A material, also called rosin sizing, 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. Alum (short for aluminum sulfate) 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, and they are responsible for 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.
Rosin is a hard, resinous material which remains when the volatile contents of turpentine are boiled away. The turpentine from which rosin is obtained is distilled from oleoresin, or crude turpentine, which is in turn obtained from the sap of coniferous trees, in particular the longleaf pine ('Pinus palustris') native to the southeastern United States. Rosin is also called colophony. A widely-used rosin, called tall oil rosin, is obtained from tall oil, a resinous byproduct of the sulfate process of paper pulp production.