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Improper storage is the greatest threat to a coin’s numismatic value. I’ll show you how to spot environmental damage on this specific alloy.
Understanding the Metal Composition of the 1928‑S Peace Dollar
In my work as a conservator, I’ve seen that the 1928‑S Peace dollar is a key‑date issue with a mintage of only 1,351,000, making it highly sought after by collectors. Struck at the San Francisco Mint, this coin features a 90 % silver and 10 % copper alloy, which provides a crisp strike and brilliant luster when in mint condition. However, the copper content also makes the alloy vulnerable to bronze disease, PVC plasticizer damage, and unwanted patina, all of which can affect its eye appeal and market value.
The Role of Copper in the Alloy
The 10 % copper in the alloy allows copper ions to migrate to the surface when the coin encounters moisture, acids, or plasticizers from old holders. Under my loupe, I search for pinkish‑brown spots that signal copper migration. Spotting these early warnings is crucial to preserving the coin’s eye appeal and overall numismatic value.
Bronze Disease: The Corrosive Menace
Bronze disease is a destructive copper‑chloride corrosion that targets silver‑copper alloys. Unlike ordinary tarnish, it appears as a powdery green‑blue deposit, forming when copper reacts with chloride ions and moisture. The resulting copper chloride is hygroscopic and acidic, fueling a self‑sustaining cycle that can eat through the metal.
Identifying Early Symptoms
When I suspect bronze disease, I check for these early indicators:
- Small, raised green or blue spots that feel powdery to the touch.
- Localized loss of luster around the spots, creating a dull, etched look.
- An acidic odor, particularly if the coin was kept in a humid
Related Resources
You might also find these related articles helpful:
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