To make it easier, we have separated the items into sub-genres.
PAPIR IS NOT JUST PAPER!
Production of Paper & What do they mean with acid or acid-free?
Paper in our time is either made from wood-based pulp or cotton. Acid-free generally means paper that if infused in water yields a neutral or basic pH (7 or slightly greater). It can be made from any cellulose fiber as long as the active acid pulp is eliminated during processing. It is also lignin- and sulfur-free. Cotton is acid-free and it's paper products are the best to use when it comes to most artistic projects, as well as framing and restoration.
Regular paper that has not had its lignin removed turns yellow, becomes brittle, and deteriorates over time. When exposed to light and/or heat, the molecules in the acidic paper will break down even faster as it oxidizes the paper. Acidic wood-pulp paper became commonplace in the late 19th century, and in the 1930's William Barrow (a chemist and librarian) published a report about the deterioration of acidic paper in the libraries. For fear of the gradual disintegration of written materials, measures have since been taken to improve the quality of paper.
Watercolour Paper is made using a mixture of water and cellulose fibers. The fibers should be 100% cotton which makes it strong but pliable. Paper products made from cotton is often referred to as "rag paper" or ''archival'. Watercolour paper is usually made by one of three processes: handmade, mold-made or machine-made. Handmade is the best and mold-made comes in a close second. Both are very durable, stable and shouldn't distort under heavy wash. They both feature irregular surface textures which are pleasing to paint on. If you buy artists' quality paper it should be either handmade or mold-made. Less expensive watercolour paper can be made using wood pulp, or sometimes a combination of cotton and other cellulose fibers, where the acids have been removed and it's been