Occluded Gas. Trapped gas bubbles in ingots or castings creating a pocket where no metal exists. If retained in the ingot, occluded gas may remain in the metal through other operations, as rolling, blanking or striking; if so they could cause laminating in blanks or struck pieces. If any metalworking activity reaches the pocket of occluded gas it may exhibit a blister in the surrounding metal. Blanks made from rolled strip that contains pockets of occluded gas would be light weight and not pass a ring test. Obviously struck pieces from these blanks would also be light weight and not ring true. Finished pieces with such condition would be rejected if caught by inspection.Occluded gas bubbles – which may be air or any other gas – can be removed by vacuum degassing if the metal can be melted or remelted. Should such blisters occur on the hardened metal of any casting or cast piece they would require chasing to smooth out the surface. Such trapped gas is a casting anomaly; it would exhibit porous surface, blowholes or pock marks. It should be avoided in any casting operation. See lamination error.
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