Paper Chemistry
 
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The Chemistry of Paper

(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)

Paper is composed of cellulose fibers. Cellulose is a polymer of the sugar glucose and is used by plants to produce cell walls. Plant matter that has been processed to create a solution consisting of cellulose filaments suspended in water can be made into paper. A screen is passed through the solution so that the filaments can collect on it and thus form a layer. This layer of cellulose fibers is then pressed and dried to produce a usable sheet of paper. The source of the cellulose fibers, and the degree to which that source is refined, determine the nature and quality of the paper produced. 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, or such chemicals as alum that have been added 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, are waterproof, 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 only 1 percent of cotton fibers. 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. The recently developed process for the removal of all lignins is being used at this time primarily to manufacture boards and storage containers used in archives, in conservation, and in museum-style framing.

Another major consideration in paper is its pH. The scientific symbol indicat­ing the concentration of hydrogen ions in a liter of solution, pH describes the acidity, alkalinity, or neutrality of something. Water, which is composed of two atoms of hydrogen that 'are attached to one atom of oxygen, is designated with the symbols H2O or HOH. A very small number of water molecules, HOH, occasionally break up and reform. During the breakup, a positively charged H ion and a negatively charged OH ion are formed. The Hs are acidic and the OHs are alkaline. Since they are in equal amounts in water, water is said to be neu­tral-neither acidic nor alkaline. Water has been assigned a pH value of 7, which represents equal concentrations of acid and alkali. If the concentration of H ions becomes greater than the number of OH ions, then the result is said to be acidic, and a lower number is assigned. Each number represents a factor of 10­ten times more or less acidic than the number above or below it. A pH of 6, for example, is ten times more acidic than water, and a pH of 5 is ten times more acidic than 6, or one hundred times more acidic than water. If the concentration of H ions becomes less than the concentration OH ions, the result is said to be alkaline and is assigned a higher number, such as 8, which is ten times less acidic than water (or ten times more alkaline than water). The scale ranges from 1 to 14.

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, rosin or 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. 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, many manufac­turers are resorting to buffering. Buffers are such chemicals as calcium or mag­nesium carbonate, which 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.

The two situations where pH information is vital is in the case of paper and boards used for archival storage and framing, and in watercolor papers. Materials used to help preserve artwork should have the highest standards. Although there is little reason for watercolor paper to meet all standards for archival use, it should be close to a neutral pH (neither acidic or alkaline) because many pig­ments used in watercolor are sensitive to significant change in pH. The whiteness of watercolor paper is another important factor because as paper acidifies it usually yellows.

Although virtually all paper products used for archival storage and framing are buffered to maintain a nonacidic pH, only a few artists' papers are buffered.

 

pH SCALE

pH1-2-3- 4 -5-6-7-8 9-10 -11 12-14

I

< <acidic < < neutral> >alkaline > >

 

With the exception of watercolor papers, it is generally considered unnecessary in papers made with 100 percent cotton because most 100 percent cotton papers will acidify only slightly and stop. It is becoming more commonplace to buffer wood-pulp art papers and boards to slow down, not stop, the discoloration and embrittlement process. The problem of maintaining a nonacidic pH with even 100 percent cotton watercolor papers is the need to add a size to reduce absorbency. Although Aquapel is a neutral size, it has not yet replaced the traditional alum/gelatin size. (For more information about sizing, see page 93.) Alum is a combination of sulfates, which tend to acidify the paper, leaving it sometimes with a pH as low as 4.5.

Exactly how much damage to a paper will be caused by a specific amount of acidity is debatable. It is clear that wood-pulp papers are more sensitive than rag or cotton papers because pulp fibers are shorter and are weakened during refinement. Yet, recently, documents produced during the Salem witch trials on paper made from linen and rags were found in the basement of an old building. They were found to have a pH near I from centuries of rat urine, yet they were still flexible and readable despite considerable yellowing. The lesson here is that pH can affect permanency and quality but is not necessarily an indication of either.

(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)

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