Shorthand term for aluminum sulfate—chemical formula Al2(SO4)3—a material that is commonly added to the papermaking furnish during refining in a process called internal sizing. Rosin is added to paper pulp to increase resistance to water and other liquids, and alum is added to help the rosin adhere to the paper fibers. The use of rosin and alum for internal sizing imparts a degree of acidity to paper, causing the rapid yellowing and crumbling with age typical of acid paper. Synthetic sizing agents, such as alkyl ketene dimer or alkyl succinic anhydride, are used instead of rosin and alum in the production of alkaline paper. See Sizing and Internal Sizing.

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