Verdigris and PVC: Saving Your Bronze Coins from Environmental Damage

Improper storage is the silent enemy of every numismatist’s collection. Let’s identify the signs of environmental damage specific to this metal type — and, more importantly, learn how to stop it before it destroys the coins we cherish.

As a numismatic conservator with decades of experience examining, preserving, and restoring coins across every metal type and era, I can tell you without hesitation that environmental damage is one of the most insidious threats a collection faces. Unlike a scratch or a cleaning scar — immediately visible and permanently recorded on a coin’s surface — environmental degradation is slow, silent, and devastatingly progressive. By the time a collector notices the telltale green crust on a Roman bronze sestertius or the hazy film on a coin stored in a PVC-laden flip, significant and often irreversible harm may have already been done.

This article tackles the specific environmental threats that plague copper and bronze coins — the workhorses of ancient and medieval numismatics — and what you can do about them. Whether you’re a seasoned collector of Byzantine folles, a beginner assembling a set of large cents, or an investor holding certified ancient bronzes, understanding these threats is essential to protecting the numismatic value and integrity of your holdings.

Understanding Why Bronze and Copper Are So Vulnerable

Before we discuss specific types of damage, it’s worth understanding why bronze and copper alloy coins are disproportionately affected by environmental factors compared to their gold and silver counterparts.

Copper is a highly reactive metal. When exposed to moisture, oxygen, chlorides, sulfur compounds, and various organic acids, copper readily forms a range of chemical compounds on its surface. Some of these compounds — like the stable, protective patina seen on ancient bronzes — can actually preserve a coin’s surface. Others, like the notorious “bronze disease,” are actively destructive and will eat through a coin’s metal if left unchecked.

Bronze, primarily an alloy of copper and tin, inherits copper’s reactivity while introducing additional complexities. The tin component can form its own oxidation products, and the interface between copper-rich and tin-rich phases within the alloy can create galvanic corrosion cells — essentially tiny batteries within the metal itself that accelerate deterioration in the presence of electrolytes like humidity.

In my experience grading and conserving ancient bronzes, I’ve seen coins that were perfectly preserved in stable burial environments for two thousand years deteriorate dramatically within just a few years of improper storage in a modern collection. The lesson is clear: the environment in which you store your coins matters as much as the coins themselves.

Bronze Disease: The Numismatist’s Nightmare

If there is one topic that every collector of ancient or historical bronze coins must understand, it is bronze disease. I’ve examined hundreds of affected coins, and I can assure you that recognizing it early is the difference between a straightforward conservation treatment and a coin that is essentially lost.

What Is Bronze Disease?

Bronze disease is a form of chloride corrosion that affects copper alloy coins, sculptures, and artifacts. It occurs when chloride ions — typically introduced during burial in soil or seawater — become embedded in the coin’s surface and subsurface layers. When the coin is later exposed to moisture and oxygen, these chlorides react with the copper to form copper chloride hydroxide (atacamite or paratacamite), which appears as bright green, powdery or crusty spots on the coin’s surface.

What makes bronze disease so dangerous is that it is self-sustaining. The chemical reaction that produces the green corrosion products also generates hydrochloric acid, which attacks fresh metal, producing more copper chloride, which reacts with more moisture and oxygen, producing more acid. This cycle continues relentlessly until either the chlorides are exhausted or the coin is completely consumed.

How to Identify Bronze Disease

In my conservation work, I look for the following diagnostic signs:

• Bright green spots or patches — Unlike the dull, flat green of a stable patina, bronze disease presents as vivid, almost fluorescent green. The color is distinctly different from the verdigris that forms naturally over centuries.
• Powdery or crusty texture — Active bronze disease feels rough to the touch (though you should never touch a coin’s surface with bare fingers). Under magnification, the green material appears as a fine powder or small crystalline bumps.
• Raised lesions — The corrosion products push upward from within the coin, creating small mounds or blisters on the surface. These can sometimes be mistaken for encrustations or deposits, but they are actually the coin’s metal being converted into corrosion products from the inside out.
• Progressive spread — If you examine a coin at different points in time, active bronze disease will visibly grow. A small green spot that was barely noticeable six months ago may have expanded into a significant area of damage.
• Association with burial deposits — Coins recovered from archaeological contexts, especially marine or waterlogged environments, are at the highest risk. I’ve seen entire lots of Roman provincial bronzes from a single excavation where every coin showed some degree of bronze disease.

