kiln scum

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kiln white, kiln scum

A white scum that has formed on a brick surface during firing as the result of drier scum and kiln atmosphere. Also see scum.

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.