Verdigris and PVC: Saving Your Collection from Environmental Damage — A Numismatic Conservator’s Guide

Improper storage is the silent enemy of numismatics. While we often worry about wear, cleaning, or mishandling, the single greatest threat to a coin’s long-term integrity is the environment it sits in. Let’s identify the telltale signs of environmental damage and, more importantly, learn how to stop it.

As a numismatic conservator with over two decades of experience examining, preserving, and restoring coins across every major metal type, I can tell you that the single greatest threat to a coin’s numismatic value is not wear, not cleaning, and not even mishandling — it is environmental damage. Every day, I see collections that have been silently ravaged by humidity, airborne chemicals, improper storage materials, and neglect. The tragedy is that most of this damage is entirely preventable.

Recently, a fascinating discussion unfolded on a collector’s forum about a 1963-D Lincoln cent bearing mysterious raised symbols on both sides. The original poster wondered whether the marks were post-mint damage (PMD) or something more exotic — perhaps a mint inspection mark that escaped into circulation. The thread generated passionate debate, with experienced collectors and experts weighing in on the nature of the marks, the physics of metal displacement, and the optical illusions that can fool even careful observers. While the consensus ultimately settled on PMD — a counterstamp applied after the coin left the mint — the discussion raised a broader and far more important question: what happens to coins like this after they’ve been altered, and how do the environments in which they’re stored determine their fate?

That question is the heart of this article. Whether you’re dealing with a counterstamped Lincoln cent, a bronze Roman provincial, or a copper-nickel Morgan dollar, the principles of environmental damage and conservation are universal. Let me walk you through the four major categories of environmental damage I encounter most frequently in my conservation work — and what you can do to protect your collection.

1. Understanding the Enemies: What Environmental Damage Actually Looks Like

Before we can fight environmental damage, we need to understand what we’re fighting. In my experience grading and conserving coins, environmental damage falls into several distinct categories, each with its own telltale signs, chemical mechanisms, and remediation strategies.

The Four Major Categories of Environmental Damage

• Bronze Disease — A progressive, self-sustaining corrosion process specific to copper and copper-alloy coins (bronze, brass, copper-nickel). It appears as bright green, powdery or crusty spots that spread if left untreated.
• PVC Plasticizer Damage — Caused by prolonged contact with polyvinyl chloride (PVC) flips, holders, and albums. It manifests as a greenish, sticky or waxy film that etches into the coin’s surface over time.
• Oxidation and Tarnish — The natural darkening or discoloration of a coin’s surface due to chemical reactions with oxygen, sulfur compounds, and other atmospheric gases. While sometimes considered “patina,” uncontrolled oxidation can destroy original luster and detail.
• Chemical Contamination — Damage from exposure to acids, salts, moisture, and pollutants. This includes fingerprints (which contain oils and acids), salt air, and off-gassing from improper storage materials.

Each of these damage types can be subtle in its early stages, which is precisely what makes them so dangerous. A coin can sit in a PVC flip for years before the damage becomes visible to the naked eye — and by then, the harm is often irreversible.

2. Bronze Disease: The Silent Killer of Copper-Alloy Coins

If I had to name the single most destructive form of environmental damage I encounter in my conservation practice, it would be bronze disease. This is not a metaphor — it is a literal, ongoing chemical reaction that will consume a copper-alloy coin from the inside out if left unchecked.

What Causes Bronze Disease?

Bronze disease is caused by the presence of chloride ions in contact with copper or copper alloys. These chloride ions can originate from a variety of sources:

• Soil contamination (coins excavated from archaeological sites are especially vulnerable)
• Exposure to saltwater or salt air
• Handling with bare hands (sweat contains sodium chloride)
• Storage in environments with high humidity, which accelerates the electrochemical reaction

The chemical process works like this: chloride ions react with copper to form cuprous chloride (CuCl). When this compound is exposed to moisture and oxygen, it undergoes a cyclical reaction that produces hydroxychloride compounds — the bright green, powdery spots you see on affected coins. Critically, this reaction regenerates the chloride ion, which then attacks more copper, perpetuating the cycle indefinitely.