Treating Bronze Disease

Treatment of bronze disease is a multi-step process that requires patience and, in many cases, professional intervention. Here is the general approach I follow in my conservation practice:

1. Mechanical removal: Under magnification, carefully remove the bulk of the green corrosion products using a wooden pick or fine bamboo tool. The goal is to eliminate as much of the copper chloride as possible without damaging the underlying metal surface. This step alone will not cure the disease, as chlorides are almost always present below the visible surface.
2. Chemical treatment — Sodium sesquicarbonate soak: The coin is immersed in a 5% solution of sodium sesquicarbonate (a mixture of sodium carbonate and sodium bicarbonate). This alkaline solution converts the insoluble copper chloride into stable, insoluble copper carbonate, effectively neutralizing the corrosive cycle. Soaks can last from several days to several weeks, with the solution changed regularly. I typically monitor the process by testing the solution for chloride ions using a few drops of silver nitrate — when no precipitate forms, the chloride extraction is complete.
3. Rinsing and drying: After chemical treatment, the coin is thoroughly rinsed in multiple changes of distilled water to remove all residual chemicals, then carefully dried. Rapid, complete drying is essential to prevent any renewed corrosion.
4. Stabilization and sealing: Finally, the treated coin is coated with a protective barrier — typically a microcrystalline wax (such as Renaissance Wax) or an acrylic lacquer like Incralac — to shield the surface from future moisture exposure.

Important caveat: If you suspect bronze disease on a valuable or historically significant coin, I strongly recommend consulting a professional conservator rather than attempting treatment yourself. Improper chemical treatment can strip original patina, alter surface detail, and significantly reduce a coin’s numismatic value and authenticity in the eyes of collectors and scholars.

PVC Plasticizer Damage: The Silent Killer in Your Collection

While bronze disease is the most dramatic form of environmental damage, PVC plasticizer damage is arguably the most common — and the most preventable. I’ve seen countless collections where decades of storage in PVC-containing flips, holders, and albums have left coins with irreversible greenish or hazy film that no amount of conservation can fully reverse.

What Is PVC Damage?

Polyvinyl chloride (PVC) is a plastic that has been widely used in coin flips, protective sleeves, and storage albums since the mid-20th century. In its rigid form, PVC is relatively stable. However, most coin storage products require flexible PVC, which is achieved by adding chemical plasticizers — typically phthalate esters — to the plastic.

Over time, these plasticizers migrate out of the PVC material and deposit onto the coin’s surface. In the presence of even small amounts of moisture, the plasticizers can break down into acidic compounds (such as hydrochloric acid and phthalic acid) that attack the coin’s metal. The result is a characteristic greenish, hazy, or sticky film on the coin’s surface that is extremely difficult to remove without risking damage to the underlying metal.

Identifying PVC Damage

Here’s what I look for when examining a coin that may have been stored in PVC:

• Greenish or bluish-green haze — This is the hallmark of PVC damage. The film often has a slightly iridescent or oily appearance and may be more pronounced in the recessed areas of the design.
• Sticky or tacky surface — In advanced cases, the migrated plasticizers create a slightly sticky feel on the coin’s surface. You may notice that dust and debris adhere to the coin more than expected.
• Association with specific holders — If a coin was stored in a soft, flexible PVC flip (as opposed to the rigid, archival-quality Mylar flips), the risk of damage is significantly higher. The older the PVC holder, the more likely damage has occurred.
• Pattern of damage — PVC damage often appears in patterns that correspond to the contact points between the coin and the holder. For example, a coin stored in a PVC flip may show more damage around the edges and in the central field where contact was most consistent.

