Paper

Paper is made of pulp that is mostly cellulose. The cellulose is usually derived from various vegetable fibers, chiefly cotton and linen, or from wood pulp. If manufacturers use wood pulp, they must separate the cellulose from other undesirable lignin components in the wood by cooking and chemical processing. If this is not done, the lignin causes the paper to darken. Cotton or linen pulp requires less refining and usually yields a stronger paper.

Manufacturers treat most papers, with the exception of filter papers and blotting papers, with a sizing material to make the paper less absorbent. Early paper makers dipped the hand-made sheets in gelatin sizing, adding small amounts of alum (potassium aluminum sulfate) as a hardener. However, in the nineteenth century, manufacturers using newly developed paper-making machinery employed a different sizing process, involving rosin and larger quantities of alum. The additional alum caused the formation of destructive acid in these machine-made papers. Since the mid-twentieth century, some sizing methods and materials have been developed that do not contribute to the paper's acidity.

The acidity of the paper is an important indicator of its potential longevity. Paper with a high acid content will age badly, darkening and becoming brittle with time. To be acceptably neutral or acid free, artists' paper should have a pH reading between 6 and 8. This requires that the cellulose pulp be neutral and that the sizing be free of ingredients that cause acidity.

Although papers made of high percentages of linen or cotton fiber are preferred for painting and drawing, some special papers with low acid content have been made from other sources, including chemically processed wood pulp as well as synthetics. Common wood-pulp papers darken and become brittle with age. Newsprint paper, a very cheap wood-pulp paper, turns yellowish brown and breaks up very quickly; in spite of this it is used very often by students as drawing paper. The student-grade white drawing papers are only slightly more expensive than newsprint, but they are an improvement on it in respect to permanence.

Paper should be stored away from heat and humidity, and it should be protected from dirt and air-born acid pollution by being kept in a cabinet.

Paper may be torn or damaged more easily than wood, metal, or fabric, but when there is concern for the fragility of a drawing or watercolor on paper, it can be glazed, matted, and backed with a rigid material, such as a heavy acid-free backing board, and thus protected against puncture or accident . . . . Properly prepared and protected, it will last as long as other permanent supports.