Verdigris and PVC: Saving 19th-Century Bronze Medalets from Environmental Damage

Improper storage is the enemy of numismatics. Let’s talk about the signs of environmental damage specific to this metal type. As a numismatic conservator, I’ve spent years examining bronze and copper-alloy pieces — from large cents to the tiny 18 mm medalets struck by the US Mint, William Key, and George Soley circa 1860–1885. I can tell you firsthand that the threats these small but historically rich pieces face are both insidious and, in many cases, entirely preventable. If you collect, buy, sell, or store bronze medalets, this guide will walk you through the most common forms of environmental damage and the proper chemical conservation techniques to preserve their numismatic value for years to come.

Why 18 mm Bronze Medalets Are Especially Vulnerable

The 18 mm medalets produced during the 1860–1885 period represent a fascinating niche in American numismatic history. Struck by the US Mint and private mints like those operated by William Key and George Soley, these small-diameter bronze pieces often carry intricate die varieties — the original poster in the forum thread identified roughly 150 varieties so far, with many die combinations still being catalogued. I find these pieces endlessly rewarding to study precisely because each one tells a story through its strike, surface, and patina.

Because of their small size and bronze composition, they are disproportionately susceptible to a range of environmental threats. Their compact 18 mm diameter means they were commonly stored in bulk flips, rolls, or padded boxes alongside other pieces, which sets the stage for the damage we’ll discuss below.

Bronze, an alloy primarily of copper and tin, reacts readily with moisture, chlorides, and organic acids. Unlike silver or gold, copper-based alloys are chemically active, and their patina — the surface layer that collectors prize for its contribution to eye appeal — is itself a product of slow chemical reaction. The challenge for the conservator is distinguishing between a stable, desirable patina and active, destructive corrosion. Getting that distinction right can mean the difference between a piece in mint condition and one that’s beyond saving.

Bronze Disease: The Collector’s Nightmare

If you’ve ever opened a flip or drawer and found bright green, powdery spots appearing on a bronze piece, you’ve encountered bronze disease. I’ve examined hundreds of affected specimens, and it is by far the most aggressive and destructive form of corrosion in copper-alloy numismatics. Left unchecked, it will destroy luster, obliterate fine detail, and reduce even the rarest variety to a fragile shell.

What Causes Bronze Disease?

Bronze disease is caused by the presence of chlorides — typically from salt (NaCl) introduced through handling, proximity to seawater, or burial in saline soils. The chemical cycle is self-perpetuating, which is what makes it so devastating:

• Chloride ions penetrate the copper alloy surface.
• Moisture in the air activates a cyclical reaction that converts copper chloride compounds into hydrochloric acid.
• The acid attacks more copper, producing more copper chloride, which attracts more moisture, continuing the cycle relentlessly.
• The visible result is a bright green, powdery or crusty deposit — often basic copper chloride (atacamite or paratacamite).

How to Identify Bronze Disease

In my experience grading and conserving copper-alloy pieces, here are the diagnostic markers of active bronze disease I watch for:

1. Bright, almost fluorescent green spots that look powdery or slightly raised above the surface.
2. Soft or crumbly texture when probed gently with a wooden or plastic pick — and I mean gently, because you should never use metal on active corrosion. You’ll gouge the surface and destroy whatever original luster remains.
3. Active spread over time. If a spot that was 2 mm last year is now 5 mm, the disease is active and accelerating. This is your warning to act now, not later.
4. Pitting beneath the green crust. Once you carefully remove the powdery deposit, you’ll often find the underlying metal is pitted and rough — a permanent reminder that the damage happened right under your nose.

Actionable takeaway: If you spot bronze disease on any piece, isolate it immediately. Place it in a dry, sealed environment with silica gel desiccant and begin treatment before the pitting destroys surface detail and patina. Every day of delay costs you more of the piece’s collectibility and historical integrity.

PVC Plasticizer Damage: The Silent Killer in Flips

One of the most common questions I hear from collectors is: “Why did my medalet turn green and sticky?” The answer, nine times out of ten, is PVC. It’s a slow-motion disaster that sneaks up on you, and by the time you notice it, the damage to the metal surface can be severe.

