Verdigris and PVC: Saving The Dalles Mint from Environmental Damage

Improper Storage is the Enemy of Numismatics

I’ve been grading and restoring coins for over twenty years now, and I can tell you this with absolute certainty: improper storage is the enemy of numismatics. I’ve watched collectors pour thousands into a rare variety, only to let it rot in a damp drawer. It’s heartbreaking. Today, let’s talk about the environmental damage that specifically targets bronze and copper-alloy coins and artifacts, using the ghost story of The Dalles Mint as our lens.

Before you tune out, consider this. While most collectors fixate on rarity or the historical punchline behind a piece, they miss the quiet, creeping damage hiding in basements, closets, and unventilated storage bins. Moisture, plasticizers, unchecked oxidation — these are the villains nobody talks about until it’s too late.

The Dalles Mint: A Brief History of a “Ghost” Facility

Before we get into conservation, let’s appreciate what we’re actually trying to save. The Dalles Mint was proposed by Senator James Nesmith and greenlit by Congress, but its location was hotly debated. Portland was in the running, sure. But the final decision put the Mint in Dalles City, right along the Columbia River, where raw ore from Idaho mines could be shipped in easily. Mary Laughlin donated the land. Harvey A. Hogue supervised construction.

Here’s that doomed timeline:

• 1864: Authorization granted on July 4th. William Logan appointed Superintendent.
• 1869: Basement and first floor completed.
• 1870: Construction suspended — delays, waning mining productivity.
• 1871: A fire rips through the surrounding city, reshuffling priorities.
• 1873: Construction officially abandoned.
• 1875: Site transferred to the State of Oregon.

Here’s the irony that keeps me up at night: the building eventually got finished, and it’s never once served as a U.S. Mint or Assay Office. Today its owner is The Mint, LLC — fate has a sense of humor. And because this branch was approved but never struck a single coin, numismatists still argue about what mintmark it would’ve carried. Dahlonega’s “D” was already gone. Denver’s “D” was years away. So proposals range from a resurrected “D” to “DC” for Dalles City (à la Carson City’s “CC”), “TD” for The Dalles, or even “DD” to dodge confusion. Any hypothetical coin from this mint would be a holy grail. Which is exactly why understanding how to protect such a piece matters so much.

Bronze Disease: The Green Plague That Eats Away at History

I’ll never forget the first time I held a bronze coin that felt gritty, almost powdery, with a vivid green crust erupting across its face. That’s bronze disease, and if you’ve seen it, you know it’s one of the most aggressive forms of corrosion out there. It preys on copper-alloy coins, especially those with high tin content.

What is Bronze Disease?

Bronze disease is progressive corrosion triggered by chloride ions trapped inside the metal. Add moisture and oxygen, and those chlorides produce hydrochloric acid. That acid gnaws into the coin from within, feeding a self-perpetuating cycle. What you end up with is a fluffy, light green or blue-green powder that can spread across the entire surface in days.

I’ve watched bronze disease obliterate pieces within weeks if nobody intervened. It’s not just ugly — it’s structural. Pitting, eroded details, full disintegration. Gone.

Identifying Bronze Disease on Your Collection

Check for these signs:

• Bright green or blue-green powder sitting on the surface.
• Pitting or cratering — the metal looks eaten away.
• A waxy or greasy feel when you touch the affected area.
• Active corrosion — brush the powder off and fresh green material blooms right back.

Imagine a hypothetical Dalles Mint coin hit with this. The details that give it numismatic value — gone. The first line of defense is always proper storage: a dry environment, desiccants, no shortcuts.

The Silent Killer: PVC Plasticizer Damage

Now here’s the one that really gets under my skin. PVC plastic. Countless collectors — especially those who came up through the hobby in the last century — stored their coins in PVC flip cases or soft plastic sleeves. Convenient? Sure. But we now know these materials are quietly devastating to numismatic items.

How PVC Damages Coins

PVC contains plasticizers, chemicals added to keep the plastic flexible. Over time, those plasticizers leach onto the coin’s surface. The result is a sticky, cloudy residue that traps dirt and moisture right against the metal. It’s a micro-environment built for corrosion — including bronze disease.

It usually starts subtle. A little cloudiness, a “foggy” look under the light. Leave it alone and it hardens into a permanent, irreversible stain. In bad cases, the residue forms a crystalline layer that’s maddeningly difficult to remove without scratching the underlying metal.

Spotting PVC Damage

Here’s what to look for:

1. Cloudiness: The coin looks hazy or foggy in the light.
2. Sticky residue: The surface feels tacky when you touch it.
3. Discoloration: An unnatural yellowish or brownish tint creeps in.
4. Imprints: You can see the outline of the plastic case or sleeve pressed into the coin.

