This information is reprinted from the For Beginners column of Hand Papermaking Newsletter #37 (January, 1997).
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Anyone who has unfolded an old newspaper clipping from memorabilia knows the importance of making paper to an archival standard. Newspaper, for instance, will become a brittle and discolored artifact in a very short time as a result of acids which remain in the paper when it is made. Acid-free paper, by contrast, is a permanent and relatively stable legacy for future generations to enjoy.
Papermakers rely on a measurement known as pH to determine if pulp is too acidic or too alkaline. The pH scale is a span of numbers from 0-14. The lowest numbers are most acidic, the highest numbers are most alkaline, and 7 is considered neutral. The pH level is easily tested with a color-changing test strip available from papermaking suppliers (also available where aquarium or pool supplies are sold).
Since we all live in an atmosphere that is slightly acidic, it is best to counter with pulp that is slightly alkaline--say 8 or 9--rather than neutral. Calcium carbonate, an alkaline buffer, can be added to adjust pH level; it also acts as a filler to make the paper more opaque. Plant fibers are often cooked in soda ash which is strongly alkaline and the resulting pulp must be rinsed well to bring the pH down closer to neutral.
There are other extraneous materials (sugars, starches, gums, etc.) that will cause discoloring and deterioration if not removed during cooking. Lignin, part of the plant cell wall, not only impedes bonding but can also be a degrading influence.
Once paper has been made to archival standards, it still requires proper storage for permanence. The pH of wrapping and packaging materials should be considered, and acid-free storage boxes are a good investment.
Copyright 1997 Hand Papermaking, Inc.