Verdigris and PVC: Saving Germany and German New Guinea Coins from Environmental Damage — A Conservator’s Guide to Bronze Disease, Oxidation, and Proper Chemical Conservation

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

As a numismatic conservator, I’ve spent decades examining coins pulled from attics, estate sales, and long-forgotten collections — and few things break my heart faster than seeing a historically significant piece of German or German New Guinea coinage slowly destroyed by environmental neglect. The forum thread that inspired this discussion featured several beautiful examples: a German New Guinea 1894-A 10 Pfennig, a 1908-G Mark, a 1927-A Bremerhaven 3 Mark, and a 1931-A Magdeburg 3 Mark. All graded within expectations, and the 1908-G Mark in particular was described as “quite attractive” in hand — reportedly the highest graded by PCGS at MS66, with only 4 MS examples on the population report.

But behind every “attractive” coin that survives to reach a grading slab lies a silent battle against environmental damage. Copper, bronze, and even silver alloys are extraordinarily reactive. The very air we breathe, the plastics we store them in, and the humidity in our homes are all waging a slow war against these pieces. In this analysis, I want to walk you through the specific environmental threats facing German Imperial and colonial coinage, how to identify them, and — most importantly — what you can do about it.

Understanding the Enemy: Why German and German New Guinea Coins Are Vulnerable

Before we get into specific damage types, it’s essential to understand why these coins are particularly susceptible. The German Mark series (½, 1, and 3 Mark denominations) and the German New Guinea colonial issues were minted in large quantities for circulation. As one forum participant noted, many circulating silver coins were stashed away at the beginning of the First World War, and since these denominations were never officially withdrawn from circulation, a reasonable number survived — though mostly in average to bad condition.

Here’s the critical point: survival does not mean preservation. A coin can survive a century and still be actively deteriorating. The metal composition of these coins — primarily silver alloys for the Mark series and copper-nickel or bronze for many colonial issues — makes them vulnerable to specific environmental threats that I see repeatedly in my conservation work.

As one collector wisely observed, many German families still have secret stashes hidden in attics, and when those attics are finally renovated — “wenn das 100-Jahre plus Dach umgebaut wird” — collectors and conservators get a chance to see what decades of improper storage have done. The results are often a mix of heartbreak and hope.

Bronze Disease: The Silent Killer of Copper and Bronze Coinage

If there’s one environmental threat that keeps me up at night, it’s bronze disease. This is not a metaphor — it’s a real, active chemical process that can destroy a bronze or copper coin from the outside in, and once it takes hold, it is extraordinarily difficult to stop.

What Is Bronze Disease?

Bronze disease is a form of corrosion caused by the interaction of copper alloys with chlorides, moisture, and oxygen. It manifests as bright green, powdery or crusty spots on the surface of the coin — often mistaken by casual collectors as simple “verdigris” or patina. But unlike stable patina, which can actually protect a coin’s surface, bronze disease is progressive and self-sustaining.

The chemical process works like this:

1. Chloride ions (from salt air, handling with bare hands, or contaminated storage materials) penetrate the coin’s surface.
2. In the presence of moisture, these chlorides react with copper to form copper chloride.
3. Copper chloride reacts with oxygen and water to form hydrochloric acid and basic copper chloride (the green powder you see).
4. The hydrochloric acid attacks more copper, creating more copper chloride — and the cycle continues.

This is why bronze disease is sometimes called “cancer of the coin.” It literally feeds on itself.

Identifying Bronze Disease on German Colonial Issues

The German New Guinea 1894-A 10 Pfennig featured in the forum thread is exactly the type of coin where I’d be on high alert for bronze disease. Colonial issues from the Pacific were often stored in tropical environments with high humidity and salt air — a perfect storm for chloride contamination.

Here’s what to look for:

• Bright green, powdery spots — not the dull, even green of stable patina, but vivid, almost fluorescent green patches that look “fuzzy” or raised.
• Pitting beneath the green spots — if you look under magnification, you’ll see that the green corrosion is sitting in small pits on the coin’s surface. This is the copper being eaten away.
• Active spread — if you see new green spots appearing over weeks or months, the disease is active and accelerating.
• A musty or acidic smell — in advanced cases, you can actually smell the hydrochloric acid being produced.

