Sizes or “sizings” for paper and cloth are additives that fill pores, impart stiffness and help maintain flatness. Without adequate sizing, even light washes can cause paper to buckle. Sizes for watercolor paper also give colors “sparkle” by keeping paint on the surface, allowing light to reflect from the white paper beneath.
A variety of materials are used as paper sizes. These can be derived from plant, animal and synthetic sources, including the same methyl cellulose used in laundry starch.
Some painters avoid animal products, and prefer not to use papers sized with gelatin. Even synthetic sizings (like AKD, for example) may not be entirely animal-free, however, since fatty acids used in production may or may not be from animal sources.
Artists who prefer not to use animal-derived art materials can select Utrecht French Premium Watercolor Paper, sized with animal-free starch sizing. http://www.utrechtart.com/Utrecht-French-Premium-Watercolor-Paper–140-lb–Dual-Surface–22×30-10-sheets-MP19980-i1008371.utrecht
The best watercolor paper is made of rag fiber. This means cotton and/or linen (today it’s mostly cotton). The term “rag” is derived from the original source of Western paper fiber: old cloth rags.
Rag paper on its own lacks the stiff, crisp quality we associate with watercolor sheets. Sizing is added at the mill to improve and condition the finished sheets.
Paper can be internally and/or externally sized. Internal sizing is added in the pulp and permeates the entire finished sheet. Internal sizing helps keep paper flat and stiff; external sizing imparts a hard, crisp surface and gives watercolors luminosity.
Unsized or lightly sized papers like many printmaking sheets have a soft, velvety surface that promotes sensitive ink transfer and gives a beautiful “crush” with relief and intaglio processes. Because they are so lightly sized, printmaking papers are too delicate and absorbent for drawing and watercolor- paper fibers lift and break apart under light erasing, and wet color sinks in, resulting in a dull appearance.
Good paper brings watercolors and gouache to their best advantage, providing a dimensionally stable surface with a brilliant reflective quality. High quality watercolor sheets permit lifting out, masking and layering without disintegrating or peeling.