Mint Error or Damaged? Decoding the Surface of 90% Silver Coins — When Verdigris, PVC, and Planchet Flaws Collide

Is that a rare lamination flaw, or did someone just scratch it with a screwdriver? Let’s break down how to tell the difference.

For over two decades, I’ve graded and authenticated U.S. silver coinage, and I can tell you that one of the most confusing scenarios in our hobby happens when surface deposits, environmental corrosion, and post-mint damage all converge on a single coin. The recent discussion around identifying and removing green crystalline deposits on 90% silver coins is a perfect case study in why surface anomalies demand forensic scrutiny before any attribution is made.

Was that green crystal a harmless PVC residue—or a copper-based corrosion product signaling a deeper metallurgical issue? And more critically, could what looks like a post-mint scratch actually be a hidden lamination flaw revealed by cleaning? I’ve seen this destroy the eye appeal and numismatic value of a coin in seconds.

Chapter 1: The Chemistry of Confusion — Verdigris, PVC, and Silver Alloys

Before we talk planchet defects, we need one foundational truth from that forum thread: verdigris simply cannot form on 90% silver coins under normal circumstances. It feels counterintuitive. After all, 90% silver coins—Walking Liberties, Franklin Halves, Peace Dollars—contain 10% copper. Shouldn’t that copper eventually produce the familiar blue-green patina we associate with bronze coinage?

Here’s what I know from years of lab work: true verdigris is a copper compound, specifically a mix of basic copper carbonate or copper acetate, formed through a purely mineral, non-living reaction. The term comes from the Old French verte grez, meaning “Greek green.” In strict metallurgical terms, verdigris needs a copper-rich environment—think bronze or pure copper coinage—where moisture and carbon dioxide interact with the copper over time. On a 90% silver alloy, the copper content is just too low and too dispersed to support verdigris formation under normal conditions.

So what was that forum participant actually seeing? The answer, as several knowledgeable members pointed out, was almost certainly PVC residue—polyvinyl chloride from older coin flips, PVC holders, and certain storage materials. PVC goo is an organic contaminant that migrates onto coin surfaces, often forming greenish or brownish deposits that beginners mistake for verdigris.

This distinction isn’t just academic. It directly impacts both authentication and valuation:

• PVC deposits are organic and can be dissolved by acetone without damaging the coin’s surface.
• True verdigris (when it does occur on silver coins under unusual circumstances) is an inorganic metal salt that cannot be removed by acetone. Its removal requires acidic solutions—such as sulfuric-acid-thiourea coin dips—and invariably causes surface damage.
• Post-mint environmental corrosion on silver coins may require ammonia-based solutions or professional conservation, and attempting DIY removal can destroy originality.

As one experienced member of that thread pointed out, acetone works rapidly on PVC—sometimes in seconds with agitation—leaving the silver surface unharmed. But the critical warning from that exchange deserves emphasis: acetone will not remove verdigris, and anyone claiming otherwise is either misidentifying the deposit or causing hidden damage that ruins the coin’s provenance.

Chapter 2: How Surface Deposits Mask Planchet Flaws — The Attribution Trap

Here’s where my expertise as an error attribution specialist comes into play. One of the most common scenarios I see in my grading room involves a coin that has been cleaned—or someone tried to clean it—and now presents surface anomalies that could be PMD or genuine planchet defects obscured by original deposits.

Consider the forum participant who mentioned a “tiny little green crystal” that fell off after gentle Q-tip contact. This is classic PVC presentation, but imagine if that same coin had instead been subjected to improper verdigris removal using an acidic dip. The resulting surface pitting and metal loss could easily be misattributed as a clipped planchet error—a minting defect where a portion of the coin blank was missing before striking—or as a lamination flaw where the metal layers of the planchet separated due to impurities or improper alloy mixing during production.

