ROSIN is a basic material used in sizing paper today. As a sizing agent it imparts water (ink) resistance to paper; however, in order for the rosin to be able to impart water resistance it must be rendered insoluble, which is the function if alum.
Although the full role of alum in the sizing of paper is not completely understood, one of its functions is to make rosin come out of solution (precipitate) while it is in close contact with the fibers of the paper-making slurry. The fibers are thus coated and impregnated with a solid and water-resistant mixture of rosin and what is probably a compound of rosin and aluminum oxide.
Although the excessive use of alum is considered detrimental to the permanence of paper, the papermaker tends to overdose with alum rather than underdose, so as to avoid soft-sizing. In addition, alum is considered by some papermakers to be a panacea for other troubles, such as frothing, sticking of the paper web to the wet presses of the papermaking machine, etc. Overdosing with alum leads to excessive acidity and, under certain circumstances, may lead to severe deterioration of the paper. While alum is not a particularly strong acid, in the presence of certain other substances it can assume a greater strength. Chlorides, which may be present in the paper as a result of bleaching processes, or natural to the water itself, can be particularly harmful. Excessive alum, in the form of aluminum sulfate, may react with chlorides present to form aluminum chloride (AlCl 3 ), which in the presence of moisture and heat, will form hydrochloric acid (HCl)—one of the most powerful of all acids in its effect on cellulose.
Alum is sometimes used in solution to wash the leaves of books; in the past it was added to paste to act as a preservative, or as a hardener to render the dry paste less water soluble. It is also used at times as a mordant for marbling colors. See also: ALUM WATER . (32 , 43 , 72 , 195 , 236 )