Error Coin Essentials: Foreign Matter

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This article is sponsored by CAC Grading, the collectors’ choice, inspiring confidence through accurate and consistent coin grading.

Error Coin Essentials introduces the core concepts behind coins that didn’t come out quite right. These articles provide a practical foundation for collectors who want to understand how errors happen, how to identify them, and how to collect them. With strong collector demand and a wide variety of examples, error coins remain one of the hobby’s most engaging fields. 

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There is no single source and no single pathway that accounts for foreign matter found embedded in coins. While most foreign matter is metallic, nonmetallic contaminants such as wood, plastic, and rubber are possible.

Early Foreign Matter Introduction 

Foreign material can be introduced at multiple points along the minting process. The earliest entry point occurs during the smelting, refining, mixing, and pouring of the molten alloy. In pure copper and copper-alloy coins, unexpected metals, such as gold and silver, can be introduced if a crucible used for a higher-value denomination isn’t thoroughly emptied. A piece of hardware with a higher melting point also might fall into the crucible and fail to completely mix with the normal constituents. The minor components of a poorly mixed alloy may persist as relatively pure streaks in the finished coin. For example, some copper-alloy cents show gray strips of unmixed tin or zinc.

Finally, bits of slag may infiltrate the alloy if it’s not skimmed off the top of the crucible. Shown below is a 1979 cent that retains about 40 percent of a large piece of slag that extends through the thickness of the coin.

The molten alloy is eventually poured into molds to form ingots. Once cooled, the ingots are rolled into long strips from which coin blanks are punched out. Foreign matter can be introduced when the strip is rolled out. Shown below is the reverse of a 1971-D cent that contains a large piece of silvery metal. We can tell it was rolled in (rather than struck in) because there is no added weight. In fact, the coin is slightly underweight at 2.98 grams, as opposed to the expected 3.11 grams. An SEM/EDS analysis (scanning electron microscopy with energy-dispersive X-ray spectroscopy) showed an elemental profile of 64-percent copper, 13-percent nickel, 13.4-percent carbon, 6.6-percent zinc, and 2.9-percent oxygen. The most prosaic explanation is that this is a piece of copper-nickel derived from a 5-cent strip. However, the other elements leave space for alternative interpretations.

Late Foreign Matter Introduction

After the metal strip is rolled to coin thickness, blanks are punched out by an array of cylindrical blanking punches. Stray bits of metal can be pressed into the upper side of the blank during this operation. As a practical matter, such an error would be almost impossible to distinguish from a foreign object that was struck into a coin.

The blank is eventually sent to the upsetting mill where it is rolled and squeezed to a smaller diameter, an action that generates the low proto-rim of the planchet. Foreign material can be rolled into the edge of the planchet during this process. Shown above is the edge of a 1968-D cent with a small pellet of gray metal that was most likely squeezed in during upsetting. Thin pieces of foil-like metal can also be pushed into the edge of the planchet and fold over onto one or both faces in the process. The overhanging metal is later struck into the surface of the coin.

For most coins, the final point at which foreign matter can be introduced is the strike. Shown above is the reverse of a 1999-D nickel with a large, struck-in fragment composed of a tungsten-carbide alloy. The added metal increased the coin’s weight to 5.25 grams from the expected 5 grams. The presence of cobalt (7.9 percent) and iron (4.1 percent) presumably accounts for the fragment’s magnetism.

Manganese-brass small dollar coins have incuse edge lettering that is applied after the strike. This presents another opportunity for foreign material to be introduced. Foreign objects interposed between the lettering die and the coin will be pressed into the edge. An example on a 2007-P Thomas Jefferson dollar was described in a September 14, 2015, Coin World article.

Conclusion

It’s important to establish whether a piece of foreign matter was poured-in, rolled-in, punched-in, squeezed-in, or struck-in. The grading services frequently fail to make these distinctions, often lazily falling back on a “struck-in” diagnosis.

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Mike Diamond has been collecting, researching, and writing about error coins since 1997.  He previously wrote a weekly column for Coin World  (“Collectors’ Clearinghouse”) that focused on minting errors and die varieties.  Prior to that, he was responsible for dozens of articles that appeared in CONECA’s specialty journal Errorscope.  He created and contributed most of the content found in The Error-Variety Ready Reference.  Over the past 28 years, he has discovered or described well over 100 new error types.

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