Verdigris and PVC: Saving Odd and Bizarre PCGS Slabbed Coins from Environmental Damage

Improper storage is the sworn enemy of numismatics. Let’s identify the signs of environmental damage specific to this metal type—before it’s too late.

As a numismatic conservator, I’ve spent decades examining coins that have survived wars, economic collapses, and generations of well-meaning but misguided collectors. Yet nothing breaks my heart quite like encountering a rare or historically significant coin—perhaps one of those wonderfully odd PCGS-slabbed pieces that make us ask “why was that slabbed?”—only to find it ravaged by entirely preventable environmental damage. The forum thread “Odd and Bizarre PCGS Slabbed Coins” showcases exactly the kind of pieces that deserve our attention: triple-dated Kennedy halves, unusual toning patterns, and coins that push the boundaries of what we consider collectible. But behind the humor and curiosity lies a serious conservation challenge, one that can make or break a coin’s numismatic value overnight.

Whether you’re a seasoned collector, a historian, or an investor, understanding how environmental factors degrade coins—particularly copper, bronze, and their alloys—is absolutely essential. In this analysis, I’ll walk you through the primary threats I encounter most often: bronze disease, PVC plasticizer damage, oxidation, and the proper chemical conservation techniques (including acetone dips) that can save these pieces from irreversible harm.

The Silent Killers: Understanding Environmental Threats to Copper and Bronze Coins

Copper and its alloys—bronze, brass, and the copper-nickel clad compositions found in modern issues—are among the most reactive metals in numismatics. Unlike gold or platinum, which remain largely inert, copper actively interacts with its environment. This reactivity is precisely what creates those beautiful rainbow toning patterns collectors adore. But it’s also what makes these coins so deeply vulnerable.

In my experience grading and conserving coins, I’ve identified four primary environmental threats that account for the vast majority of damage I encounter:

• Bronze disease – A self-perpetuating chemical reaction that can destroy a coin entirely
• PVC plasticizer damage – Caused by improper storage in vinyl holders
• Oxidation and toning progression – Natural but potentially destructive surface changes
• Atmospheric pollutants – Sulfur compounds, chlorine, and moisture in the air

Each of these threats requires different identification techniques and conservation approaches. Let’s examine them in detail.

Bronze Disease: The Numismatic Nightmare

What Is Bronze Disease?

Bronze disease is perhaps the most feared condition in copper and bronze coin conservation. Despite its name, it doesn’t affect only bronze—it can strike any copper alloy, including the copper-nickel clad Kennedy halves that feature so prominently in those “odd PCGS slabbed” discussions. The condition is caused by the presence of chlorides on or within the coin’s surface, which react with moisture and oxygen to form copper hydrochloride and other corrosive compounds.

Here’s what makes it truly terrifying: the chemical reaction is autocatalytic. That means it produces the very conditions that accelerate further decay. Once bronze disease takes hold, it will continue to destroy the coin until the chlorides are neutralized or removed—or until there’s simply nothing left to destroy.

Identifying Bronze Disease

From a conservator’s perspective, early detection is absolutely critical. Here are the telltale signs I look for during every examination:

• Bright green, powdery spots – Unlike stable verdigris (which forms a protective patina), bronze disease appears as raised, pale green, almost fuzzy deposits
• Pitting beneath the green deposits – When you carefully remove the powdery growth, you’ll find active pitting in the metal beneath
• Progressive spread – The spots multiply and expand over time, even in seemingly dry conditions
• Acidic smell – In advanced cases, you may detect a faint acidic odor when the coin is warmed slightly

I’ve examined coins from shipwrecks, buried hoards, and even some of those unusual PCGS-slabbed pieces that spent decades in humid environments. Bronze disease doesn’t discriminate based on rarity or value—it will attack a common cent just as readily as a rare pattern piece with exceptional provenance.

Why PCGS-Slabbed Coins Can Still Develop Bronze Disease

One might assume that encapsulation in a PCGS slab would protect against bronze disease. Unfortunately, that isn’t always the case. If chlorides were present on the coin’s surface at the time of encapsulation—perhaps from handling with bare hands, previous burial, or storage in a contaminated environment—the sealed environment can actually trap moisture and accelerate the reaction.

