Blister
Blister. A casting surface imperfection caused by dirt in the mold. It is removed on the cast piece by chasing. Breen reports blisters on the U.S. cents struck on copper-coated zinc composition introduced in 1959; it was cured somewhat by annealing.
References: NC9 {1988} Breen, p 234.excerpted with permission from
An Encyclopedia of Coin and Medal Technology
For Artists, Makers, Collectors and Curators
COMPILED AND WRITTEN BY D. WAYNE JOHNSON
Roger W. Burdette, Editor