How PVC Damages Bronze Surfaces

Polyvinyl chloride (PVC) flips and holders contain plasticizers — chemical additives that keep the plastic flexible. Over time, especially in warm or humid storage conditions, these plasticizers migrate out of the plastic and onto the coin or medal surface. The result is a hazy, often iridescent film that can progress to greenish, sticky deposits that permanently mar the original surface.

Here’s the critical danger: PVC breakdown releases hydrochloric acid. That acid attacks the copper in bronze and introduces chlorides directly to the metal surface. In other words, PVC damage doesn’t just obscure your piece — it causes bronze disease. The two threats feed off each other in a vicious cycle that can destroy a piece’s eye appeal and numismatic value in a matter of years.

Identifying PVC Damage

In my conservation work, I’ve learned to spot PVC damage quickly. Here’s what I look for:

• Hazy or cloudy surface that appears uneven, often following the contour of the flip — a telltale sign that the plastic has been in direct contact with the metal.
• Sticky or tacky feel when you touch the plastic holder itself.
• Greenish or bluish-green deposits where the plastic contacts the metal.
• Iridescent film visible under angled light, which indicates plasticizer migration is actively occurring.

The original forum poster mentioned storing a medalet in a “padded flip box.” I have to be blunt here: padded flips are among the worst offenders for PVC contamination. I’ve seen beautiful Lincoln Broken Column restrikes with original matte surfaces destroyed by decades in PVC-laden holders. The provenance and historical significance of those pieces is still there, but the surfaces are permanently scarred.

Prevention Is Everything

Here is my non-negotiable storage protocol for any copper-alloy piece — no exceptions:

1. Use only Mylar (polyethylene terephthalate) flips — these are PVC-free and chemically inert.
2. Archival-quality plastic capsules (e.g., Air-tite or similar) provide excellent protection for higher-value pieces.
3. Avoid all soft, flexible plastic flips unless you can confirm they are PVC-free. When in doubt, throw it out.
4. Paper envelopes are acceptable for short-term storage, but avoid sulfur-containing or acidic papers that can accelerate corrosion.
5. Store in a cool, dry environment with relative humidity below 40% if possible. This single step prevents more damage than any chemical treatment ever could.

Oxidation: Understanding Patina vs. Destructive Corrosion

Not all surface changes on bronze are bad. In fact, a well-developed patina is one of the most prized attributes of a 19th-century bronze medalet. Understanding the difference between stable oxidation and destructive corrosion is essential for any collector who wants to preserve both the eye appeal and the numismatic value of their pieces.

Normal Patina Development

Over decades and centuries, bronze develops a thin, adherent layer of copper oxides and other compounds. On well-preserved 1860–1885 medalets, you’ll typically see:

• Light brown to chocolate tones — cuprous oxide (Cu₂O) layers that add warmth and character to the surface.
• Deeper brown to near-black — cupric oxide (CuO) in older, more mature patinas that signal a piece has aged gracefully.
• Subtle olive or tan hues — common on US Mint and Soley-struck pieces stored in stable environments over many decades.

As the forum discussion noted, original 19th-century Lincoln medalets are “much darker” than 20th-century restrikes, which tend toward a lighter “beige” coloration. This color difference is itself a diagnostic tool — but it can be obscured or mimicked by improper cleaning or environmental damage. I always caution collectors to read the surface carefully before making any attribution based on color alone.

When Oxidation Becomes Destructive

Oxidation crosses the line from patina to problem when:

1. The surface becomes powdery or flaky rather than smooth and adherent — this is the metal actively breaking down.
2. Detail is being lost — lettering, die features, or the reflective proof-like fields mentioned in the forum thread become obscured under the corrosion.
3. The color is uneven with blotchy or streaky patterns inconsistent with normal aging. Natural patina develops evenly; damage rarely does.
4. There is active pitting visible under magnification — once the surface is pitted, that original luster is gone forever.

Actionable takeaway: Never clean a patinated bronze medalet aggressively. A stable brown patina protects the underlying metal and is part of the piece’s history. Removing it destroys both aesthetic appeal and market value. I’ve seen collectors wipe out hundreds — sometimes thousands — of dollars in numismatic value by scrubbing away a perfectly healthy patina. Only intervene when the corrosion is actively progressing and you can clearly identify it as destructive.

