Occluded Oxides. These are small particles or clumps of oxide (often copper oxide or lead oxide) that occasionally are embedded in coinage ingots. If rolling brings the particles to the surface, the rolling mill operation can cut out part of the strip. Occasionally, particles will appear only after the coin or medal is struck.
excerpted with permission from
For Artists, Makers, Collectors and Curators
COMPILED AND WRITTEN BY D. WAYNE JOHNSON