acid free

   A characteristic of inert materials; especially said of papers with a 7 pH, or very close to 7 pH. Below 6.5 pH or above 8.5 pH is not considered acid-free. Acid free materials are more permanent, less likely to experience acid migration — to discolor, or to deteriorate materials they are placed with over time. Works on paper, and the mats, mounts, etc. with which they are framed, are best acid free. This term is sometimes used incorrectly as a synonym for "alkaline" or "buffered." Such materials may be produced from virtually any cellulose fiber source (cotton and wood, among others), if measures are taken during manufacture to eliminate active acid from the pulp. However free of acid a paper or board may be immediately after its manufacture, over time the presence of residual chlorine from bleaching, aluminum sulfate from sizing, or pollutants in the atmosphere may lead to the formation of acid unless the paper or board has been buffered with an alkaline substance. The presence of alpha cellulose in paper or board is an indication of its stability or longevity. Non-cellulosic components of wood are believed to contribute to the degradation of paper and board. Also see adhesives, archival image, art conservation, buffer, glassine, polypropylene, and storage.