|
A measurement used to describe the level of acidity or alkalinity of paper (and many other substances). The scale used for indicating pH, or potential for Hydrogen, has readings of 0 through 14. The neutral point is 7. (Distilled water has a pH of 7.) Readings below a pH of 7.0 indicate that a substance is acidic. The lower the number is, the higher the acidity level is. Readings above a pH of 7.0 indicate that a substance is alkaline. The higher the number is, the higher the alkalinity level is. Each single digit of the pH scale relates to a measure of 10, so a paper stock with a pH of 4.0 is 10 times more acidic than paper with a pH of 5.0. A paper stock with a pH of 6.0 is 10 times less acidic than paper with a pH of 5.0. Most paper manufactured during the first half of the 20th century was made with an acid base. Paper with an alkaline base became more available in the last half of the 20th Century and is now quite common. Acidic paper deteriorates in a relatively short period of time, which is why old newspapers appear yellow and brittle. Newsprint has a pH of about 4.5, which becomes even lower once it has been used for printing. Paper with an acid base should never be used for documents that must remain intact for many years. Since the 1970's, most of the paper used for book publishing and other printed materials in which permanence is of importance, has been produced with an alkaline base. Alkaline paper is manufactured with fillers such as calcium carbonate, which bring the pH above 7. Alkaline paper is said to be permanent, but actually, papers with a neutral pH are best for preserving items such as photographic materials and matte boards for artwork.
|