I’ve seen PCGS-slabbed coins where bronze disease developed inside the holder, creating those bizarre green blobs that make collectors question why the coin was slabbed in the first place. The slab protects against physical handling damage and some atmospheric exposure, but it cannot reverse chemical contamination that occurred before encapsulation. That’s a crucial distinction every collector needs to understand.

PVC Plasticizer Damage: The Hidden Threat in Your Collection

The Chemistry of PVC Degradation

Polyvinyl chloride (PVC) was once the standard material for coin flips, albums, and storage pages. It’s flexible, transparent, and inexpensive—qualities that made it enormously popular from the 1960s through the 1980s. But PVC contains plasticizers (typically phthalates) that migrate out of the plastic over time, especially in warm or humid conditions.

These plasticizers react with the metal surface of coins, creating a distinctive green, sticky, or waxy residue. Unlike bronze disease, PVC damage is not autocatalytic—it won’t continue to destroy the coin once the PVC source is removed. However, the chemical reaction can etch the coin’s surface, causing permanent loss of detail and luster that no amount of conservation can fully reverse.

Recognizing PVC Damage

In my conservation work, I identify PVC damage through several characteristic markers:

• Green or bluish-green residue – Often appears as a film or deposit, particularly in recessed areas of the design
• Sticky or tacky surface – The plasticizer residue feels distinctly different from normal toning or dirt
• Distinctive odor – PVC off-gassing has a characteristic chemical smell, especially noticeable when the coin is first removed from its holder
• Associated with specific storage types – Coins stored in soft PVC flips, older albums, or certain plastic tubes are most commonly affected

Those odd PCGS-slabbed coins that collectors love to discuss? Many of them were rescued from PVC holders before encapsulation. The toning patterns you see—sometimes bizarre, sometimes beautiful—may actually be the result of PVC interaction with the coin’s surface. Collectibility often depends on distinguishing between naturally developed patina and chemically induced damage.

The PCGS Slab Advantage

Modern PCGS slabs are manufactured from inert, PVC-free plastics. When a coin is properly slabbed by PCGS, it’s protected from further PVC exposure. However, any damage that occurred before encapsulation remains. This is why I always recommend that collectors inspect coins carefully before purchasing slabbed pieces, especially those with unusual toning or surface deposits that could be masking deeper problems.

Oxidation and Toning: Beauty and Danger in Equal Measure

The Science of Coin Toning

Oxidation is the natural process by which copper reacts with oxygen to form copper oxide. This thin layer of oxide is what we call toning, and it’s responsible for the spectacular color patterns that make some coins extraordinarily valuable. The thin-film interference effect—the same phenomenon that creates oil slicks on water—produces the rainbow hues that collectors prize for their eye appeal.

But not all oxidation is benign. In my experience, I’ve categorized toning into three distinct types:

1. Stable patina – A thin, even layer of oxidation that protects the underlying metal and enhances the coin’s appearance
2. Active toning – Oxidation that continues to progress, potentially obscuring details and reducing eye appeal
3. Destructive oxidation – Heavy corrosion that physically damages the coin’s surface and reduces its grade

When Toning Becomes Damage

The line between beautiful toning and destructive oxidation can be razor-thin. I’ve examined coins with stunning rainbow toning that was actually the early stages of bronze disease. I’ve also seen coins with what appeared to be ugly, dark toning that was actually a stable patina protecting the surface beneath.

For those odd PCGS-slabbed coins with unusual toning—the ones that make collectors ask “why was that slabbed?”—the answer often lies in the toning itself. Some of these coins display toning patterns so unusual or aesthetically striking that they warrant encapsulation despite (or because of) their unconventional appearance. But collectors need to know: that same toning could be a ticking time bomb.

Environmental Factors That Accelerate Oxidation

Several environmental conditions can push oxidation beyond the desirable rate:

• High humidity – Moisture is essential for most oxidation reactions
• Elevated temperatures – Heat accelerates chemical reactions, including corrosion
• Sulfur compounds – Present in some papers, fabrics, and atmospheric pollution
• Salt exposure – Particularly dangerous for copper alloys, as it introduces chlorides
• Acidic environments – Some storage materials off-gas acids that attack metal surfaces

Proper Chemical Conservation: The Acetone Dip and Beyond

When to Intervene

As a conservator, my first principle is simple: do no harm. Not every coin needs chemical treatment, and improper conservation can cause more damage than the original problem. I’ve seen well-intentioned collectors ruin mint condition surfaces with aggressive cleaning.