If you suspect PVC damage, do not rub or scrub. You’ll drive the plasticizer deeper into the metal. Get a professional conservator involved. Seriously.

Oxidation and Patina: When Beauty Becomes a Threat

Not all discoloration is a death sentence. A stable, even patina on a bronze coin can be genuinely beautiful — it adds character, depth, eye appeal. But there’s a razor-thin line between stable patina and active oxidation, and crossing it means trouble.

The Difference Between Patina and Oxidation

Stable patina is a thin, adherent corrosion layer that’s reached equilibrium. Usually a dull green or brown. It doesn’t flake. It doesn’t spread. On ancient coins, it’s often a mark of authenticity — and yes, it can add to provenance and collectibility.

Active oxidation is the opposite. It’s ongoing, destructive corrosion:

• Uneven coloring — patches of different shades competing for space.
• Flaking or powdery texture — the surface is unstable.
• Brittleness — the coin feels fragile, like it might crumble.

For a coin from a proposed mint like The Dalles, telling these apart is everything. A stable patina whispers centuries in the ground. Active oxidation screams recent neglect.

Assessing Oxidation on Your Coins

When I examine a coin, I check:

• Color consistency: Uniform, or are there ugly spots?
• Texture: Smooth, or gritty and rough?
• Sound: Tap it. A healthy coin rings clear. A heavily oxidized one sounds dull, dead — almost hollow.

Chemical Conservation: Acetone Dips and Other Techniques

Spot damage? Act fast. As a conservator, I reach for a handful of chemical techniques to halt and reverse corrosion — but these demand precision. One wrong move and you’ve done more harm than the original problem.

The Acetone Dip: A Common Tool in My Kit

The workhorse for removing PVC residue and light surface corrosion is the acetone dip. Acetone dissolves organic contaminants — including PVC plasticizers — without attacking the underlying metal.

Here’s how I do it:

1. Preparation: I examine the coin under magnification first. Note any flaking or unstable areas. You don’t want surprises.
2. Solvent bath: Pure acetone, not nail polish remover. Glass container. Submerge the coin.
3. Monitoring: A few minutes max. Watch the contaminants dissolve and float to the surface.
4. Rinsing: Distilled water, immediately. Residual acetone left on the surface can be just as harmful as the original problem.
5. Drying: Soft, lint-free cloth or a gentle stream of compressed air for delicate pieces.

Important: Acetone is flammable and the fumes are no joke. This is a job for experienced conservators in a ventilated space. Please don’t try this at home without proper training.

Other Conservation Techniques

Beyond acetone, there are a few other tools I keep in my kit:

• Electrolysis: Great for deep corrosion on copper and bronze. A low-voltage current runs through the coin submerged in an electrolyte solution. Effective, but you need precise control or you’ll damage the surface.
• Rosin packing: The coin gets embedded in heated rosin to stabilize fragile, corroded areas. Perfect for structurally compromised pieces.
• Consolidants: Acrylic resins or similar materials bonded to flaking surfaces to lock the corrosion layer to the metal and stop further loss.

Actionable Takeaways for Collectors and Dealers

Whether you’re buying, selling, or just trying to keep your collection alive, here’s what I tell every client:

• Inspect before you buy. Magnification is your best friend. Look for PVC damage, bronze disease, active oxidation. Ask the seller how the coin was stored.
• Never use PVC storage. Switch to inert materials — 2-mil polypropylene, Mylar flips, or hard cases designed for coins.
• Control your humidity. Cool, dry environment. Silica gel packets or a dehumidifier. Keep relative humidity under 40%. That alone will cut your bronze disease risk dramatically.
• Handle by the edges. Oils and acids from your skin accelerate corrosion over time. Respect the strike.
• Get professional help when needed. A botched cleaning can destroy more value than the original damage. Don’t DIY your way into regret.

Conclusion: The Legacy of The Dalles Mint and the Importance of Preservation

The Dalles Mint is one of those stories that sticks with you. Ambition, loss, progress marching on — and a building that stands today as a monument to what might have been. It never produced a coin, but its legacy fuels debate over mintmarks and mint condition possibilities that keep numismatists arguing at every show.

Environmental damage — bronze disease, PVC leaching, unchecked oxidation — is the silent enemy of every collection. I’ve seen it claim pieces far more historically significant than a hypothetical Dalles coin. By knowing what to look for and how to respond, we protect the stories these objects carry. Store wisely. Inspect regularly. Act the moment you spot trouble.

The current owner of the site, The Mint, LLC, probably isn’t striking gold there today. But for the collector who understands that true treasure lives in preserved details — luster intact, patina stable, provenance clear — that’s where the real payoff is.

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