drier scum

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scum, scumming

1. A surface deposit sometimes formed on clay bricks; caused either by soluble salts in the clay which migrate to the surface as moisture escapes during drying (drier scum) or by the formation of deposits during kiln firing (kiln scum).
2. A mass of organic matter which floats on the surface of sewage.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.