How to Identify Bronze Disease

In my experience, bronze disease is often mistaken for ordinary verdigris or patina. Here’s how to tell the difference:

• Ordinary verdigris is typically dark green, blue-green, or brownish-green, and forms a relatively stable layer on the coin’s surface. It does not actively spread once the coin is removed from the corrosive environment.
• Bronze disease appears as bright, almost neon green spots or patches that are powdery or crusty to the touch. These spots will grow over time, even in seemingly dry conditions, because the reaction generates its own moisture.
• Active bronze disease often has a slightly wet or slimy appearance under magnification, even when the coin feels dry.

Treating Bronze Disease

Treatment depends on the severity of the infection:

1. Mechanical removal — For mild cases, carefully scraping away the powdery corrosion under magnification using a wooden or plastic pick (never metal, which can scratch the surface). This removes the visible corrosion but does not address chloride ions trapped beneath the surface.
2. Chemical treatment with sodium sesquicarbonate — This is the gold standard for bronze disease treatment. The coin is soaked in a 5% solution of sodium sesquicarbonate, which converts the unstable cuprous chloride into stable cuprous oxide. Soaking times can range from days to weeks depending on severity.
3. Electrolytic reduction — For severe cases, I sometimes use a controlled electrolytic process to strip away corrosion products. This is an advanced technique that should only be performed by experienced conservators, as it can damage the coin’s original surface if done incorrectly.
4. Sealing with benzotriazole (BTA) — After treatment, I apply a BTA solution, which forms a protective complex on the copper surface, inhibiting future corrosion. This is a critical step that many amateur restorers skip, leading to recurrence.

Conservator’s Note: If you suspect bronze disease on a valuable coin, do not attempt to treat it yourself without proper training. Improper treatment can cause far more damage than the disease itself. Consult a professional numismatic conservator.

3. PVC Plasticizer Damage: The Slow Burn

Of all the forms of environmental damage I see, PVC damage is perhaps the most insidious because it is so preventable. For decades, PVC-containing flips and holders were the standard in the coin industry. Millions of coins were stored in these materials, and the damage accumulated silently over years and decades.

How PVC Damages Coins

PVC (polyvinyl chloride) is a plastic that requires the addition of plasticizers — typically phthalate compounds — to make it flexible. Over time, these plasticizers migrate out of the PVC material and deposit onto the coin’s surface. The process is accelerated by:

• Heat — Higher temperatures increase the rate of plasticizer migration
• Humidity — Moisture acts as a catalyst for the chemical reaction between plasticizers and metal surfaces
• Time — The longer a coin remains in PVC, the worse the damage
• Pressure — Coins pressed tightly against PVC (as in album pages) suffer more damage than those loosely stored

The plasticizer residue reacts with the coin’s metal surface, creating a greenish, sticky or waxy film that is extremely difficult to remove. On silver coins, this often appears as a hazy, iridescent discoloration. On copper coins, it manifests as a greenish-blue film. On gold coins — which are more resistant to chemical attack — PVC damage is less common but can still occur, particularly on 22-karat and lower alloys.

Identifying PVC Damage

Here are the key indicators I look for when examining a coin for PVC damage:

• A greenish or bluish-green haze on the coin’s surface, often most visible in the fields (the flat areas around the design)
• A sticky or tacky feel when the coin is handled (though this may not be apparent if the coin has been out of PVC for some time)
• Iridescent or rainbow-like toning that appears unnatural or uneven
• On high-grade coins, a loss of original mint luster that cannot be attributed to wear or cleaning
• Damage that is concentrated on one side of the coin — the side that was in direct contact with the PVC holder

Treating PVC Damage: The Acetone Soak

This is where the topic of proper chemical conservation becomes critical. The most common and effective treatment for PVC damage is an acetone soak — and I want to be very clear about what this means, because the term “acetone dip” is widely misunderstood in the collecting community.

Acetone (CH₃COCH₃) is a powerful organic solvent that dissolves PVC plasticizer residue without affecting the metal of the coin itself. Pure, reagent-grade acetone will not harm a coin’s original surface, luster, or patina. This is a crucial distinction: acetone removes foreign material (the plasticizer film) without attacking the coin’s own metal.

