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Acid (adj. acidic)

In chemistry, a material that can form hydrogen (H+) ions when dissolved in water. Acids damage cellulose in paper, board and cloth by weakening or breaking molecular bonds which leads to embrittlement. Acidic materials can be introduced during the manufacture of library and archive materials, or may be present in the raw material. Acids can also be introduced by migration or from atmospheric pollution. Acids can be neutralised by an alkali to form a salt.

Acid deterioration

The weakening of paper or board structure by an acid, through hydrolysis or other means. This results in breaking down the chain length of the material with a subsequent loss of strength. This can become so severe that the paper may have almost no residual strength left. It is then said to be brittle.

Acid free

In chemistry, materials that have a pH of 7(neutral) or higher (alkaline). Acid free paper is often alkaline buffered. Since cellulose is damaged by acids, acid free materials are desirable in library and archive preservation.

Acid migration

The transfer of acid from an acidic material to a less acidic, neutral, or alkaline material. This may occur when two materials are in direct contact or indirectly by vapour transfer. It can cause staining, weakening and embrittlement. The actual mechanisms of acid migration are not well understood, and the term is sometimes erroneously applied to any transfer of a stain to an adjacent surface.

Acrylic

A plastic material noted for its transparency, weather resistance and colour fastness. Acrylics are important in preservation because of their stability and resistance to chemical change. Acrylics are available in sheets, films and resin adhesives. Some common trade names for sheet acrylics are Perspex, Lucite and Plexiglas. Ultraviolet absorbing acrylic sheet is used in preference to glass for glazing framed materials because it is less likely to break and the additional ultraviolet absorbers protect the framed objects from light damage.

Adhesive

A substance used to join two materials together, by chemical or mechanical action. Generally applied as a liquid, or as a solid activated by heat or pressure. A desirable characteristic of adhesives used in conservation is reversibility.

Adhesive tape

Paper or fabric tape with an adhesive layer applied. The adhesive layer is generally activated by pressure, or by the application of heat or water. Ordinary pressure sensitive or 'sticky' tapes should not be used for materials intended for long term preservation, since the adhesive can degrade and yellow and the adhesive residues can become impossible to remove. Some archival adhesive tapes are safer to use, but caution should still be exercised especially on very valuable materials.

Alkali (adj: alkaline)

In chemistry, a material that can form hydroxyl (OH-) ions when dissolved in water. Alkaline materials are sometimes added to conservation materials to neutralise acids or to provide an alkaline reserve or buffer for the purpose of counteracting acids which may form in the future. While a number of chemicals may be used as alkaline buffers, the most common ones used in paper conservation are magnesium carbonate or calcium carbonate. Alkalis can be neutralised by an acid to form a salt.

Alum / rosin size

Chemicals commonly used to size paper. Alum/rosin sizes were used extensively in the past, and have contributed significantly to the brittle book problem because they leave an acidic reserve in the paper.

Ambient conditions

The existing conditions of temperature and humidity in any building or room

Archival quality

A term suggesting that a material, product or process is durable and/or chemically stable; that it has a long life; and can therefore can be used for preservation purposes. The phrase is not quantifiable; no standards exist that describe how long an 'archival' material will last. The word 'permanent' is sometimes used to mean the same thing. Some organisations, for example ANSI, are now using the term life expectancy (LE) - LE = 100 years, LE = 500 years etc.