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Pumice.  A natural abrasive used in polishing. Pumice is an irregularly shaped, light-weight volcanic glass (expelled from volcanos as lava but which water escaped leaving many minute cavities). When ground into powder the gray or white hard particles have many angular corners or projections to form an excellent abrasive. The dry pumice powder is mixed with water to form a slurry with the consistency of very thick soup. The pumice slury is used extensively in medallic finishing; it is dolloped on to a medallic surface and relieved by buffing with a wet wheel. Pumice is also suitable for polishing silver (more so in a paste form).  See buffing and polishing, finish and finishing.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators


Roger W. Burdette, Editor

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