A watercolor paper is made from pulp, cotton rag, or a combination of both. Because today's production technology is so high-quality, it is no longer decisive how much rag a paper must contain in order to be classified a high-quality paper. However, many papers contain a 10% to 30% proportion of rag.
The fibre of watercolor paper should interweave in every direction, so that it does not curl when moist. After flooding the paper 2-3 times, it should not extend in any direction anymore. It is also important that the watercolor paper is acid-free; this property ensures a high resistance to aging and prevents changes in color.
For the artist, the strength of the paper is also highly important. The more water an artist uses for watercolor, the thicker the paper should be. In the so-called dry technique, one should not use papers under 180 to 200 g / m². For wet-on-wet, the paper should be more than 250 / m² to avoid buckling.