Treating PVC Damage: The Acetone Dip

The most effective treatment for PVC damage is an acetone soak. Acetone is an excellent solvent for the phthalate plasticizers that cause the damage, and it evaporates completely without leaving residue. Here is the procedure I use:

1. Use pure, reagent-grade acetone. Do not use nail polish remover or hardware-store acetone, which may contain water, oils, or other additives that can harm the coin’s surface. In my lab, I use ACS-grade acetone.
2. Soak the coin in a glass container filled with acetone. The duration depends on the severity of the damage — typically 15 minutes to several hours. For heavy PVC contamination, I may change the acetone once or twice as it becomes saturated with dissolved plasticizer.
3. Remove the coin and allow it to air-dry in a well-ventilated area. Acetone evaporates rapidly, so drying is quick. Do not rinse with water, as this is unnecessary and could introduce moisture to a coin that may have other vulnerabilities.
4. Inspect under magnification. If any greenish film remains, repeat the process. In my experience, most PVC damage can be fully removed with one to three acetone soaks.

Critical warning: Acetone will not damage the metal of a copper or bronze coin, but it will dissolve any wax, lacquer, or other coatings that may have been applied to the coin’s surface. It can also damage certain types of coin holders, albums, and display materials. Always remove the coin from its holder before treatment, and never use acetone on coins that have been artificially toned or coated with any substance.

Furthermore, acetone is highly flammable and should be used only in a well-ventilated area away from heat sources. Wear appropriate gloves and eye protection.

Oxidation and Toning: When Is It Damage and When Is It Beauty?

One of the most nuanced topics in numismatic conservation is the distinction between harmful oxidation and attractive, value-enhancing toning. This is an area where dealer descriptions become critically important — and where the line between honest representation and misleading hyperbole can be razor-thin.

The Chemistry of Copper Oxidation

Copper oxidizes readily in the presence of oxygen, forming first a thin layer of cuprous oxide (Cu₂O), which appears as a reddish or pinkish film on the coin’s surface. With continued exposure to oxygen and moisture, this converts to cupric oxide (CuO), which is black. In the presence of sulfur compounds (from air pollution, certain papers, or rubber bands), copper forms copper sulfide (CuS), which produces the dark brown to black toning seen on many older coins.

On bronze coins, the situation is more complex due to the presence of tin, which forms its own oxide layer (SnO₂). This tin oxide is generally stable and protective, which is one reason why some ancient bronzes survive in remarkably good condition — often retaining their original luster and surface quality in ways that defy their age.

Natural Toning vs. Environmental Damage

In my experience, the key distinction between desirable toning and harmful oxidation lies in stability, uniformity, and aesthetics:

• Natural, stable toning develops slowly over decades or centuries. It tends to be uniform or gradually transitional in color, with a smooth, satiny appearance. On silver coins, this produces the rainbow effects that collectors prize. On copper and bronze, natural toning typically manifests as an even brown, chocolate, or mahogany color that is consistent across the coin’s surface — enhancing eye appeal without obscuring the strike.
• Harmful oxidation tends to be uneven, blotchy, or spotty. It may appear as dark, irregular patches that obscure design details, or as a dull, matte surface that lacks the luster of original mint state. In severe cases, oxidation can progress to active corrosion — particularly if chlorides are present.
• Artificial toning — whether intentionally applied by a dealer to enhance a coin’s appearance or accidentally induced by improper storage — often has an unnatural color (such as vivid blues, purples, or oranges on copper coins) and may appear in patterns that don’t correspond to natural environmental exposure.

What Dealers Should Disclose

This brings us to the heart of the original forum discussion: the ethics of dealer descriptions. As a conservator who has examined coins from every level of the market, I can tell you that the most reputable dealers are those who describe their coins with precision and transparency.

