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.
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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
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.