The following is the Collectors' Clearinghouse column from the July
27 issue of Coin World:
It’s long been recognized that areas of a coin thinned by a strike
may show strong persistence of first-strike elements in the aftermath
of a second strike. This is because the region of reduced thickness
may fall close to, or beneath, the minimum die clearance of the press.
This principle is beautifully embodied in the illustrated 1975
50-centavo coin of Mexico. When I first saw it in an online auction,
I thought it was a centered double strike with slight movement between
strikes and with a 40 percent indent on the second strike. Such errors
feature flattened, expanded first-strike elements in the floor of the
indent. A double-struck indented 1982 1-peso coin of Mexico
demonstrates this effect.
But closer study of the photos showed my first impression was wrong.
The indentation covers the upper half of the eagle, which dominates
the obverse design. But the floor of the depression shows the lower
half of the eagle in relief. As with most Mexican coins, the obverse
die functioned as the hammer die in both of these coins.
I then surmised that this was a partial counterbrockage on the
second strike — a very rare error.
The internal margin of the counterbrockage lines up with a
disruption in the die-struck reverse design, with duplicated design
elements appearing on either side of the discontinuity. This led me to
believe that I was also looking at an “invisible strike.” It looked as
if areas lying outside the partial counterbrockage on both faces had
escaped the second strike because the minimum die clearance had
increased beyond the thickness of the coin.
The problem with this diagnosis is that there weren’t any
first-strike design elements beneath the counterbrockage. Some of the
original design should have persisted, especially given the sharply
defined feathers of the stylized eagle and the fact that the coin was
partly confined by the collar. The collar limits expansion of the coin
metal and thereby facilitates preservation of first-strike details.
The confusion cleared up once the coin arrived. As anticipated, it
proved to be a centered double strike, with slight movement between
strikes. But the partial counterbrockage was produced during the first
strike, and there was no invisible strike.
After the first strike, the coin would have looked like the
illustrated 1980 20-peso piece. This coin features a 75 percent
partial counterbrockage of the reverse design on the reverse face. In
this example, the reverse face was struck by the hammer die.
Returning to the 50-centavo coin, it’s clear that in the aftermath
of the first strike, all areas lying beyond the counterbrockage were
weakly-struck on both faces. For example, on the obverse face we can
see the faint tops of the letters CE (from CENTAVOS) sitting just
above the sharply-struck versions generated by the second strike. The
only strong die-struck elements generated by the first strike were
those lying directly opposite the counterbrockage, where the double
thickness boosted the effective striking pressure. These elements
include the date, the M Mint mark, the letters AVOS, and the neck and
terminal plumes of the Mayan king.
The second strike was die-struck on both faces. However, the area of
the coin thinned by the partial counterbrockage happened to be thinner
than the minimal die clearance. The clearance was nevertheless within
normal limits, since the area outside the counterbrockage was
well-struck on both faces. It also appears that minimum die clearance
decreased slightly during the second strike, as the die-struck design
on the thicker portion of the coin is much stronger than in the first strike.
The counterbrockage was almost completely untouched by the obverse
die during the second strike. Only the edge of the die contacted the
rising lateral floor of the counterbrockage, leaving behind a design
rim. Opposite the counterbrockage, die-struck reverse design elements
from the first strike were untouched by the second strike. Outside the
borders of the counterbrockage, the weak first-strike design was
nearly obliterated by the second strike on both faces.
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