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In the past, aluminum sulfate was used in making paper, although synthetic agents have mostly replaced it. The aluminum sulfate helped to size of the paper. In this process, aluminum sulfate was combined with rosin soap to change the absorbency of the paper. This changes the ink-absorbing properties of the paper. Using aluminum sulfate means that the paper was made under acidic conditions. Use of synthetic sizing factors means that acid-free paper can be produced. Acid-free paper does not break down as fast as paper sized with acid. Though you may not have known it, aluminum sulfate is used to make many products you use daily. Even the water you drink is filtered using aluminum sulfate!
