Alum

Alum
A common chemical used in acid hardening fixing baths.
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-Alum - chemical used in acid hardening fixing baths.
-Aluminum compounds - groups of chemicals often used as hardeners in fixing baths.

-Chrome alum - alternative term for potassium chromium sulfate.
-Chromogenic development - process in which the oxidation products of development combine with color couplers to form dyes during processing.

The most common hardening agent is potassium alum. The hardening and toughening of the gelatin by the alum stops the tendency of the emulsion to swell but leaves it expanded and rigid enough for the washing process.

Hardeners
Are chemicals often used with a fixing bath to strengthen the physical characteristics of an emulsion. The most common hardeners are potassium or chrome alum.

See also: Fixing, Photograph, Fixing bath, Silver, Back