That said, when environmental damage threatens a coin’s preservation, intervention becomes necessary. I recommend chemical conservation when:

• Bronze disease is actively progressing
• PVC residue is present on the surface
• Destructive oxidation is obscuring important details or the coin’s strike
• The coin’s long-term preservation is at risk

The Acetone Dip: Technique and Precautions

Acetone (CH₃COCH₃) is the workhorse solvent in numismatic conservation. It’s effective at removing organic contaminants—including PVC plasticizer residue, oils, and adhesives—without reacting with copper or its alloys. Here’s the proper technique I use in my conservation practice:

1. Use pure, reagent-grade acetone – Hardware store acetone often contains additives that can leave residues. Purchase 99.9% pure acetone from a chemical supply company.
2. Work in a well-ventilated area – Acetone fumes are flammable and potentially harmful. Never use near open flames or in enclosed spaces.
3. Immerse the coin completely – Place the coin in a glass or stainless steel container and cover with acetone. Do not use plastic containers, as acetone will dissolve many plastics.
4. Soak for 15-30 minutes – For light contamination, a brief soak is sufficient. For heavy PVC residue, multiple soaks may be necessary.
5. Agitate gently – Swirl the container occasionally to help dislodge contaminants. Do not scrub the coin’s surface under any circumstances.
6. Allow to air dry – Remove the coin and place it on a clean, lint-free surface. Allow the acetone to evaporate completely. Do not rinse with water.
7. Inspect and repeat if necessary – Examine the coin under magnification. If residue remains, repeat the process with fresh acetone.

What Acetone Cannot Do

It’s important to understand the limitations of acetone treatment, because unrealistic expectations lead to frustrated collectors:

• Acetone will not remove bronze disease – The chlorides that cause bronze disease are not soluble in acetone. Specialized treatment with benzotriazole (BTA) or sodium sesquicarbonate is required.
• Acetone will not reverse etching – If PVC damage has already etched the coin’s surface, acetone can remove remaining residue but cannot restore lost metal.
• Acetone will not stop active oxidation – While acetone removes organic contaminants, it doesn’t create a protective barrier against future oxidation.
• Acetone may affect some toning – In rare cases, acetone can alter certain types of toning, particularly those caused by organic compounds.

Advanced Conservation Techniques

For coins with severe environmental damage, more aggressive conservation may be necessary. These are the methods I turn to when standard approaches aren’t enough:

• Benzotriazole (BTA) treatment – A corrosion inhibitor that bonds with copper surfaces to prevent further oxidation and bronze disease progression
• Sodium sesquicarbonate soak – Used to remove chlorides from coins affected by bronze disease, particularly effective for excavated specimens
• Electrolytic reduction – A technique used for heavily corroded coins, particularly those from marine environments
• Mechanical cleaning – Under magnification, using wooden picks or specialized tools to remove corrosion products without damaging the underlying surface

I want to emphasize that these advanced techniques should only be performed by experienced conservators. Improper application can cause irreversible damage to rare and valuable coins. When in doubt, seek professional help.

Prevention: The Best Conservation Strategy

Proper Storage Materials

The most effective conservation is prevention. Based on my decades of experience, I recommend the following storage practices without reservation:

• Use only inert storage materials – Mylar (polyester) flips, archival-quality cardboard holders, and PVC-free albums are essential
• Avoid paper and cardboard directly against coins – Standard paper contains sulfur and acids that can cause toning and corrosion
• Consider slabbed storage for valuable pieces – PCGS, NGC, and other reputable grading services use inert plastics that provide excellent long-term protection
• Never use rubber bands, plastic bags, or adhesive mounts – These materials contain compounds that react with copper alloys

Environmental Controls

Maintaining proper environmental conditions is equally important:

• Control humidity – Maintain relative humidity below 50%, ideally between 30-40% for copper and bronze coins
• Stabilize temperature – Avoid rapid temperature fluctuations that can cause condensation on coin surfaces
• Minimize air exposure – Store coins in sealed containers when possible, particularly in polluted urban environments
• Use desiccants – Silica gel packets in storage containers help maintain low humidity
• Avoid basements and attics – These areas typically have the most extreme temperature and humidity fluctuations

