Acid-free refers to paper and paper products with a pH value of 7 or a bit higher. pH is a standard, scientific scale from 0 to 14 used to measure the level of acid in a substance. Values between 0 and 7 are considered acidic and values higher than 7 are basic.
Paper is made using a wood-based pulp that naturally contains lignin. This chemical causes paper to yellow and deteriorate over time. When paper that is exposed to heat or light, the lignin in the paper causes the molecules to break down at a faster rate. William Barrow, who published a report on the deterioration of acidic paper, first documented the significance of this process in the 1930s. As a librarian, he was deeply concerned that entire collections of valuable written materials would be lost.
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As a result of his efforts, and widespread recognition of the importance of printed materials, acid-free paper is now the industry standard. Acid-free pamphlet boxes and other specialty products are available to libraries and museums to help preserve documents that were created on paper with a high acid content. In addition to these steps, the paper surface can be treated to minimize the deterioration over time. Paper material that can accept this chemical treatment must meet a certain standard of quality. Very old paper is usually best preserved by placing it is a case away from light, heat and people.
In the paper making process, acid-free paper is treated with calcium or magnesium bicarbonate. These chemicals neutralize the natural acids in the wood pulp. The chemical mixture is absorbed by the pulp and helps to stop new acid from forming. Once treated, this paper has an alkaline reserve of two percent or greater. The paper materials made with this process will be preserved for at least 100 years.
The shift to acid-free paper has an additional environmental benefit. This process uses chalk instead of clay as filler in the pulp. The changes in materials has resulted in fewer corrosive chemicals, both in the factory and in the wastewater. Creating paper requires a significant amount of water, and the acid-free process allows the same water to be treated and reused.
Specially designed archival paper that is 100% acid-free is often used for legally significant or historical documents. This type of paper has a longer life span than the 100 years provided by acid free paper. Typically made of cotton or other fabric material, this paper is more expensive, thicker and is sold through archival supply stores.