How do sizing materials help prevent ink from spreading through paper?

In papermaking, sizing is the process of treating paper to prevent aqueous solutions, such as ink, from soaking into the fibers.1

Because cellulose fibers are naturally hydrophilic (water-attracting), untreated paper is highly absorbent, which would cause ink to spread or "feather" upon application. To prevent this, sizing materials—such as rosin soap dispersions—are added to the paper stock. Since rosin soap does not naturally bond to cellulose, a coupling agent like alum (aluminum sulfate) is used. The acidity of the alum precipitates the rosin, allowing the sizing to attach firmly to the fiber surfaces. This treatment creates a surface that lacks an affinity for water, effectively resisting the penetration of aqueous inks and ensuring they remain on the surface of the paper rather than spreading through its porous structure.21