Acid is very common in cellulosic materials and is a major factor in the aging of paper artifacts. It is especially insidious because the only obvious symptom is often a yellowish, reddish or brownish cast, sometimes accompanied by embrittlement. The detail of a 15th c. almanac shown here had a brownish tinge before conservation treatment and a creamier appearance after the acidic content had been reduced.
Paper usually becomes more acidic as it ages. Papers made from plain wood pulp are especially at risk because the fiber degrades and becomes acidic relatively quickly. Papers made from other types of plant fiber (such as cotton or linen) often age much more slowly. Other factors that may contribute to acidity in a paper artifact include: manufacturer's additives, the media used (ink, paint, etc.), air pollution, fluctuations in relative humidity and temperature, exposure to light (especially ultraviolet), and low quality housing and framing materials.
A number of conservation techniques are used to reduce the acid content of degraded papers. These include immersion bathing, blotter washing and suction table purging. Buffering solutions made with alkaline salts may also be applied to some papers in order to neutralize existing acids and counteract their formation in the future. This process is called "alkalization." In general, paper artifacts that are stored in a slightly alkaline condition (pH 7.0 to pH 8.5) are more stable than those stored in acidic conditions, although there are exceptions.
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