Handling Best Practices

Even with perfect storage, improper handling can introduce contaminants that start the corrosion clock ticking:

• Always handle coins by the edges – Fingerprints contain oils, salts, and acids that can initiate corrosion
• Wear cotton or nitrile gloves – Particularly important for copper and bronze coins with delicate surfaces
• Never clean coins with commercial products – Most coin cleaners are abrasive or acidic and cause permanent damage
• Inspect coins regularly – Early detection of environmental damage allows for prompt intervention before the problem spirals

Case Studies: Environmental Damage in Odd and Bizarre PCGS-Slabbed Coins

The Triple-Dated Kennedy Half

Those fascinating triple-dated Kennedy halves that appear in “odd PCGS slabbed” discussions present unique conservation challenges. The multiple dates create additional surfaces and edges where corrosion can initiate. I’ve examined specimens where bronze disease had begun at the date area, likely due to moisture trapped against the high points of the design during improper storage.

For these coins, I recommend:

• Immediate removal from any PVC-containing holders
• Careful inspection under magnification for early signs of bronze disease
• Acetone treatment if PVC residue is present
• Encapsulation in inert holders for long-term preservation

Unusually Toned Specimens

The bizarre toning patterns that make some PCGS-slabbed coins conversation pieces often result from environmental exposure. While some of this toning is stable and aesthetically pleasing—genuinely enhancing the coin’s eye appeal—other patterns indicate active corrosion that’s eating away at the surface. I’ve learned to distinguish between the two by examining the surface texture closely. Stable toning is smooth and adherent, while active corrosion often appears raised or powdery.

For collectors acquiring these pieces:

• Request detailed photographs before purchase, especially close-ups of the toning
• Examine the coin in person if possible, using a loupe or microscope
• Ask about the coin’s storage history and provenance
• Consider professional conservation assessment for valuable pieces or rare varieties

The Investment Perspective: How Environmental Damage Affects Value

For investors and collectors focused on financial returns, environmental damage represents a significant and often underestimated risk. A coin’s grade—and therefore its numismatic value—can be dramatically affected by corrosion, PVC residue, and improper cleaning. I’ve seen rare coins lose 50% or more of their value due to environmental damage that could have been prevented with basic storage precautions.

Key considerations for investors:

• Slabbed coins aren’t immune – PCGS encapsulation protects against future damage but doesn’t reverse existing problems lurking beneath the surface
• Unusual toning can be a red flag – While some toning enhances value, bizarre patterns may indicate active environmental damage
• Conservation can restore value – Professional treatment can stabilize damaged coins and preserve their investment potential
• Documentation matters – Coins with documented conservation history often command premium prices from knowledgeable buyers who understand what proper preservation means

Conclusion: Preserving Numismatic History for Future Generations

Those odd and bizarre PCGS-slabbed coins that spark so much discussion in collector forums represent more than curiosities—they’re pieces of numismatic history that deserve our protection. Whether it’s a triple-dated Kennedy half, an unusually toned cent, or any other copper-alloy coin that catches your eye, understanding and preventing environmental damage is essential to preserving these artifacts for future generations.

Bronze disease, PVC plasticizer damage, and destructive oxidation are serious threats, but they’re not inevitable. Through proper storage, careful handling, and appropriate chemical conservation when necessary, we can protect these coins from the environmental factors that would otherwise destroy them. The acetone dip, when performed correctly, remains one of the most effective tools in our conservation arsenal for removing organic contaminants without harming the underlying metal.

As both a conservator and a collector, I find that the most rewarding aspect of this work is rescuing coins from environmental damage and restoring them to stable condition. Every coin we preserve—no matter how odd or bizarre it may appear—contributes to our understanding of numismatic history and ensures that future collectors can appreciate these remarkable pieces.

The next time you encounter one of those “why was that slabbed?” coins, look beyond the humor and consider the conservation story it might tell. With proper care and attention, even the most environmentally damaged coins can be stabilized and preserved, maintaining their historical significance, eye appeal, and collectible value for years to come.

Related Resources

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