Materials added to the papermaking furnish at the beginning of the paper machine's forming section to improve the paper's resistance to water or other liquids. Substances used for internal sizing include rosin and printer's alum, which is short for aluminum sulfate (which helps the rosin adhere to the paper fibers). Rosin used for sizing is slightly acidic, so alkaline paper is sized using synthetic alkaline sizing agents. Although internal sizing is added to prevent water penetration, it does not make the paper waterproof. Internal sizing is also called beater sizing, as it is often added in the beater, a device used for paper pulp refining. Papers that have a minimal amount of internal sizing are called slack-sized, while papers that have a large amount of internal sizing are called hard-sized. Paper destined for sheetfed offset printing need to be hard-sized in order to withstand repeated exposure to the press's water-based dampening system. Unsized papers used for paper towels, blotter papers, or other uses that require high levels of water absorbency are called waterleaf.
There are a number of tests to determine the degree of a paper's internal sizing. (See Feathering Test, Contact Angle Method, Dry Indicator Method, Ink Flotation Method, Curl Test Method, Cobb Size Test, and Water Immersion Method.) Internal sizing differs from external sizing, also called surface sizing. See Sizing.