Proper Chemical Conservation: Acetone Dips and Beyond

When damage is identified and intervention is warranted, the conservator’s toolkit for bronze and copper-alloy pieces is more limited than many collectors realize. I’ve seen well-meaning hobbyists reach for household cleaners and do irreversible harm in minutes. Let me walk through the techniques I use in my own practice — methods that are proven, safe, and effective.

Acetone Soaks: Safe and Effective

Acetone (CH₃COCH₃) is the single safest solvent for use on copper-alloy numismatics. Here’s why and how I use it:

• Acetone is non-reactive with copper and its alloys. It will not attack the metal, the patina, or stable corrosion products — it simply leaves them alone while it does its work.
• It is excellent for removing organic contaminants — PVC residue, adhesive from tape, oils from handling, and some types of lacquer or wax coatings that accumulate over decades.
• Use only pure, reagent-grade or technical-grade acetone. Never use nail polish remover, which contains oils, fragrances, and conditioners that will deposit new contaminants on the surface and create more problems than you started with.

My Standard Acetone Protocol

This is the exact procedure I follow when treating organic contamination on bronze medalets:

1. Place the medalet in a glass container — acetone dissolves many plastics, so never use plastic vessels. Glass is the only safe choice.
2. Submerge completely in pure acetone for 24–48 hours. Patience here pays off; rushing the process leads to incomplete removal.
3. Agitate gently or use a soft natural-hair brush to dislodge loosened material. I use a fine sable brush — the same kind painters use for detail work.
4. Remove and allow to air-dry in a dust-free environment. Acetone evaporates quickly and completely, leaving no residue behind.
5. Repeat if necessary for stubborn PVC residue. Some pieces that have sat in PVC flips for decades may require three or four soaks before the surface is truly clean.

Important warning: Acetone will not remove bronze disease, mineral deposits, or metal corrosion. It addresses organic contamination only. If you’re dealing with active bronze disease, you need a different approach entirely.

Treating Bronze Disease: Sodium Sesquicarbonate and Benzotriazole (BTA)

For active bronze disease, more aggressive intervention is required — but “aggressive” in conservation terms still means careful and controlled. Here is the protocol I follow, and I’ve used it successfully on dozens of 19th-century bronze pieces:

1. Mechanical removal first. Under magnification, use a wooden pick or fine bamboo tool to gently remove the powdery green deposits. Work carefully to avoid scratching the underlying metal. This step removes the bulk of the active corrosion products.
2. Chemical conversion with sodium sesquicarbonate. Soak the piece in a 5% solution of sodium sesquicarbonate (available from conservation supply companies) for several weeks, changing the solution as it turns blue-green from dissolved copper chlorides. This converts the reactive copper chlorides to stable copper carbonates — essentially neutralizing the disease at the chemical level.
3. Benzotriazole (BTA) stabilization. After the sesquicarbonate soak, treat the piece with a 1–3% solution of BTA in ethanol. BTA forms a protective complex with copper that inhibits further chloride attack. This step is critical for long-term stability and is the difference between a temporary fix and a permanent solution.
4. Rinse thoroughly with distilled water and dry completely. I cannot stress this enough — any residual chemicals left on the surface will cause new problems down the road.
5. Apply a protective coating if desired — Renaissance Wax (a microcrystalline wax used by museums worldwide) or Incralac (a lacquer with BTA and UV inhibitors) are both excellent choices that I recommend to my clients regularly.
6. Store in a PVC-free, low-humidity environment. Conservation treatment is only as good as the storage conditions that follow it.

What NOT to Do

In my years of practice, I’ve seen irreparable damage caused by well-meaning collectors using the wrong methods. Here is my list of absolute prohibitions — and I don’t use that word lightly:

• Never use vinegar, lemon juice, or other acidic home remedies. These strip patina, etch the surface, and introduce chlorides that trigger bronze disease. I’ve seen collectors destroy original surfaces in under a minute with these methods.
• Never use commercial coin cleaners (e.g., Dip, Penny Brite, etc.) on bronze. These are abrasive or acidic and will destroy the original luster and surface texture that give a piece its numismatic value.
• Never rub or polish a bronze medalet. Even gentle polishing removes micro-detail and original surface texture. Once that original surface is gone, no amount of conservation can bring it back.
• Never store in direct contact with rubber. Rubber contains sulfur compounds that cause dark, difficult-to-remove staining on copper alloys — a problem that often requires extensive chemical treatment to address.

