The application of various materials either to wet paper pulp (called internal sizing) or to the surface of partially dried finished paper (called surface sizing) to provide a desired degree of resistance to water or other fluids and to increase surface strength. Papers that are unsized (called waterleaf) readily absorb moisture, and are used for towel, tissue, and blotting papers. Internal sizing typically consists of an acid- or alkaline-based rosin and papermaker's alum (used to help the sizing adhere to the paper fibers) added to the papermaking furnish during the stock preparation phase prior to its reaching the forming wire of the papermaking machine. (Sizing that is added during the refining stage is sometimes called beater sizing, after the vessel—the beater—in which the process occurs.) Internal sizing does not make a paper waterproof, or increase its wet strength, but it does hinder the penetration of water through the paper and contributes greater dimensional stability to the paper. Slack-sized paper has a minimal amount of internal sizing, while hard-sized paper has a greater degree of internal sizing. Hard-sizing is used for papers that must withstand repeated exposure to offset press dampening systems without absorbing significant levels of moisture. Sizing is also added to prevent glues and other adhesives from penetrating the paper when applied to labels and packaging.
Surface sizing is added to partially dried paper on the size press, located near the end of the paper machine's drying section. Surface sizing typically consists of a starch solution added to the surface of the paper web by rollers. Surface sizing is typically applied as a surface fiber sealant, improving strength and stress resistances, as well as impeding the penetration of ink far into the surface of the paper. Variations in the surface-sizing process and the materials used confer different advantages on the paper, such as increased water resistance and the ability to repel grease, oil, or other substances.
The use and degree of both internal and surface sizing varies according to paper type and the desired characteristics. Papers used in offset lithography, as well as bond and writing papers, and cover, index, and bristol papers are both internally and externally sized. Papers used in high-speed web offset lithography are usually not surface-sized but may be internally sized, while papers used in letterpress printing don't need to be sized at all.
Measures of the degree of sizing are related to tests to determine water resistance. There are three basic groups of tests to measure the degree of sizing and water resistance: those that measure surface water absorption (see Feathering Test and Contact Angle Method), those that determine how long it takes liquids to penetrate the paper (see Dry Indicator Test, Ink Flotation Method, and Curl Test Method), and those that determine how much liquid a quantity of paper can absorb in a set period of time (see Cobb Size Test and Water Immersion Method).