A brockage is an incuse, mirror-image version of a coin’s design.
It is generated when a coin is struck into a planchet or when two
previously-struck coins overlap within the striking chamber.
The most desirable kind of brockage is a first impression
(“first-strike brockage”) that is sharply defined and shows little or
no expansion. Many first-strike brockages are expanded and distorted.
The subset that exhibits minimal expansion emerges under special
circumstances. These original-size, incuse impressions are often
termed “mirror brockages.” Regrettably, the term “mirror brockage” has
been indiscriminately applied to brockages that fall far short of the
strict standards maintained here.
A bona fide mirror brockage can arise under several circumstances:
Confinement of both discs within the collar: If both discs are
confined by the collar, lateral expansion is blocked, and the brockage
that results will show no expansion. The illustrated 1962 Lincoln cent
shows a mirror brockage of the obverse design on the reverse face. The
planchet represented by this coin was fully confined by the collar.
Since it was on top, it follows that the bottom coin (the
“brockage-maker”) was confined as well.
This brockage is incomplete because the design of the bottom coin
was incomplete. The bottom coin entered the striking chamber as a
split planchet. This resulted in reduced effective striking pressure
and the failure of some details to strike up.
Confinement of the bottom disc within the collar: When a coin is
driven into a planchet, the two discs are temporarily locked together
by frictional forces. Therefore, even if only the bottom disc is
confined by the collar, this will still limit expansion of the top
disc, although not always as effectively. Still, the result will often
be a brockage with minimal expansion, as shown on the illustrated 1980
Lincoln cent. The planchet represented by this coin was fully confined
by the collar, while the coin that generated the brockage was situated
above the collar. Lateral expansion of the intrusive portion of the
top coin was constrained by frictional forces.
The area of overlap was subjected to increased effective striking
pressure owing to the double thickness. This caused some coin metal
from the bottom coin to squeeze over the top of the collar in the form
of “horizontal lipping.”
Weak (low-pressure) strike: When two discs overlap within the
striking chamber, effective striking pressure is boosted well above
normal “ram pressure” — at least under normal circumstances. “Ram
pressure” is the tonnage applied to a planchet of normal thickness.
Since two overlapping discs have an increased aggregate thickness,
they are subjected to additional tons of pressure. This is why many
first-strike brockages are grossly expanded and distorted when the two
discs are struck out-of-collar.
At times, however, the presence of overlapping discs coincides
with a press malfunction that either reduces ram pressure or increases
the minimum die clearance. The latter is the gap that exists between
the dies at their closest approach in the absence of a planchet.
The double-struck 1972-D Roosevelt dime shown here displays a
mirror brockage of Roosevelt’s head on the reverse face of the second,
off-center strike. Both the top coin and the bottom coin (the
brockage-maker) were struck out-of-collar. Yet the brockage of
Roosevelt’s head shows very little expansion. Striking pressure seems
to have been great enough to generate a clear impression, but too weak
to generate much expansion.
The Denver Mint produced many similar mirror brockages in the
years 1968 to 1973, mainly among 5-cent coins, with some dimes,
quarter dollars and half dollars thrown into the mix. Clearly,
something odd was affecting the coinage presses at the Denver Mint
during this six-year span. Oddly enough, I’ve never seen a Denver Mint
cent from the same six-year period that shows this type of brockage.
Some mirror brockages owe their existence to a confluence of
factors, the most common being a weak strike in combination with
confinement of the bottom disc by the collar.
Additional factors come into play when you begin talking about
mirror brockages from struck fragments, split-after-strike lamination
flakes and other nonstandard brockage-makers. But space does not
permit consideration of these exotica.
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