When I examine a coin for a client, I provide a detailed written assessment that includes:

• The type and extent of any toning, with specific reference to color, distribution, and whether it appears natural or artificial
• Any evidence of environmental damage, including PVC film, bronze disease, or active corrosion
• The presence of any conservation treatments, past or present
• An honest assessment of the coin’s grade, including any factors that may limit its grade (such as weak strike, porosity, or surface issues)

A dealer who describes a coin with active bronze disease as simply “having attractive green toning” is not engaging in harmless hyperbole — they are committing a form of misrepresentation that can cost the buyer significant money in conservation treatment. Conversely, a dealer who notes that a coin “has some light, even toning consistent with long-term storage in a stable environment” is providing useful, honest information that helps the buyer make an informed decision and assess the coin’s true collectibility.

Proper Storage: Prevention Is Always Better Than Cure

After decades of conservation work, I am firmly convinced that the single most important thing a collector can do to protect their coins is to store them properly from the outset. No conservation treatment can fully restore a coin that has suffered severe environmental damage — the goal is always to stabilize and prevent further deterioration, not to return the coin to its original state.

Best Practices for Copper and Bronze Coin Storage

Here are my recommendations, based on both professional conservation standards and practical collecting experience:

• Use only archival-quality holders. Mylar (polyester) flips, such as those made by Capital Plastics, are the gold standard for individual coin storage. They are chemically inert, do not off-gas, and will not react with the coin’s metal. Never use soft PVC flips, vinyl sleeves, or rubber-band-secured groupings.
• Control humidity. Relative humidity should be maintained below 40% for copper and bronze coins. In my lab, I use silica gel desiccant packets in storage cabinets and check humidity levels regularly with a digital hygrometer. For particularly valuable or sensitive coins, consider using a sealed display case with controlled atmosphere.
• Avoid sulfur-containing materials. Certain papers, cardboards, and fabrics contain sulfur compounds that can tarnish and corrode copper alloys. Use only acid-free, sulfur-free tissue paper and storage materials.
• Minimize handling. The oils and salts on human skin can initiate corrosion on copper surfaces. Always handle coins by the edges, and consider wearing nitrile gloves when examining valuable pieces.
• Inspect regularly. I recommend examining your collection at least twice a year under good lighting and magnification. Early detection of bronze disease, PVC damage, or other environmental issues is critical to successful treatment.
• Isolate problem coins. If you identify a coin with active bronze disease, remove it from your collection immediately and store it separately. The chlorides responsible for bronze disease can potentially spread to adjacent coins through airborne moisture.

The Role of Third-Party Grading and Dealer Accountability

The forum discussion that inspired this article raised important questions about dealer honesty and the role of third-party grading (TPG) services. As someone who has worked with both dealers and grading services, I want to offer some perspective on this intersection.

Third-party grading provides a valuable baseline of authenticity and condition assessment. When a coin is encapsulated in a PCGS, NGC, or ANACS holder, the buyer has a reasonable expectation that the coin has been examined by trained professionals and assigned a grade within an established framework. However, TPG grades are not infallible, and they do not capture every aspect of a coin’s condition.

Specifically, TPG services generally do not assess:

• The presence or extent of PVC damage (unless it is severe enough to affect the grade)
• Whether toning is natural or artificial
• The presence of active bronze disease (particularly on ancient coins, which are less commonly submitted to major TPG services)
• Whether a coin has been previously conserved or treated

This is where dealer descriptions become essential. A responsible dealer should disclose any known environmental damage, conservation treatments, or other condition issues that are not fully captured by the TPG grade. The best dealers I’ve worked with go beyond the grade — they describe the coin’s color, surface quality, eye appeal, and any known issues in detail, often supplementing their written descriptions with high-resolution photographs or TrueView-style images.

As one forum participant wisely noted: “A fair description paints an image in the buyer’s mind that does not disappoint when the coin is received.” I could not agree more. In my view, the standard for dealer descriptions should be informed transparency — not perfection, not legalistic precision, but a genuine effort to convey the coin’s true condition to a buyer who may not have the opportunity to examine it in person. This is especially critical when dealing with a rare variety or a coin with significant provenance, where the stakes of misrepresentation are highest.