Here is the proper procedure I use in my conservation lab:

1. Use only 100% pure acetone — Hardware store acetone often contains water, oils, or other additives that can leave residue on the coin. I use reagent-grade or ACS-grade acetone exclusively.
2. Soak the coin in a glass container — Never use plastic containers, as acetone will dissolve many plastics. A glass jar with a glass or PTFE (Teflon) lid is ideal.
3. Soak for 5–15 minutes — For mild PVC damage, a short soak is usually sufficient. For heavy damage, I may soak for up to 30 minutes, changing the acetone if it becomes discolored.
4. Remove and allow to air-dry — Acetone evaporates rapidly and completely, leaving no residue. Do not rinse with water. Do not rub or wipe the coin.
5. Repeat if necessary — For severe cases, multiple soaks may be required.

Critical Warning: Do not use nail polish remover as a substitute for pure acetone. Most nail polish removers contain water, fragrances, dyes, and conditioning oils that will deposit on the coin’s surface and cause additional damage. Only use 100% pure acetone from a reputable chemical supplier.

It is also important to understand what acetone cannot do. Acetone will not:

• Remove original toning or patina (if the toning is natural and bonded to the metal surface)
• Repair scratches, gouges, or other physical damage
• Remove heavy corrosion or bronze disease
• Restore mint luster that has been lost to wear

Acetone is a cleaning agent, not a restoration agent. It removes contaminants; it does not rebuild what has been lost.

4. Oxidation and Tarnish: The Inevitable Process

Oxidation is the most universal form of environmental damage. Every metal coin, regardless of composition, will react with its environment to some degree. The question is not whether oxidation will occur, but how quickly and how severely.

Metal-Specific Oxidation Patterns

Different metals oxidize in different ways, and understanding these patterns is essential for proper conservation:

• Copper and Bronze: Copper reacts with oxygen to form cuprous oxide (Cu₂O, red) and cupric oxide (CuO, black). In the presence of moisture and carbon dioxide, it forms copper carbonate (CuCO₃, green) — the classic “verdigris” patina. While a stable, even patina can be desirable on ancient coins and can actually enhance eye appeal, uneven or active oxidation is destructive.
• Silver: Silver reacts with sulfur compounds in the atmosphere to form silver sulfide (Ag₂S), which appears as yellow, brown, or black tarnish. This is the most common form of environmental damage on silver coins and is generally reversible through proper conservation techniques.
• Gold: Pure gold is highly resistant to oxidation, which is one reason it has been prized as a monetary metal for millennia. However, gold coins are rarely pure — most contain copper or silver alloys that will oxidize. This is why you sometimes see discoloration or “copper spots” on gold coins.
• Nickel: Nickel forms a thin, protective oxide layer that actually helps prevent further corrosion. However, in humid or salty environments, nickel alloys can develop a greenish corrosion product similar to bronze disease.

Controlling Oxidation Through Proper Storage

The key to controlling oxidation is controlling the coin’s environment. Here are the storage conditions I recommend:

• Relative humidity below 40% — This is the single most important factor. High humidity accelerates every form of environmental damage. Use silica gel packets or a dehumidifier in your storage area.
• Stable temperature — Avoid large temperature fluctuations, which cause condensation on coin surfaces. A consistent temperature between 65–72°F (18–22°C) is ideal.
• PVC-free storage materials only — Use Mylar (polyethylene terephthalate) flips, archival-quality cardboard holders, or inert plastic capsules. Avoid all PVC-containing products.
• Minimal handling — Always handle coins by the edges, and consider wearing nitrile gloves (not cotton, which can catch on sharp details and transfer oils).
• Avoid paper and cardboard — Standard paper and cardboard contain sulfur compounds and acids that can cause toning and corrosion over time. Use only archival-quality, acid-free materials.

5. The Forum Case Study: What the 1963-D Cent Teaches Us About Damage and Perception

Returning to the forum thread that inspired this article, the discussion about the 1963-D cent with mysterious symbols offers several valuable lessons for collectors — not just about PMD versus mint errors, but about the broader challenges of coin examination and conservation.

