The papermaking industry relies heavily on a range of chemical additives to achieve desired paper properties, and aluminum sulfate stands out as a particularly significant one. Historically known as papermaker's alum, its primary role is as a sizing agent. Internal sizing, which involves adding chemicals to the pulp before sheet formation, is essential for imparting resistance to water, ink, and other liquids. Aluminum sulfate, when combined with sizing agents like rosin soap, helps these agents adhere to the cellulose fibers. This interaction creates a more water-resistant surface, crucial for printing and writing papers.

Beyond its direct sizing capabilities, aluminum sulfate also plays a role in pH adjustment. Many modern sizing agents require a mildly acidic environment to function optimally, and aluminum sulfate helps create these necessary conditions. This dual action contributes to improved ink holdout, better sheet formation, and enhanced paper strength and durability. While newer synthetic sizing agents exist, aluminum sulfate continues to be a widely used and cost-effective additive across various paper types, including cultural, packaging, and industrial papers. Its consistent performance and beneficial chemical properties ensure its continued importance in producing high-quality paper products. The careful application of aluminum sulfate allows manufacturers to achieve superior results, contributing to the overall efficiency and quality of the papermaking process.