PAPER PRIMER: HOW pH, COTTON, LIGNIN, OBA'S & BUFFERING EFFECT PERMANENCY
Fine art papers that have no OBA's, that are acid-free, lignin-free, 100% cotton, and that are buffered (usually with a calcium carbonate reserve of about 1.5% - 2.0%) have an estimated paper life expectancy of 500 years or more (based on the American National Standards Institute and the American Society for Testing Materials).
The two most important factors that affect the quality of paper are the presence of impurities and an acidic pH. Finished papers may contain natural impurities, such as lignins that have not been removed during processing, unnatural impurities, such as residual chemicals, like sulfites, not washed out during final processing.
Lignins, which are the combined glues that hold plant cells together, are undesirable in a finished paper product. They age poorly, turn brown, become acidic over time, and resist the natural bonding of cellulose fibers to each other. If lignins are not removed and are left in contact with the surrounding cellulose fibers in paper, their acidity will break down the cellulose and the paper will become brittle.
Lignins comprise 20 to 30 percent of wood, but less than 1 percent of cotton fibers (despite this <1%, cotton is considered "lignin-free"). Because of the high concentration of lignins in wood, papers made from wood pulp discolor and eventually self-destruct. Although there are methods for the removal of most or all of the lignins, unless the residual chemicals used in these processes are also dealt with, embrittlement and acidification will only be postponed. For this reason, wood pulp papers are generally avoided for permanent artwork. Because it is nearly lignin-free, paper made from 100 percent cotton is most desirable.
However, "alpha-cellulose" papers (which are mostly lignin-free wood pulp papers) offer an inexpensive compromise to cotton. Our Duo-Brite Matte papers are an example of an acid-free, and mostly lignin-free, alpha-cellulose wood pulp papers that offer a greater degree of permanence than regular wood pulp papers, but without the higher expense and greater permanency of cotton. Moab Lasal is another alpha-cellulose paper, but both papers are NOT OBA-free (see below). Hahnemuhle sells three fine art papers that have a mixture of cotton and mostly lignin-free wood pulp (however, this substitution does not seem to have reduced their expense that much ;o).
WHY 100% COTTON?
Paper is composed of plant cellulose fibers. Cellulose is a polymer of the sugar glucose and is used by plants to produce cell walls. The source of the cellulose fibers, and the degree to which that source is refined, determine the nature and quality of the paper produced. Cotton fiber is up to 10 times stronger than cellulose fibers made from wood, and cotton is naturally acid and lignin free.
pH describes the acidity, alkalinity, or neutrality of something. Distilled water has been assigned a pH value of "neutral" 7, which represents equal concentrations of acid and alkali. Each whole number represents a factor of 10-ten times more or less acidic than the number above or below it. The more acidic a paper, the faster the cellulose will break down, resulting in a shorter lifespan. A number of factors can influence the pH of a paper. Residual acids from processing, alum sizing, fillers used to create bulk, oils used to make paper transparent, optical brighteners, atmospheric sulfur dioxide, and the presence of lignins can all result in a pH of 4.5 or lower.
WHY BUFFER THE PAPER?
Recent study has shown that even the purest cotton papers will become slightly acidic, even though they left the mill at pH ranging between 6.5 and 7. This may be due to the nature of the paper itself, or because of exposure to air polluted with sulfur dioxide and oxides of nitrogen --common pollutants caused by the burning of fossil fuel, which turn water molecules into sulfuric acid and nitric acid. To cope with the natural and unnatural acidification of paper, some manufacturers add buffers to the paper. Buffers such as calcium carbonate can absorb a significant amount of acid. Buffered papers are often slightly alkaline with a pH around 8.5. A pH moderately higher than 7 is not considered harmful in paper.
"OBA" is an acronym for "optical brightening agent". Many paper substrates have optical brighteners added to increase their apparent whiteness. The cellulose fibers comprising paper have a natural yellow color that is bleached during manufacturing, but some slight yellow remains. To counteract this yellow color, a "bluing" agent is added to paper. The bluing agents are actually ultraviolet dyes that work by fluorescing the invisible ultraviolet light into visible light (OB's absorb ultraviolet light and re-emit it as visible blue and violet wavelengths), thereby making the paper to appear brighter or whiter. OB's are known as "fluorescent agents" because they strongly fluoresce under "black light" (a good test to see if a paper has OBA's).
However, many paper makers believe that optical brighteners interfere with permanence, because they can break down over time and can cause irregular yellowing of the paper (or the inkjet coating), or cause acidity in the paper, which can lead to a premature deterioration of the paper structure.
In fact, the Library of Congress defines an archival paper to be OBA-free.