The dominant material in paper is cellulose fibers. For most, work-a-day paper, paper those fibers are derived from wood. But wood also contains other materials, notably lignin, which do not contribute to the longevity of paper. So most high quality art papers are made from cotton fibers, which are almost pure cellulose. In recent years, new processes have allowed better removal of the acids and lignins from wood fibers, resulting in a material called “High Alpha Cellulose” which has archival characteristics similar to cotton fibers, but is typically much less expensive. There are two types of cotton fibers used in papermaking. Linters are short fibers that have a relatively large diameter. These are the fibers found closest to the seed in the cotton boll. They find their way into the papermaking process directly, as virgin fibers as they are not useful for making thread. The second type of fiber is Rag. These are longer, thinner fibers that come from off cuts in the garment industry. Hence the term. These scraps are processed and beaten to separate the fibers into pulp.
Another common fiber found in papers used in printmaking is Kozo or Mulberry. This is a very long fiber and is usually found as a dominant component of Japanese papers.
In addition to the fibers, there are a couple of other components that are important. The first of these is a buffer. Acids are the enemy of longevity of artwork. A cotton or alpha cellulose paper might start out neutral, but there may be acids present in the inks or other materials and there are environmental factors as well that will tend to pull the ph down over time. To combat this, the paper manufactures include buffer material (such as calcium carbonate) to absorb and neutralize these acids, delaying this change and lengthening the life of the paper.
The second common additive is sizing. Sizing serves two purposes: it helps control absorbency and provides protection from oily media. Sizing may also aid in dimensional stability. Sizing can be internal (added to the pulp before the sheet is formed) or external (sprayed or otherwise applied to the formed sheet). Sizing can be any one of a number of materials, from starches, to natural or synthetic glues. Heavily sized papers can be a problem for intaglio printing as the high pressure may tend to squeeze the sizing out of the paper making the press blankets stiff over time and interfering with the trapping of the ink. In fact, in a three blanket system, the thinnest blanket which is against the paper is referred to as the sizing catcher.