Treating Bronze Disease: A Conservator’s Protocol

If you suspect bronze disease on a coin in your collection, do not ignore it. Here’s the protocol I recommend:

1. Isolate the affected coin immediately. Bronze disease can potentially spread to nearby coins through airborne chloride particles. Place the coin in a separate, dry container.
2. Do NOT attempt mechanical removal. Scraping or brushing the green spots will damage the coin’s surface and may push chlorides deeper into the metal.
3. Begin with a distilled water soak. Submerge the coin in distilled water (never tap water, which contains chlorides) for 24-48 hours. Change the water every few hours. This helps draw out soluble chlorides through osmosis.
4. Consider a benzotriazole (BTA) treatment. BTA is a corrosion inhibitor that bonds with copper and creates a protective layer. I typically use a 3% BTA solution in ethanol for 24 hours. This is a standard conservation treatment used by major museums.
5. After treatment, dry the coin thoroughly and store it in a low-humidity environment with silica gel packets.

For severe cases, I always recommend consulting a professional conservator. The German New Guinea gold and colonial series is rare enough that amateur treatment attempts can do more harm than good.

PVC Plasticizer Damage: The Hidden Threat in Your Storage Materials

While bronze disease is the most dramatic environmental threat, PVC plasticizer damage is arguably the most common — and the most preventable. I cannot overstate this: the way you store your coins matters as much as the coins themselves.

How PVC Damages Coins

Many coin flips, albums, and storage boxes manufactured before the 1990s — and some still produced today — contain polyvinyl chloride (PVC) as a plasticizer. PVC is a soft, flexible plastic that off-gasses acidic vapors over time. When a coin is stored in direct contact with PVC, these vapors react with the metal surface and create a distinctive greenish or cloudy film.

The damage progresses in stages:

• Stage 1 (Early): A faint greenish or iridescent haze appears on the coin’s surface. This is often visible only under magnification or at certain angles. At this stage, the damage may be reversible.
• Stage 2 (Moderate): The haze becomes more pronounced, with visible green streaks or spots following the contours of the coin’s design. The coin’s original luster begins to diminish.
• Stage 3 (Severe): Thick green corrosion covers significant portions of the coin’s surface. The metal underneath may be pitted. At this stage, the damage is permanent and the coin’s grade — and value — is significantly reduced.

Why German Mark Coins Are Especially at Risk

The German Mark series — including the 1908-G Mark, 1927-A Bremerhaven 3 Mark, and 1931-A Magdeburg 3 Mark from the forum thread — were heavily collected throughout the 20th century. Many of these coins spent decades in PVC-based albums and flips before collectors became aware of the danger.

As one forum participant noted, collecting graded coins is not yet common among German collectors, which means many potentially high-grade examples are still sitting in old, potentially damaging storage materials. This is both a risk and an opportunity: the coins may be ungraded gems, but they may also be silently deteriorating.

Identifying PVC Damage

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

• Greenish film or streaks that follow the coin’s design elements rather than appearing randomly (which would suggest bronze disease).
• A “sticky” or tacky feel to the coin’s surface — the plasticizer residue can create a slightly adhesive quality.
• Cloudiness or hazing that cannot be wiped away with a soft cloth.
• Damage concentrated on one side — if one side of the coin shows significantly more damage than the other, it was likely stored face-down against a PVC surface.

Prevention and Remediation

The good news is that PVC damage is almost entirely preventable. Here are my recommendations:

1. Audit your storage materials immediately. Check every flip, album page, and storage box. If it’s soft, flexible, and has a slight “plastic” smell, it likely contains PVC. Replace it with archival-quality, PVC-free alternatives (Mylar flips, acid-free cardboard, or inert plastic holders).
2. Remove coins from suspected PVC holders carefully. Wear nitrile gloves to avoid transferring oils and chlorides from your skin.
3. For early-stage PVC damage, an acetone dip can be remarkably effective. I’ll discuss this in detail in the next section.
4. Never use commercial coin cleaners on PVC-damaged coins. Many contain acids or abrasives that will worsen the damage.

Oxidation and Tarnish: The Ever-Present Challenge

Beyond bronze disease and PVC damage, oxidation is the most universal environmental threat to coinage. All metals oxidize when exposed to oxygen, moisture, and atmospheric pollutants. The rate and appearance of oxidation vary significantly by metal type, storage conditions, and the coin’s history.

Silver Oxidation: The Case of the German Mark Series

The German ½, 1, and 3 Mark coins are silver alloys (typically .900 fine for pre-1915 issues and .500 fine for later issues). Silver oxidation produces a range of colors — from light yellow to deep purple, blue, and black — depending on the specific compounds formed and the thickness of the tarnish layer.

