The following is the Collectors' Clearinghouse column from the Aug.
29, 2016, issue of Coin World:
Error collectors new to the hobby quickly encounter coins with
unexpected bumps in the field and design.
These anomalous elevations are a constant source of confusion due to
their many causes and similar appearance. Surface elevations can
reflect defects on the die face or defects that arise from within the planchet.
Let’s run down the most common suspects and their key diagnostics.
1. Die chips, interior
Small pieces of the die face can break off, leaving a void into
which coin metal rises. Smaller die breaks are called die chips, while
those larger than 4 square millimeters are called interior die breaks.
Both are usually located in areas vulnerable to brittle failure, such
as the edges of the design and narrow interstices within and between
design elements. The edges of these die breaks tend to be sharply
defined and at least somewhat irregular.
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2. Blebs (die erosion pits)
The surface of a worn die will sometimes become pitted, possibly as
a result of decarburization of the die steel. These pits are expressed
on the coin as low, flat elevations with relatively soft, irregular
margins. Blebs are usually surrounded by obvious signs of die wear,
such as radial flow lines or an orange peel texture. Blebs are usually
found in the field.
3. Die subsidence
(sunken die) errors
The surface of a die will sometimes sink in, leaving a recess into
which coin metal rises. This form of die deformation is presumably the
result of abnormally soft die steel. The zone of subsidence will
sometimes show cracking along its margin. In the absence of such
cracks, the edge will be softly defined. The design may be indistinct
where it crosses the zone of subsidence. It rather depends on the
recess’ size, depth, and degree of deformation. Die subsidence errors
are often associated with wide die cracks and split dies.
4. Die dents
A die face can be dented by foreign objects at any point before or
after installation. Die dents vary enormously in size, shape, depth,
and texture. Edges tend to be clearly defined and the surface usually
displays a rough or peculiar texture. The edges of a die dent may show
cracking or the development of a pressure ridge. They can occur
anywhere in the field and design.
5. Plating blisters
Plating blisters develop in the coin in the immediate aftermath of
the strike, as gas expands between the core and poorly bonded plating.
Among domestic coins, plating blisters are the exclusive province of
copper-plated zinc cents. Blisters are generally small and
subcircular, with a smooth surface and soft outline. They can occur
anywhere on the field and design. The design continues uninterrupted
as it crosses a blister.
6. Occluded gas bubbles
Solely the province of solid-alloy coins, occluded gas bubbles form
just beneath the surface and push up the overlying metal immediately
after the strike. Like plating blisters, the surface is smooth and the
edges soft. The design is uninterrupted.
7. Corrosion domes
Contaminants trapped beneath or penetrating the surface of
aluminum, plated zinc, and plated steel coins can react with
surrounding metal to form an expanding front of spongy, corroded
metal. The resulting solid dome will superficially resemble a hollow
plating blister or occluded gas bubble. In many cases the corroded
metal bursts through and may fall out, leaving a crater.