Coins and planchets that split along the horizontal plane are often
lumped into a single category, the “clamshell split.”
However, this term should really be restricted to solid-alloy
coins like cents, 5-cent coins and silver-alloy coins minted before
the 1965 compositional change. In the case of clad-composition coins,
the proper term should be “clamshell separation.”
While clamshell splits and clamshell separations are similar in
appearance, these two planchet errors arise from entirely different causes.
A clamshell split is an alloy error in which a planchet or coin
cleaves along the horizontal plane. The illustrated 1945-P Jefferson,
Wartime Alloy 5-cent coin is a representative example. Many splits of
this nature are thought to be caused by the presence of contaminants
in the alloy.
Inadequate mixing of metals
A horizontal split can also be caused by an inadequate mixing of
the constituent metals. This can leave a zone of weakness at the
boundary between layers containing different proportions of metals.
Wartime Alloy 5-cent coins — composed of 56 percent copper, 35 percent
nickel and 9 percent manganese — are especially prone to clamshell
splits and assorted types of “lamination” errors. The illustrated
specimen clearly shows a poorly mixed alloy. The surface is marked by
numerous lamination cracks and missing lamination flakes.
More than one plane of weakness can occasionally develop in a
solid alloy coin. The illustrated 1944-P Jefferson 5-cent coin shows a
double clamshell split on the edge.
When a split extends completely across a planchet, the two halves
go their separate ways. If one or both hemi-planchets eventually make
their way into a coinage press, you end up with a
“split-before-strike” error. If a coin splits into two halves after
the strike, you then have a pair of “split-after-strike” errors.
Clamshell splits are closely related to clamshell lamination
flaps, like that seen in the accompanying 1981 Lincoln cent. The only
difference is the depth at which separation occurs. A clamshell split
generally develops at a depth between one-third and one-half of the
distance between one face and the opposite face. Clamshell lamination
flaps form much closer to the surface and are therefore much thinner.
Because it’s so thin, a lamination flap can bend and fold over (as in
the illustrated example), while a clamshell split will break off if
it’s folded up.
By definition, the line of cleavage in a clamshell split or
clamshell lamination flap must extend in from the edge without
emerging on either face. If the line of cleavage does invade the
coin’s surface in the form of a crack, the clamshell label is dropped.
The defect is instead referred to as a retained lamination flake.
As mentioned earlier, clamshell separations are the exclusive
province of clad-composition coins. A clamshell separation is a
bonding error — a failure of one clad layer to bond to the core. In
the bonding process used for contemporary, circulation-quality dimes,
quarter dollars and half dollars, two coils of thin copper-nickel clad
strip are bonded to a thicker coil of pure copper strip. Bonding
occurs well above the melting point of both copper and nickel, as the
three strips are forced together under enormous pressure between two rollers.
If pressure is inadequate, or if the facing surfaces of adjacent
strips are not meticulously cleaned, a secure bond may not form. Once
the blank is punched out, the poorly bonded clad layer can separate
before or after the strike. A partial separation is the same thing as
a clamshell separation. A typical example is shown here in a 1965
When a bond fails completely on a planchet, the clad layer assumes
an independent existence. If it manages to reach a coinage press, it
will emerge as a “struck clad layer.”
If a clad layer separates after the strike, it is also often
called a struck clad layer. However, it would be more appropriately
classified as a “separation-after-strike” error.
On very rare occasions a copper core will split along the
horizontal plane, creating a “split core” error. Since the split
occurs within a zone of solid metal, these kinds of errors are
rightfully classified alongside clamshell splits and split planchet errors.
Coin World’s Collectors’ Clearinghouse department does
not accept coins or other items for examination without prior
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Clearinghouse without prior permission will be returned unexamined.
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