not powder.

On a Roman bronze, a smooth, dark green or brown patina that follows the contours of the design is often exactly what collectors want. It proves authenticity and age. It preserves the strike. It gives the coin real eye appeal.

Destructive oxidation, by contrast, is porous. Uneven. Active. It lifts. It powders. It pits. It is often accompanied by the bronze disease spots described above. Or by a rough, crusty texture that obscures detail.

Environmental Factors That Accelerate Oxidation

1. Humidity: Sustained humidity above 50% accelerates all copper corrosion. Below 35% is ideal for long-term storage of copper alloys.
2. Temperature swings: Attics and garages cause condensation inside holders. That microscopic dew is more damaging than a single soak.
3. Air pollutants: Sulfur compounds from cardboard, wood, and even some foams accelerate tarnishing. I never store valuable bronzes in original cardboard flips or wooden boxes without an inert barrier.
4. Fingerprints: The salts and oils from skin introduce chlorides directly. I have seen a perfect fingerprint etched in corrosion on a large cent within a year of careless handling.

Conservator’s rule: If the color is hard, stable, and does not transfer to a cotton swab, leave it alone. The most damaging thing you can do to a stable patina is to try to “improve” it. My mantra in the lab is: first, do no harm.

Chemical Conservation: Acetone Dips and When to Stop

Forum threads often ask, “Can I just dip it in acetone to save it?” The answer is: sometimes. Carefully. And only for the right problem.

Acetone is a solvent, not a magic cleaner. It does not remove corrosion. It removes organic materials — PVC residue, old lacquers, adhesives, and surface grime.

In my experience, a proper acetone dip is the first step in treating PVC damage. It dissolves the plasticizer film without attacking the metal. But it will not cure bronze disease. It will not remove stable patina.

Using the wrong chemical at the wrong time can strip value and destroy evidence of authenticity. I have seen it ruin coins that could have been saved.

My Conservator’s Protocol for PVC and Surface Contamination

When a coin arrives in my lab with suspected PVC damage, I follow a strict sequence:

1. Documentation: Photograph obverse, reverse, and edge under magnification before any treatment. Note all spots of green activity.
2. Dry assessment: Gently test with a soft brush. If powder comes off, I treat it as active bronze disease first, not PVC.
3. Acetone soak: I use pure acetone (not nail polish remover, which contains oils and fragrances) in a glass dish. The coin is fully submerged for 5-10 minutes, agitated gently, then removed with inert tweezers.
4. Rinse: A second bath in fresh acetone, then air-dry on clean blotting paper. No heat, no hair dryer.
5. Inspection and re-housing: Under 10x magnification, I check for remaining green spots in pits. If present, the coin is not clean — it is still contaminated and needs further treatment or long-term monitoring.
6. Stabilization: For coins with a history of chloride exposure, I may apply a microcrystalline wax barrier after complete drying, but only if the coin is fully stable. Wax on active disease traps moisture and makes it worse.

What never to do: Never use vinegar, lemon juice, olive oil, baking soda, or commercial coin dips on a bronze you intend to preserve. I have seen all of them strip patina, pit the surface, and leave residues that accelerate future corrosion. Never use a wire brush, a Dremel, or a tumbler. Never dip a coin with active bronze disease in acetone and put it straight back in a holder — you must address the chlorides and dry it thoroughly, ideally in a desiccated environment for several days.

Acetone is safe for the metal. It is not safe for your lungs or for lacquered surfaces. Work in ventilation, away from flame. Understand that acetone will remove any original toning or lacquer that might be part of the coin’s history. That is why I only recommend it when PVC contamination is confirmed.

Storage, Slabs, and the Fake Slab Problem

The forum thread raises a critical point for modern collectors: what happens when a coin sits for years in a counterfeit slab? From a conservation perspective, a fake slab is worse than no slab at all.

A genuine third-party slab is sonically sealed. It uses inert plastics. It includes a gasket that limits moisture exchange. A counterfeit slab is often a two-part shell glued or heat-sealed. With air gaps, unknown plasticizers, and no barrier against humidity.

It creates a microclimate — warm, humid, and chemically active — directly against the coin’s surface. I have opened counterfeit holders for modern copper cents and found the coins hazed green. Identical coins in genuine holders from the same year were bright and stable. The difference was not the coin. It was the plastic.

Best practices I recommend to every client:

• Store copper alloys at 30-40% relative humidity, stable temperature, in inert holders only
• Use Mylar flips, polyethylene bags, or archival slabs — never soft vinyl, never cardboard without a barrier
• Include a small silica gel packet in storage boxes, and replace it when saturated
• Inspect your collection annually under magnification, especially any coin that has changed holders
• Quarantine any new acquisition for 30 days and inspect for active spots before filing it with your main collection

What This Means for Buyers, Sellers, and the Authenticity Guarantee

The eBay Authenticity Guarantee discussed in the forum — where items over $500 are sent for authentication — is a valuable tool against counterfeits. But as a conservator, I must add a warning: authentication is not conservation.

A coin can pass an authenticity check and still be environmentally compromised. A genuine bronze in a PVC flip, returned to a buyer in that same flip, will continue to corrode. The guarantee protects against fakes. It does not protect against bad storage.

For buyers, this means you must inspect beyond the label. Ask sellers: How was this stored? In what holder? For how long? If a seller cannot answer, assume the worst and budget for re-housing and inspection upon arrival.

For sellers, disclosure is not just ethical. It is financial. A coin with early PVC damage that is disclosed and conserved can retain much of its value. The same coin sold undisclosed, which then blooms with bronze disease in the buyer’s cabinet, will result in returns, disputes, and reputational damage far exceeding the original sale price.

The forum members were right to laugh at the idea that a bad actor will simply re-list under a new name. The same applies to bad storage habits. Without education, the cycle repeats. Coin in vinyl. Coin damaged. Coin sold. Coin damaged further.

Conclusion: Collectibility Is Preservation

Collectibility is not just rarity, grade, or a genuine label. In my experience grading and conserving for over two decades, collectibility is stability.

A moderately rare bronze with a stable, original patina will always be more desirable — and more valuable — than a rarer variety actively powdering with bronze disease or slimed with PVC residue. Its numismatic value lives in its surface.

Improper storage is the enemy of numismatics because it is silent, cumulative, and often irreversible. Bronze disease eats from within. PVC plasticizers impregnate the surface with chlorides. Oxidation, when uncontrolled, destroys detail that took centuries to form.

But with proper identification, proper chemical conservation like a careful acetone dip for organic contamination, and proper long-term storage in inert materials at controlled humidity, we can arrest that process.

The “worse example” from that forum thread should be a lesson for all of us. Whether the slab is fake or genuine, whether the guarantee catches the counterfeit or not, the coin’s survival depends on what we do after it arrives. Examine it. Smell the holder. Look under magnification. Re-house it correctly. Document it.

Coins that survive us are not the ones we lock away and forget. They are the ones we actively preserve. As conservators and collectors, that is our shared responsibility — to ensure that the history in our hands outlives the plastic it was stored in.

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