There are many routes by which foreign matter can end up embedded in a coin. (In fact, I've written about it before.) But regardless of its nature and origin, one generally expects embedded foreign matter to take and hold an image when struck. This is, in fact, one of the features you look for when evaluating the authenticity of embedded foreign matter. It should carry the image impressed into it by the die and its edges should lie flush with the surrounding field and the design.

When it's just right

These expectations are fulfilled in our first example, an off-center Jefferson 5-cent coin with a roughly triangular piece of copper or copper alloy that was struck into the reverse face. The design elements are sharply struck and the edges of the copper fragment lie in the same plane as the surrounding normal surface.

Connect with Coin World:  

• Sign up for our free eNewsletter
• Like us on Facebook  
• Follow us on Twitter

It’s rather remarkable that both the 5-cent planchet and the copper fragment ended up lying in the same off-center position. Severe damage found on the unstruck portion of both faces could have been inflicted before or after the strike.

As with many rules governing errors, there are a few exceptions to this one. Once in a great while, embedded material proves too hard to take an image. In other words, it experiences no plastic deformation when struck. In other cases the substance is too soft and springy, so that any image transferred to the foreign material disappears shortly after the coin is ejected.

When it's too hard

Embedded in the reverse face of a 1999-D Jefferson 5-cent coin is an angular piece of broken, dark gray metal that is strongly attracted to a magnet. It was clearly struck in, as the obverse design rim shows finning and the edge shows horizontal lipping along an arc closest to the foreign object. This is an indicator of excessive localized striking pressure caused by the extra thickness between the dies in this area. The coin is also overweight at 5.25 grams (normal is 5 grams). A small collar break located at 11:00 has no obvious connection to the embedded metal.

None of the letters of E PLURIBUS UNUM have struck up where the metal crosses the motto. The metal appears to be pig iron, which has a high carbon content and is very hard and brittle. Cracks in the brittle metal show microscopic bits of brown and white granular material that looks like quartz or sand. This grit testifies to the crude nature of the alloy. I can’t imagine where it came from.

Slag (detritus left over from the smelting process) is also too hard and brittle to take an impression. Such is the case with a 1979 Lincoln cent that contains a large piece of slag that rises to the surface on both faces. Over half of the slag inclusion fell out after the strike.

Slag derived from copper ore has a relatively high iron content, and so it’s not surprising that this piece of slag is strongly attracted to a magnet.

Other stiff substances that reluctantly take an image include hardened, compacted die fill (“grease”) and metal derived from the working face of the collar.

When it's too soft

As mentioned earlier, a substance can also be too soft to hold an image, provided that it is also resilient. Resilience is the tendency for a substance to return to its original shape after it has been deformed. Items with this property would include rubber (say from bushings, gaskets, or O-rings), rubber cement, and various resins and plastics.

I have seen few such errors and only in photos. I was therefore unable to identify the precise nature of the foreign matter.

One cannot expect to find many examples of resilient material attached to a coin or even found in loose association with a coin containing a “struck-through” error. As the material returns to its original shape after the strike, it also tends to pop out of its recess. Furthermore, matching a loose piece of rubbery material to any particular recess is problematical as the shapes will not match.

The shape of a struck-through error will correspond to the shape of the material under full compression, when it’s maximally flattened and expanded.

After the die lifts off the surface of the coin, the material will spring back to a smaller, more compact shape that may bear little resemblance to the recess it produced.