7 types of surface bumps that confuse new collectors
Published: Aug 18, 2016, 6 AM
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 die breaks
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.
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