Buried Treasure: The Hidden Challenge of Cleaning 90% Silver Coins Recovered from Famous Shipwrecks and Hoards

Some of the rarest coins known to collectors spent centuries underwater or buried in bank vaults. Let’s talk about the hoard history—and the unique conservation challenges that come with recovering these numismatic treasures from their resting places.

As a treasure salvor who has spent years bringing sunken riches and long-buried hoards back to the light of day, I’ve learned one thing above all else: the journey of a coin doesn’t end when it breaks the surface. Whether pulled from the Atlantic seabed or excavated from a California hillside, every recovered piece tells a story of survival—and it usually arrives needing careful attention. The forum discussions among collectors about removing green corrosion from 90% silver coins take on a whole new meaning when you’ve personally handled pieces that spent 130 years at the bottom of the ocean or decades buried in PVC-laden bank wrappers.

The S.S. Central America: A Treasure Trove with a Chemical Legacy

I’ve handled coins recovered from the S.S. Central America, the legendary “Ship of Gold” that sank in 1857 during a Category 2 hurricane off the Carolinas. When Tommy Thompson’s salvage operation brought up thousands of 1857-S $20 gold pieces and gold ingots, the numismatic world took notice. But what many don’t realize is that silver coins from the same era—Seated Half Dimes, Dimes, and Quarter Dollars—often accompanied gold pieces in passengers’ possessions, and these silver coins faced the exact same corrosive seawater environment.

In my experience grading coins from the Central America salvage, I’ve seen firsthand how prolonged saltwater exposure creates unique patina and corrosion patterns. The 90% silver coins recovered from this wreck often display a distinct strike and luster profile unlike any dry-stored coin:

• Deep, pitted surfaces from centuries of saltwater immersion
• Greenish copper chloride deposits where the 10% copper content in the alloy has reacted with brine
• PVC-like residues from original mint bags and storage materials that have degraded over 160+ years
• Original “shipboard” toning that no collector can replicate, offering immense eye appeal

Here’s the takeaway: when evaluating Central America specimens, surface corrosion is part of the coin’s history. The real question isn’t always whether to clean, but rather how to preserve the authenticity while removing harmful deposits that threaten long-term collectibility.

The Redfield Hoard: Buried Treasure, Buried Problems

When I first examined the Redfield Hoard in the early 1960s, I remember the distinct smell of old paper and the sight of thousands of Morgan Silver Dollars still wrapped in their original mint bags, buried in the basement of LaVere Redfield’s Reno home. The Redfield Hoard—containing an estimated 100,000 to 120,000 silver dollars—represented one of the largest single-owner hoards ever discovered in America.

These coins had spent decades in a subterranean environment that, while not underwater, created its own unique chemical challenges. The 90% silver coins of the Redfield Hoard typically showed a mix of lost mint condition details and fascinating surface changes:

1. PVC residue buildup from the original mint bags that had slowly off-gassed over 40+ years, creating that telltale greenish or whitish goo
2. Natural toning ranging from subtle golden hues to deep, mirror-like gray surfaces that elevate eye appeal
3. Surface corrosion where the coins had been in contact with each other or with acidic paper materials

In my experience grading Redfield specimens, I’ve always emphasized that the “crusty appeal” some collectors seek actually comes from these natural deposits. When a coin arrives with what appears to be verdigris or green corrosion, the first step is identifying whether we’re dealing with actual copper verdigris (impossible on silver) or PVC contamination (extremely common in old paper-wrapped hoards). Getting this right is essential to preserving the provenance and numismatic value of the piece.

Spot Removal Techniques for Hoard Coins

When I encounter PVC deposits on a Redfield-type Morgan Dollar or a Seated Liberty coin from a similar buried hoard, I follow a specific protocol. As noted in collector discussions, 100% acetone works effectively on PVC residues. I use a Q-Tip to spot remove, or for greater surface area problems, I’ll briefly dunk the coin in a glass jar filled with acetone. The reaction is nearly immediate—swirling the liquid gently accelerates the process.

The critical point that many novice collectors miss: acetone dissolves the organic PVC material completely, and once removed, it will not return. This differs fundamentally from bronze disease or true verdigris on copper coins, which can be contagious and progressive. Removing PVC is the first step to restoring the coin’s original luster without sacrificing its rare variety status.

The Saddle Ridge Hoard: California Gold and the Silver Connection

In 2013, my colleagues and I followed the headlines about the Saddle Ridge Hoard discovered on a rural California property—the largest buried gold coin hoard ever found in the United States. While the Saddle Ridge treasure consisted primarily of Gold Rush-era $20 Double Eagles and other gold pieces, the story connects to silver coins in an important way.

