Low-pressure indents, partial brockages, and “invisible strikes” are
relatively rare errors. These errors reflect press malfunctions in
which striking pressure is abnormally low.
Low striking pressure can be caused by a precipitous drop in ram
pressure (the tonnage applied to a planchet of normal thickness).
However, most low-pressure (weak) strikes seem to be caused by a
dramatic increase in minimum die clearance, a term that refers to the
closest approach the dies make to each other in the absence of a planchet.
In a low-pressure indent or partial brockage, there is little or no
expansion of the indented metal despite the aggregate double-thickness
within the striking chamber. Ordinarily, an intrusive coin or planchet
will greatly elevate effective striking pressure, causing significant
lateral expansion of the indented metal (if the disc is struck
out-of-collar) or dramatic finning and horizontal lipping beneath the
indentation (if the disc is struck in-collar).
Invisible strikes are often associated with low-pressure indents and
partial brockages. An invisible strike occurs when minimum die
clearance exceeds the thickness of a planchet. In such circumstances
an unaccompanied planchet will remain unstruck. However, if a second
planchet or a coin intrudes into the striking chamber, it takes up
enough excess space to allow a die-struck design to form on the face
opposite the indent or partial brockage. The rest of the planchet
In the Denver Mint, low-pressure multi-errors of the sort just
described became somewhat more common during the Nixon era (1968 to
1974). A respectable number of 5-cent coins and dimes feature such
errors, with 5-cent coins dominating. Denver Mint quarter dollars with
such errors seem primarily restricted to the year 1973. Half dollars
carrying the D Mint mark came late to the game (possibly no earlier
than 1974) and also stayed later (at least until 1977).
A rather fetching example of such an error is presented here. It’s a
1972-D Jefferson 5-cent coin with an in-collar double strike and no
rotation between strikes. The first strike was normal while the second
strike features a 20 percent low pressure indent at the coin’s
northern pole and a 40 percent low-pressure partial brockage on left
side of the coin. Remarkably, this coin belongs to a Denver Mint
portion of an Uncirculated Mint set and still resides in its plastic
pocket. The planchet responsible for the indent would have ended up as
an off-center strike with an unexpanded (“mirror”) brockage of the
obverse design on its reverse face. The coin responsible for the
partial brockage would have received an off-center second strike with
an unexpanded brockage of the obverse design overlapping the original
raised reverse design.
It’s not clear why these types of errors proliferated during this
narrow time period. I can only assume that a modification was made to
some of the Denver Mint presses that increased the likelihood of such
errors. Since these errors are almost the exclusive province of 5-cent
coins and dimes during the first six years of the phenomenon, it seems
that these modified presses were used primarily for these two
denominations. Use of these modified presses then seems to have
expanded to quarter dollars in one year only (1973), while their final
application was consigned to the half dollar denomination.
Another intriguing pattern is that, in 5-cent coins and dimes,
low-pressure indents and brockages most often occurred when two
previously struck coins overlapped. This may be due, in part, to the
use of dual and quad presses for these denominations. In quarter
dollars and half dollars, these errors most often involve an
overlapping coin and planchet.
Perhaps the oddest pattern is that the denomination produced in
greatest abundance — the cent — seemed for the longest time to have
been immune to this sort of error during the period of time under
discussion. This drought was recently broken by my acquisition of a
double-struck 1968-D Lincoln cent. The first strike was normal. The
second strike occurred out-of-collar, and with a planchet inserted
between the coin and the reverse (anvil) die. That second strike
generated a low-pressure indent on the reverse face, an opposing oval
of die-struck design on the obverse face (containing Lincoln’s
shoulder), and an invisible strike elsewhere.
Both strikes were delivered by the same die pair, as the close
positioning of the two strikes would imply.