Actionable Takeaways for Buyers and Sellers

Whether you’re buying, selling, or simply maintaining a collection, here are the key points to remember:

For Buyers:

• Always purchase coins with a reasonable return privilege, especially when buying online or by mail.
• Examine coins under magnification upon receipt, looking specifically for signs of bronze disease, PVC damage, and unnatural toning.
• Ask dealers direct questions about storage history, conservation treatments, and any known environmental issues.
• If a coin shows signs of active bronze disease, factor the cost of professional conservation into your purchase price — or negotiate a lower price to account for the risk.
• Be skeptical of descriptions that rely entirely on superlatives (“MONSTER TONE A+++++”) without substantive detail about the coin’s actual condition.

For Sellers and Dealers:

• Describe coins honestly and specifically. Note any environmental damage, conservation treatments, or condition issues that a reasonable buyer would want to know.
• Provide accurate, well-lit photographs that represent the coin’s true appearance. Avoid excessive image editing that alters color or conceals surface issues.
• Offer clear return policies. A generous return policy is not a sign of weakness — it is a sign of confidence in your product and a powerful tool for building long-term customer relationships.
• Educate yourself about the specific environmental risks associated with the metal types you sell. A dealer who cannot identify bronze disease or PVC damage is a dealer who cannot accurately describe their inventory.

For All Collectors:

• Invest in proper archival storage materials. The cost of Mylar flips and acid-free holders is trivial compared to the numismatic value of the coins they protect.
• Control your storage environment. Keep coins in a cool, dry, stable location away from direct sunlight, temperature fluctuations, and sources of chemical contamination.
• Inspect your collection regularly. Early detection of environmental damage is the key to successful treatment.
• When in doubt, consult a professional conservator. Improper DIY treatment can cause far more damage than the original problem.

Conclusion: Protecting Our Numismatic Heritage

The coins in our collections are more than financial assets — they are tangible links to the civilizations, economies, and artisans who produced them. A Roman bronze sestertius that survived the fall of an empire, centuries of burial, and the modern collector market deserves to be preserved with the same care and respect that we would accord to any other historical artifact.

Environmental damage — whether from bronze disease, PVC plasticizers, or uncontrolled oxidation — is a threat that every collector of copper and bronze coins must take seriously. But it is also a threat that is largely preventable through proper storage, regular inspection, and informed purchasing decisions.

As a conservator, I have seen the devastating effects of neglect, but I have also seen the remarkable results that careful, professional treatment can achieve. The key is awareness: knowing what to look for, understanding the risks, and taking action before minor issues become major problems.

The forum discussion that inspired this article touched on a fundamental truth about our hobby: trust between buyers and sellers is the foundation of a healthy market. That trust is built on honest descriptions, transparent business practices, and a shared commitment to preserving the coins that we are privileged to hold in trust for future generations. Whether you are a dealer describing a coin for sale or a collector evaluating a potential purchase, remember that the most valuable thing you can offer is the truth.

Take care of your coins, and they will take care of you — both as investments and as irreplaceable pieces of human history.

Related Resources

You might also find these related articles helpful:

• Buried Treasure: How the S.S. Central America, Redfield Hoard, and Saddle Ridge Hoard Reveal the Hidden Life of America’s Greatest Coin Discoveries – Some of the finest known examples of certain coins spent centuries underwater or buried in bank vaults. Let’s look…
• Starting a Coin YouTube Channel: How to Turn Wild Dealer Stories and Rare Finds Into Viral Numismatic Content – The coin collecting hobby is absolutely exploding on social media right now. If you have been thinking about turning you…
• Early vs. Late Die State: Evaluating Strike Quality, Die Wear, and Dealer Descriptions in Numismatics – A coin struck from a fresh die looks completely different than one struck from a dying one. Let’s look at the die …