Here’s what I’ve observed in my conservation work with German silver coinage:

• Light toning (golden to light blue) is generally stable and can actually enhance a coin’s eye appeal. Many collectors prize naturally toned silver coins.
• Heavy, dark toning (deep brown to black) can obscure design details and reduce a coin’s grade. This is particularly problematic for the Mark series, where die polish lines can already be challenging to distinguish from hairlines.
• Uneven toning — dark on one side, light on the other — often indicates that the coin was stored in an environment with uneven air circulation, such as a closed album or a coin that was partially wrapped.

As one astute forum participant noted, grading German Mark coins can be particularly challenging because die polish lines can resemble hairlines. Under higher magnification, die polish lines will be raised (part of the coin’s original surface), while hairlines are incised (scratches that remove metal). This distinction is critical for accurate grading, and environmental damage can make it even harder to tell the difference.

Copper and Bronze Oxidation

For the German New Guinea bronze and copper-nickel issues, oxidation presents differently. Copper oxidation initially produces a thin, protective layer of copper oxide (which appears as a dull brown or reddish film). Over time, this can develop into copper carbonate — the familiar green verdigris.

The key distinction I always emphasize is between stable patina and active corrosion:

• Stable patina is even, adherent, and does not change over time. It actually protects the underlying metal from further oxidation. Many ancient coin collectors consider patina desirable.
• Active corrosion (including bronze disease) is uneven, powdery, and progressive. It will continue to damage the coin if left untreated.

Proper Chemical Conservation: The Acetone Dip and Beyond

Now let’s talk about what you can do when environmental damage has already occurred. As a conservator, I approach chemical treatments with great caution — the wrong treatment can cause irreversible damage. But when applied correctly, certain chemical conservation methods can be remarkably effective.

The Acetone Dip: Your First Line of Defense

Acetone (CH₃COCH₃) is the single most useful chemical in a coin conservator’s toolkit. It’s a powerful solvent that dissolves many organic compounds — including PVC plasticizer residue, adhesive from old album pages, oils from fingerprints, and various organic contaminants — without reacting with the metal itself.

Here’s my standard acetone dip protocol:

1. Use only pure, reagent-grade acetone. Hardware store acetone often contains water and other additives that can cause spotting or additional corrosion. I source mine from laboratory supply companies.
2. Work in a well-ventilated area. Acetone fumes are flammable and can cause dizziness. Never use acetone near an open flame.
3. Submerge the coin in a glass container of acetone. Do not use plastic containers — acetone will dissolve most plastics.
4. Soak for 5-15 minutes for light contamination, up to several hours for heavy PVC residue. For severe cases, I may change the acetone multiple times as it becomes saturated with dissolved contaminants.
5. Remove the coin with plastic or wooden tongs (never metal). Allow it to air-dry on a clean, lint-free surface. Do not wipe or rub the coin.
6. Repeat if necessary. Stubborn PVC residue may require multiple soaks.

Critical warnings:

• Acetone will not remove active bronze disease, mineral deposits, or metal corrosion. It only removes organic contaminants.
• Acetone will not restore original luster to a coin that has been worn or polished away. If the surface detail is gone, no chemical treatment will bring it back.
• Acetone can damage certain coin holders and capsules. Remove the coin from any plastic holder before treatment.
• Never use acetone on coins with artificial toning or coatings. It will strip them away.

When to Call a Professional

While the acetone dip is a safe and effective treatment for many common environmental damage issues, there are situations where I strongly recommend professional conservation:

• Active bronze disease on rare or valuable coins. The German New Guinea series, in particular, deserves professional treatment given its historical significance and relative scarcity.
• Coins with uncertain metal composition. Some treatments that are safe for silver can damage copper alloys, and vice versa.
• Coins with numismatic value that could be affected by any treatment. When in doubt, consult a professional before doing anything.
• Coins with active corrosion that has penetrated below the surface. Surface treatments won’t address subsurface damage.

The Surviving Population Mystery: Why Environmental Damage Matters for Valuation

The forum discussion touched on something that I think is critically important and often overlooked: the mystery of surviving populations. As one collector noted, it’s very difficult to gauge the surviving population at various grades for many German and German New Guinea issues. Population reports from grading companies like PCGS provide some data, but they only tell part of the story.

