Alum can serve several functions in papermaking. The single most important use of alum is to precipitate rosin size on pulp fibers prior to their being formed into a sheet of a paper. Paper is sized primarily to make it resistant to penetration by water or aqueous liquids. For example, writing ink must not be permitted to blur or feather when applied to writing paper. Another major product, paper bags, must resist moisture absorption in order to retain strength.

Mills producing bleached pulp and paper grades require high water quality. Alum is frequently used as a coagulant to remove turbidity and color. For effluent treatment in pulp and paper mills, alum has been highly effective as a flocculant and coagulant in removing suspended solids and color.

Alum in the Paper Industry Technical Paper