Paper is made by beating wood or cotton fibers into a pulp, suspending them in water and drawing out sheets of wet fiber onto screens. The resulting sheets are pressed to remove moisture, either by hand or machine, and left to dry. Wood fibers produce the bulk of the paper we see in our lives, from newspapers to cardboard boxes. Drawing papers that are made from wood fibers are well-priced, but not suitable for permanent work because the acids they contain cause the paper to discolor and turn brittle. In more expensive papers, acids are removed or neutralized. Cotton fibers interlock and weave better than wood fibers, creating a structure of strength and flexibility in cotton fiber or "rag" papers. Papers can also be made from combinations of wood and cotton, or cotton and synthetic fibers, or even from polypropylene!
Many factors play into deciding what paper to use. One must consider variations of grade, weight, texture and finish. The grade, or quality of paper can range from inexpensive newsprint to 100% rag. The weight of paper is perhaps the most confusing issue. Paper is measured by its basis weight (144 "full size" sheets). The weight is then used to describe a single sheet, i.e. 140lb. The texture of paper is determined by touch. All papers have texture in variations of smooth to rough. Cold-pressed and rough papers have a coarse texture with pits and valleys making them good for watercolors. Hot-pressed paper is smooth and good for drawing or opaque painting techniques. The finish of a sheet is determined by how the sheet is flattened by a machine.
Paper has a natural acidic level that can destroy its longevity. Makers of quality paper use a scale of pH 1 (highly acidic) to pH 14 (highly alkaline) and strive to achieve a neutral level of pH 7. Only pH 7 is truly neutral, but 6 and 8 can be considered neutral. Neutral pH papers are typically labeled "pH-neutral", "acid-free" or "archival".