Here’s where environmental damage analysis becomes directly relevant to valuation:

1. Many “surviving” coins are actively deteriorating. A coin that was mint-state 50 years ago may now be corroding in someone’s attic. The population reports don’t account for coins that have degraded since being graded.
2. Ungraded coins may be hiding in old, damaging storage. As the German collector in the thread noted, many German collectors still buy “junk” ½ and 1 Mark coins at spot price. Some of these may be high-grade examples that have been sitting in PVC albums for decades, slowly accumulating damage.
3. Environmental damage can mask true rarity. The 1908-G Mark from the forum thread is reportedly the highest graded by PCGS at MS66, with only 4 MS examples on the pop report. But how many potentially high-grade examples are out there, hidden in old collections, their surfaces clouded by PVC damage or active bronze disease? The true surviving population may be significantly higher than the graded population suggests.
4. Condition rarities command premium prices — but only if the condition is genuine. As one collector observed, prices don’t always match rarity, especially in high grades. A coin that has been improperly cleaned or treated may appear to be in high grade but will never achieve the value of a genuinely original, well-preserved example.

This is why I always tell collectors: preservation is an investment. The cost of proper storage materials — archival-quality flips, acid-free boxes, silica gel packets — is trivial compared to the value of the coins they protect. And the cost of professional conservation, when needed, is almost always less than the value lost to environmental damage.

Actionable Takeaways for Collectors, Buyers, and Sellers

Let me leave you with a concrete action plan based on everything we’ve discussed:

For Collectors:

• Audit your storage materials today. Replace any PVC-containing flips, albums, or boxes with archival-quality alternatives.
• Inspect your coins regularly. Look for early signs of bronze disease (bright green powdery spots), PVC damage (greenish haze or streaks), and active corrosion. Early detection is everything.
• Control your storage environment. Aim for low humidity (below 50% relative humidity), stable temperature, and good air circulation. Silica gel packets are your friend.
• Handle coins with nitrile gloves. Bare hands transfer oils, salts, and moisture that accelerate corrosion.
• Consider the acetone dip for coins with suspected PVC damage. It’s safe, effective, and can dramatically improve a coin’s appearance and long-term stability.

For Buyers:

• Ask about storage history. When purchasing German or German New Guinea coins, ask the seller how the coin has been stored. Coins from old collections may have hidden environmental damage.
• Examine coins under magnification before purchasing. Look for the signs of bronze disease, PVC damage, and active corrosion that we’ve discussed.
• Be skeptical of “too good to be true” prices on high-grade examples. If a coin appears to be in unusually high grade for its date and mint mark, investigate whether it has been improperly cleaned or treated.
• Factor conservation costs into your purchase price. If a coin shows early signs of environmental damage, the cost of professional treatment should be reflected in what you’re willing to pay.

For Sellers:

• Preserve before you sell. A coin that has been properly stored and shows no signs of environmental damage will command a significantly higher price than one with visible corrosion or PVC damage.
• Be transparent about condition. Disclose any known environmental damage or previous treatments. Buyers — especially experienced collectors and dealers — will appreciate the honesty, and it protects you from disputes.
• Consider professional grading for high-value examples. As the forum discussion illustrates, population reports can significantly affect valuation. A professionally graded coin with a documented grade and authentication is worth more than an ungraded example of uncertain quality.

Conclusion: Preserving History, One Coin at a Time

The German Imperial and German New Guinea coinage series represents a fascinating chapter in numismatic history — from the unified coinage of the Kaiserreich to the colonial issues minted for territories on the other side of the world. The 1894-A 10 Pfennig from German New Guinea, the 1908-G Mark (reportedly the finest known at MS66), the 1927-A Bremerhaven 3 Mark, and the 1931-A Magdeburg 3 Mark featured in the forum thread are all pieces of living history, connecting us to the economic, political, and cultural realities of their time.

But history doesn’t preserve itself. Every coin in your collection is engaged in a slow chemical battle with its environment, and without proper care, the enemies we’ve discussed — bronze disease, PVC plasticizer damage, oxidation, and active corrosion — will eventually win.

As a conservator, I’ve seen too many historically significant coins destroyed by preventable environmental damage. I’ve also seen coins rescued from the brink of destruction by timely, appropriate conservation treatment. The difference almost always comes down to knowledge and vigilance.

Know the signs of environmental damage. Store your coins properly. Act quickly when you detect a problem. And when in doubt, consult a professional. The coins in your collection aren’t just metal — they’re pieces of history, and they deserve to be preserved for the next generation of collectors, historians, and conservators who will study and appreciate them.

The surviving population of German and German New Guinea coinage may always hold some mysteries, as the forum discussion suggests. But one thing is certain: the coins that survive in the best condition will be the ones that were protected from environmental damage. That’s not just good conservation — it’s good collecting.

Related Resources

You might also find these related articles helpful:

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