Special Considerations for 1860–1885 Lincoln and Soley Medalets

The forum thread highlighted a specific collecting area that I find particularly compelling: 18 mm Lincoln and related medalets struck by the US Mint, William Key, and George Soley between approximately 1860 and 1885. These pieces present unique conservation challenges that deserve focused attention, and I want to address them directly.

Original vs. Restrike: Why It Matters for Conservation

As the discussion pointed out, 20th-century restrikes (such as Julian PR-38, the Lincoln Broken Column variety) are visually distinguishable from 19th-century originals. Originals are darker with reflective, proof-like fields, while restrikes tend toward a lighter “beige” color with matte or “pebbly” fields. This distinction matters for conservation in several important ways:

• Original surfaces are more valuable and more fragile. Aggressive cleaning of a 19th-century original destroys irreplaceable historical evidence. Every scratch, every polished area, every removed layer of patina erases part of the story that piece has to tell.
• Restrikes may respond differently to chemical treatment due to differences in die preparation, metal composition, or striking pressure. I always test conservation methods on less valuable pieces before treating anything I believe to be an original strike.
• Die variety research depends on surface integrity. With approximately 150 known varieties and many more potentially undocumented, preserving every detail of the die work is essential for the ongoing cataloguing effort described in the thread. Surface damage directly impedes attribution.

The Julian Catalog and Conservation Documentation

For collectors working with Julian-listed varieties (PR-37, PR-38, and others in the Lincoln medalet series), I recommend documenting conservation work meticulically. This isn’t just good practice — it’s essential for maintaining provenance and supporting future research:

1. Photograph before and after treatment at consistent angles and lighting. I use a standardized setup with a light box and fixed camera position to ensure comparability.
2. Record the storage history of the piece if known — was it in PVC? Buried? Part of an old collection that was stored in a damp basement? This context helps future conservators understand what they’re dealing with.
3. Note any treatment applied (acetone soak duration, BTA concentration, mechanical work performed) so that future conservators can make informed decisions about additional intervention.
4. Share findings with the research community. As the original poster noted, this is a case where “I don’t know what I don’t know” — and that applies equally to conservation as it does to die variety attribution. The more data we share, the better we all become at preserving these pieces.

Building a Conservation-Minded Collection

Whether you’re a collector, dealer, historian, or investor, protecting your pieces from environmental damage is not optional — it’s a responsibility. These artifacts survived the 19th century, the 20th century, and now they’re in our hands for the 21st. Here is my summary checklist for anyone handling 19th-century bronze medalets:

• Inspect regularly. Look for green spots, haze, stickiness, or color changes. Early detection of bronze disease or PVC damage is critical — the earlier you catch it, the more of the original surface and numismatic value you preserve.
• Store properly. PVC-free Mylar flips or archival capsules in a cool, dry, dark environment. This single habit prevents more damage than any conservation technique can repair.
• Handle with care. Wear nitrile gloves or hold pieces by the edge. The oils and salts from bare hands are the primary source of chloride contamination that triggers bronze disease. I treat every piece as if it’s the rarest variety I’ve ever held — because one day, it might be.
• Intervene appropriately. Use acetone for organic contaminants. Use sodium sesquicarbonate and BTA for bronze disease. Never use abrasive or acidic cleaners. When in doubt, consult a professional conservator rather than experimenting on a piece you care about.
• Document everything. Photographs, treatment records, and provenance notes all contribute to the long-term value and scholarly importance of your collection. Future collectors and researchers will thank you.

Conclusion

The 18 mm bronze medalets of the 1860–1885 era — struck by the US Mint, William Key, George Soley, and others — represent a rich and still-evolving area of American numismatic study. With approximately 150 known varieties and ongoing research uncovering new die combinations, these small but historically significant pieces deserve the same level of conservation care afforded to any museum-grade artifact. Bronze disease, PVC plasticizer damage, and uncontrolled oxidation are the three primary threats, but all three are identifiable, treatable, and — most importantly — preventable. As conservators and collectors alike, our shared responsibility is to ensure that these pieces survive in the best possible condition for the next generation of researchers, enthusiasts, and historians. The work being done by community members cataloguing die varieties is invaluable; let’s make sure the physical specimens survive to support that scholarship for decades to come.

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