The Optical Illusion Problem

The original poster was convinced that the symbols on his coin were raised — standing above the coin’s surface — which would have been consistent with a die modification rather than a post-mint punch. Multiple experienced collectors examined the images and concluded the symbols were actually incuse (sunken into the surface), consistent with a counterstamp or punch mark. The original poster eventually confirmed under microscope examination that the symbols were indeed incuse, admitting he had been “fooled by an optical illusion.”

This is a critically important lesson for all collectors: lighting direction matters enormously when examining coin surfaces. As one forum member pointed out, the lighting on the date and “LIBERTY” came from above, but the apparent lighting on the “&” symbol appeared to come from below — a telltale sign that the symbol was actually incuse, not raised. The brain interprets shadows based on expected lighting direction, and when an unexpected feature appears, it can easily reverse the perceived relief.

In my conservation work, I use raking light — light directed at a very low angle across the coin’s surface — to reveal surface details that are invisible under normal lighting. This technique is invaluable for identifying:

• Counterstamps and punch marks
• Surface scratches and tooling marks
• Environmental damage (PVC film, corrosion products)
• Die characteristics and mint marks

The PMD Question and Its Conservation Implications

The forum consensus was that the symbols were post-mint damage (PMD) — specifically, a counterstamp applied using a punch and hammer after the coin left the mint. One expert explained the physics clearly: a well-braced punch applies downward force only on the area directly underneath the punch, displacing metal downward and outward. This creates a raised area on the opposite side of the coin — which is exactly what was observed on the reverse, where a flattened spot appeared on the memorial building directly underneath the punch mark on Lincoln’s face.

Another collector described an alternative method: punching a design into soft steel, placing the coin over the punched area, placing a soft material (like wood) on top, and striking it with a hammer. This drives the coin’s surface into the punched design, creating a raised image on the coin. The original poster noted that the symbols on the obverse and reverse appeared slightly different, suggesting they may not have been made by the same tool — consistent with a hand-applied counterstamp rather than a die modification.

From a conservation standpoint, counterstamped coins present unique challenges:

• The displaced metal around a counterstamp is often work-hardened and more susceptible to corrosion than the surrounding surface.
• The sharp edges of a counterstamp can trap moisture and contaminants, creating localized corrosion cells.
• Counterstamped coins stored in PVC holders may develop accelerated PVC damage in the crevices of the stamp, where plasticizer residue accumulates.

6. Proper Chemical Conservation: Beyond the Acetone Soak

While acetone soaks are the most commonly discussed chemical conservation technique, they are just one tool in a conservator’s arsenal. Here is an overview of the chemical treatments I use in my practice, organized by damage type:

For PVC Plasticizer Removal

• Acetone soak — As described above, the standard treatment for PVC residue.
• Isopropyl alcohol (99%) — A milder alternative for coins with light PVC damage or for coins that may have delicate toning that you want to preserve. Less effective than acetone for heavy PVC contamination.

For Oxidation and Tarnish Removal

• Acetone — Effective for removing light tarnish and organic contaminants from silver coins without altering the underlying metal.
• Thiourea-based solutions — Used for more aggressive tarnish removal on silver. These solutions dissolve silver sulfide without attacking the metallic silver beneath. Use with extreme caution — over-treatment can strip original luster.
• Formic acid solutions (dilute) — Occasionally used for copper coins with heavy oxide buildup. This is an advanced technique that requires precise concentration control and timing.

For Bronze Disease Treatment

• Sodium sesquicarbonate soak — The standard treatment, as described in Section 2.
• Benzotriazole (BTA) application — Used as a preventive treatment after bronze disease removal, and as a standalone protective coating for copper-alloy coins in long-term storage.
• Silver oxide packing — For severe bronze disease on small areas, I sometimes apply a paste of silver oxide and water directly to the affected area. This converts the unstable chloride compounds to stable silver chloride, which is then removed mechanically.

Conservator’s Rule #1: When in doubt, do nothing. A coin with environmental damage is almost always worth more than a coin that has been improperly “conserved” by an untrained hand. If you are unsure about a treatment, consult a professional.