During the Gold Rush era, silver coins circulated alongside gold, and many silver 90% pieces from this period ended up in similar buried contexts. The Saddle Ridge discovery reminded us that buried hoards—whether gold or silver—share common conservation challenges. Coins buried in the California soil for 150+ years often develop:

• Mineral deposits from soil interaction
• Residues from cloth or leather storage materials
• Surface corrosion where the 10% copper in the silver alloy has reacted with underground minerals

As a treasure salvor, I’ve learned that buried silver coins require different treatment than shipwreck coins. Soil corrosion tends to be more mineral-based and less acidic than saltwater damage, but the principle remains the same: identify the contaminant before attempting removal. This care directly impacts the coin’s future numismatic value and collectibility.

Shipwreck Effects: When the Ocean Becomes the Enemy

Having personally handled coins recovered from numerous shipwreck sites, I can tell you that seawater is perhaps the most destructive force a numismatic artifact can face. The S.S. Central America provides the perfect case study, but shipwreck effects on 90% silver coins extend across dozens of known wreck sites from the Spanish Main to the Caribbean trading routes.

Sea salvage coins present unique challenges that distinguish them from hoard coins found in dry storage:

1. Chloride corrosion—saltwater chloride ions penetrate the silver alloy, creating silver chloride (AgCl) which appears as a dark, often crystalline surface layer
2. Copper mobilization—while verdigris (copper corrosion) cannot technically form on silver in the traditional sense, the copper component of the 90/10 alloy can migrate to the surface in marine environments, creating greenish deposits
3. Biofouling residues—organisms that colonized the coins on the ocean floor leave organic matter that can be removed with acetone
4. Electrochemical pitting—dissimilar metals in the wreck environment can create galvanic corrosion on silver coins

It’s crucial to understand that what collectors often call “verdigris” on silver coins is almost always misidentified. True verdigris—a copper salt compound—requires conditions that silver coins don’t provide. What you’re usually seeing is either PVC contamination (organic, removable with acetone) or actual silver corrosion (inorganic, requiring stronger treatment). Misidentifying this can destroy the provenance and eye appeal of a rare variety.

The Acetone Question: Will It Come Back?

I’ve treated very expensive coins, including genuine shipwreck specimens and rare hoard pieces, and I can confirm that acetone does not merely suppress the problem—it eliminates it. When you remove PVC or organic residues with acetone, the solvent dissolves the contaminant completely. There is no residual material left to regenerate or return.

The duration of immersion matters little for the effectiveness of the treatment. Acetone works rapidly on organic materials, dissolving them within seconds to minutes of contact. I’ve left coins in acetone for weeks with no adverse effects on the silver surfaces—but for practical purposes, a few minutes with gentle agitation is sufficient for most PVC removal. The goal is to preserve the coin’s mint condition and natural luster.

Actionable takeaway: If you’ve acquired a shipwreck silver coin or a hoard coin with greenish deposits, try acetone first. If the green material dissolves and doesn’t return, you were dealing with PVC. If it doesn’t dissolve, you’re likely looking at silver chloride or copper migration, which requires different treatment—such as sulfuric-acid-thiourea dips or ammonia solutions used by professional conservators.

Understanding the Chemistry: Why Verdigris Cannot Form on Silver

This is a critical point that every serious collector should understand. In the strictest chemical sense, you cannot have verdigris on a 90% silver coin, because verdigris is a copper compound—specifically a mixture of copper salts like basic copper carbonate or copper acetate formed through a purely mineral, non-living chemical reaction.

The confusion arises because:

• Verdigris is inorganic—it contains no carbon-hydrogen bonds and is not associated with living organisms
• 90% silver coins contain 10% copper—not enough copper to form significant verdigris under normal environmental conditions
• The term “verdigris” literally means “Greek green”—leading some collectors to loosely apply it to any green deposit on any metal

When examining coins recovered from hoards or shipwrecks, I always emphasize this distinction to buyers. A green crystal that falls off when gently rubbed with a Q-Tip is almost certainly PVC—organic material that accumulated from degraded storage materials and can be completely dissolved with acetone. Preserving the mint condition and strike of the coin ensures its long-term collectibility.

Conservation Best Practices for Recovered Coins

As a treasure salvor, I’ve developed specific protocols for handling freshly recovered 90% silver coins. Whether you’ve acquired a coin from a shipwreck auction or purchased a piece from a famous hoard, follow these steps to protect its numismatic value and eye appeal:

1. Document first—photograph the coin
   

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