US Coins

Occluded gas bubbles can be mistaken for die defects

Readers of this column are well aware that a raised blemish on a coin can have any number of explanations.

The 1949 Lincoln cent shown here has two elevations in the field, one above and one below LIBERTY. If solid, they could represent some kind of die defect. Some support for this conjecture can be found in the failure of either mound to flex when pressed with the tip of a toothpick.

The two mounds could be unusually well-defined die dents. However, their smooth surfaces and soft borders leave room for doubt. Die dents that penetrate the die face as deeply as these lumps imply generally show an altered surface texture and sharper margins.

The smooth surfaces and soft borders of the two elevations are also quite incompatible with freestanding interior die breaks.

What they likely aren’t

The two mounds are unlikely to be die subsidence errors (sunken die errors) as these are seldom, if ever, restricted to the field and have never been found arranged in pairs.

They’re clearly not “blebs” (die erosion pits). The latter are associated with late die states, have very low relief, and have very irregular borders.

The undamaged reverse face (not shown) demonstrates that these lumps are not the result of impacts on the reverse face.

Likewise, they don’t appear to be the result of intense heat applied outside the Mint, as the toning and surface texture are completely natural.

Occluded gas bubbles

The only remaining possibility is that these are occluded gas bubbles. In this rare planchet defect, heat generated by the strike causes a pocket of gas to form and expand below the surface of the coin.

An “occluded” gas bubble is simply one whose roof has remained intact instead of rupturing from the internal pressure. If the roof is of sufficient thickness, it could conceivably resist pressure from a toothpick.

The occluded gas bubble hypothesis is supported by a microscopic surface texture that is identical to the surrounding field. Even microscopic die flow lines continue across the two bulges without interruption. This supports the idea that the die face was intact and undisturbed when the coin was struck.

Occluded gas bubbles are restricted to solid-alloy coins and should be distinguished from plating blisters that form in copper-plated zinc cents. Blistered plating occurs when there is a poor bond between the copper plating and the zinc core. Heat generated by the strike causes gas expansion beneath the plating, pushing it up. Several unusually large blisters are seen in the accompanying 1986-D Lincoln cent.

I recently acquired an example similar to the 1949 cent that would seem to break the logjam of competing hypotheses. The illustrated 1958-D Lincoln cent has a much larger elevation located in the field above the date. Its relief is similar to the two elevations seen in the 1949 cent. Like that cent, there is no change in microscopic surface texture and the margins of the mound are soft. As with the previous example, the bulge failed to flex when pressed with the tip of a toothpick. The color is natural and no damage appears on the reverse face.

Although I wasn’t able to produce flexion or an indentation in the surface of the mound, a stronger impact received during its years of circulation managed the task. A microscopic transverse crease/scratch crosses the apex of the mound. Above and below the crease, the surface of the mound is warped subtly downward, indicating that the mound is probably hollow.

The perfectly normal reverse face provides another clue that this elevation formed at or after the strike. Any recess in the die face this broad and deep should have caused at least a little bit of weakness on the opposite face. The totality of the evidence indicates that this elevation is probably an occluded gas bubble.

By extension, I now believe that the two elevations on the 1949 Lincoln cent are also occluded gas bubbles.

My previous experience with occluded gas bubbles involved much smaller blemishes, as on the reverse face of the illustrated 1941-D Winged Liberty Head dime. Here a circular bubble has pushed up the M of AMERICA and the adjacent field. Its status as a gas bubble is confirmed in the lower half of the reverse face, which shows many fine linear gas bubbles.

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