7. Prevention: The Best Conservation Is No Conservation

The most important lesson I can share with collectors is that prevention is infinitely preferable to treatment. Every chemical treatment, every mechanical cleaning, every conservation intervention carries some risk. The goal should always be to store your coins in conditions that minimize the need for intervention in the first place.

The Ideal Storage Environment

Here is my recommended storage protocol for collectors at every level:

1. Individual coin holders — Each coin should be stored in its own inert holder. For raw coins, use Mylar flips (not PVC), Air-tite capsules, or archival-quality cardboard flips. For certified coins, the slab itself provides protection, but store slabs in a cool, dry environment away from direct sunlight.
2. Climate control — Maintain a stable environment with relative humidity below 40% and temperature between 65–72°F. Use a hygrometer to monitor conditions. In humid climates, a room dehumidifier is essential.
3. Avoid basements and attics — These areas typically have the most extreme temperature and humidity fluctuations. A closet on an interior wall of your home is usually the best storage location.
4. Regular inspection — Examine your collection at least twice a year under good lighting. Look for early signs of environmental damage: green spots (bronze disease), hazy film (PVC damage), or unusual discoloration (oxidation or chemical contamination).
5. Keep records — Document the condition of your coins with photographs when you acquire them. This makes it much easier to detect changes over time and provides valuable provenance information for future buyers.

Materials to Avoid

Based on my experience examining damaged collections, here are the storage materials I tell my clients to avoid at all costs:

• PVC flips, holders, and albums — The number one cause of preventable environmental damage in numismatics.
• Rubber bands — Rubber contains sulfur compounds that cause rapid tarnishing of silver and copper coins.
• Paper envelopes and standard cardboard — Contain acids and sulfur compounds that cause toning and corrosion.
• Adhesive labels and tape — The adhesive can chemically react with coin surfaces and leave permanent residue.
• Cotton gloves — Contrary to popular belief, cotton gloves are not recommended for coin handling. They reduce dexterity, can catch on sharp edges, and transfer oils and moisture to the coin surface. Bare, clean hands (washed and dried) or nitrile gloves are preferred.

8. When to Call a Professional Conservator

Not every coin needs professional conservation, and not every conservation treatment should be performed by a professional. Here is my guidance on when to seek expert help:

Definitely Consult a Professional If:

• The coin is high-value (worth more than you can afford to lose if something goes wrong)
• You suspect bronze disease on a copper-alloy coin
• The coin has heavy corrosion that obscures design details
• You are considering any chemical treatment on a coin with original mint luster or desirable toning
• The coin is an ancient or historical artifact with archaeological significance

You Can Probably Handle It Yourself If:

• The coin has light PVC damage and you have access to pure acetone
• The coin is a common-date, low-value piece that you are willing to risk
• You are performing preventive conservation (applying BTA, re-holding in inert materials)

Conclusion: Protecting the Past for the Future

The story of the 1963-D cent with its mysterious counterstamp is, in many ways, a microcosm of the broader challenges facing coin collectors. It is a story about the difficulty of accurate identification, the ease of optical illusion, the importance of expert consultation, and the passion that drives collectors to seek answers. But it is also a story about what happens after the identification — about how a coin is stored, preserved, and protected for future generations.

Whether your collection consists of modern Lincoln cents or ancient bronze sestertii, the principles are the same: control the environment, use inert materials, inspect regularly, and intervene only when necessary and with proper knowledge. Environmental damage is not a sudden catastrophe — it is a slow, cumulative process that can be halted and, in many cases, prevented entirely with proper care.

As a conservator, I have seen coins that were nearly destroyed by decades of improper storage, and I have seen coins that survived centuries in the ground and emerged with their surfaces largely intact. The difference is almost always the environment. Give your coins the environment they deserve, and they will reward you with decades — or centuries — of beauty, collectibility, and value.

The next time you pull a coin from a PVC flip and notice that telltale greenish haze, or you spot a bright green spot on a bronze coin that wasn’t there last year, you’ll know exactly what to do. And if you’re ever in doubt, remember: when it comes to conservation, patience and caution are always the right choices.