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Paper
Acidity and Alkalinity
Acids in paper contribute significantly to its deterioration. They attack and break apart the bonds which hold the cellulose chains, the microfibrils, and the fibres together.
Where do the acids come from?
Acids are by-products of the breakdown of impurities such as lignin, and of the cellulose fibres themselves.
Alum-rosin sizing is a major cause of acid deterioration in paper. Rosin, or pine resin, is applied to paper to make it less absorbent, that is, to size it. Because rosin does not attach readily to paper, papermakers' alum, also known as aluminium sulphate, is used to form a link between the paper and the rosin. Rosin breaks down to produce acids, and papermakers' alum produces sulphuric acid.
Atmospheric gases and pollutants, such as sulphur dioxide which forms sulphuric acid, can attack paper.
Acids can migrate from adjacent acidic materials.
Acidity, alkalinity and pH
The strength of acidity or alkalinity of any substance is given as a measure of pH. The pH value is a measure of the availability of free hydrogen ions.
The pH scale runs from 0 to 14. pH 0 is very acidic and pH 14 is very alkaline or basic. pH 7 is a neutral value: neither acid nor alkaline.
Papers which are buffered with an alkaline substance such as calcium carbonate usually have a pH measurement of about 8.
Paper and paper-based products suitable for use in storage and display of flat paper should be acid-free; but in order to be meaningful, the pH value of a paper must be used in conjunction with other selected specifications for archival papers.
A pH testing pen can be useful for spot-testing the acidity or alkalinity of storage materials but this is not a foolproof method.
| CAUTION: |
| Do not use a pH testing pen to test items in your collection because it will leave a strong stain in the paper. |
If you have a problem relating to the care of flat paper, contact a conservator. Conservators can offer advice and practical solutions.
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