This is the attribution trap. Surface damage from cleaning attempts can create features that convincingly mimic genuine mint errors. I’ve seen coins slabbed as error varieties that were actually victims of PVC removal gone wrong, and I’ve seen genuine laminated planchets dismissed as PMD because a previous owner had scrubbed the surface with abrasives.

My diagnostic approach involves a systematic examination protocol:

1. Magnification at 10x and 30x: Examine the surface under controlled lighting. PMD scratches typically follow irregular, random patterns. Lamination flaws follow the metal flow lines of the planchet and often appear as parallel lines or raised ridges beneath the surface.
2. Edge examination: Clipped planchets show a distinctive curved clip line at the coin’s rim. Post-mint nicks and scratches crossing the rim are random and do not follow the coin’s original curvature.
3. Surface texture analysis: PVC deposits leave a waxy, slightly raised residue. Verdigris creates crystalline formations that are hard and embedded in the metal surface. True planchet flaws are part of the coin’s metal structure and feel smooth to the touch of a fingernail.
4. Acetone spot test: A small application of acetone on a cotton swab can reveal whether a green deposit is organic (PVC) or inorganic (metal salt). If it dissolves, it was PVC. If it remains, further identification is needed before any removal attempt.

Chapter 3: Lamination Errors — What They Are and How to Identify Them

Lamination errors are among the most fascinating and valuable planchet defects in the numismatic world. They occur when the layers of metal in a coin blank—or planchet—fail to bond properly during annealing and rolling. The result is a coin with visible separation between metal layers, often appearing as raised lines, blisters, or incomplete areas on the surface.

For 90% silver coins, lamination errors are particularly noteworthy because the alloy composition (90% silver, 10% copper) was specifically designed to produce a durable, attractive coinage metal. When that bond fails, it speaks to a manufacturing irregularity at the mint—a deviation from standard production that makes the coin a genuine mint error rather than a damaged specimen, significantly boosting its collectibility.

Key identifiers of genuine lamination errors:

• Flow-line alignment: Lamination lines follow the directional flow of the metal during planchet production. They are parallel to each other and consistent across the coin’s surface.
• Raised or recessed texture: Depending on whether the separation occurred before or after striking, the lamination may appear as a raised ridge or a recessed line. Both forms are legitimate errors.
• No disruption to design elements: A genuine lamination flaw does not distort the coin’s design. The portrait, lettering, and motifs remain intact and properly struck, with the lamination existing as a separate surface feature. This preservation is key to maintaining mint condition and eye appeal.
• Consistency across multiple coins: When a lamination error is caused by a rolling mill irregularity, you may find similar flaws on multiple coins from the same production run. This is a strong indicator of a genuine mint error rather than post-mint damage.

I’ve personally examined Walking Liberty Halves from the Philadelphia Mint in the 1940s that exhibited lamination errors traceable to specific heat-treatment irregularities during annealing. These coins command significant premiums precisely because they represent a documented deviation from normal mint operations—a true rare variety for any serious collection.

Chapter 4: Clipped Planchets — The Anatomy of a Rim Defect

A clipped planchet error occurs when a portion of the metal blank is missing before the coin is struck. This can happen for several reasons: a misaligned blank in the punching press, a worn die clipping the edge, or a feeding malfunction that partially shears the blank before it enters the striking chamber.

The defining characteristic of a clipped planchet is the curved clip line at the coin’s edge. This line follows a smooth, concave or convex arc that corresponds to the curvature of the missing metal. It is not a straight line, nor is it random—it traces the path of the shearing force that removed the material.

Distinguishing a genuine clipped planchet from a post-mint rim nick is critical for accurate attribution:

• Genuine clips maintain the coin’s original rim profile. The metal that remains is smooth and undisturbed, with the clip line showing the clean shear mark from the minting process.
• Post-mint rim damage often shows striations, burrs, or compression marks indicating the coin was struck or scraped after leaving the mint. The edge may be rough, irregular, or show evidence of contact with another hard
  

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