3.1 Water/humidity
Water is the greatest natural enemy for all audiovisual carriers. It has direct chemical and indirect influences on the stability of carriers. Direct chemical influences are hydrolysis and oxidation of several carrier components as well as dissolution of some carrier materials.
3.1.1 Hydrolysis is a chemical reaction involving water, omnipresent in form of humidity in the air, as the central agent. Some polymers are prone to hydrolysis. Acids and metal ions act as catalysts in supporting such processes. The reaction changes the chemical and the physical properties of the original polymer, often producing a by-product that acts as an auto-catalyst that enhances the destructive process. Some hydrolytic processes are (partly) reversible; others are not.
3.1.1.1 Vinegar syndrome. A widely known hydrolytic polymer breakdown is the so-called vinegar syndrome. This process that deteriorates cellulose acetate (CA) films beyond retrievability, was first observed in the late 1940s. It has, however, been more widely experienced since the 1980s, particularly in tropical film archives. Acetic acid is one of the products of the process of hydrolysis of CA and acts as an auto-catalyst to accelerate the chemical reaction. Because of its smell it has become known as vinegar syndrome. With the advancement of the process, affected films lose their structure and become unplayable.
Magnetic film sound carriers with a base of CA are particularly at risk because of the catalytic properties of metal in the form of the iron oxide used as the magnetic pigment. Cellulose acetate audio tapes are also affected, but not, however, to the same disastrous degree as films because of their sub-critical mass. Apart from smell, acid detection strips assist to objectively assess the process. CA hydrolysis is not reversible.
3.1.1.2 Pigment binder degradation. Some binders of modern magnetic tapes (2.2.1.1.2) are also prone to hydrolysis causing the tapes to become sticky. Because the symptoms of this kind of hydrolysis are reversible to some degree, such tapes can generally be reconditioned for replay by exposing them to low humidity and elevated temperatures or a combination thereof. For details see IASA-TC04, 5.4.3.3. Recent research has, however, revealed that hydrolysis of binder is only one of several reasons for the so-called sticky shed syndrome (2.2.1.1.2).
3.1.2 Direct contact with water. Direct short contact with water is only dangerous for some kinds of instantaneous discs such as those made from gelatine, cardboard etc, and for hard disk drives. For other carriers water is not immediately dangerous, as long as the contact is short, the carriers are carefully cleaned if the water was dirty, and the carriers are thoroughly dried soon after coming into contact with the water. In fact, cleaning with de-ionised water is a recommended step in preparing vinyl and shellac discs for replay using a professional disc-cleaning machine (IASA-TC 04, 5.2.3, 5.3.3).
The major problem with carriers exposed to water influx is the logistical challenge of cleaning and drying the contaminated carriers, particularly for magnetic tape cassettes. Another logistic problem is the separation of carriers from paper and carton materials, such as LP albums, and the drying of those, before they become affected by mould. If greater quantities are affected, vacuum freeze drying, successfully developed for the rescue of paper and book materials, may be the only chance to safeguard paper and carton materials accompanying audiovisual material. The applicability on audiovisual carriers themselves and specifically on magnetic tape, has, as yet, not been sufficiently investigated (for influx prevention see 4.2).
3.1.3 Oxidation is another chemical reaction triggered by water. It is a potential threat to non-oxidic, pure metal particle magnetic pigments as used for compact cassettes type IEC IV, for R-DAT and for most digital video formats (2.2.1.1.1.2). Oxidation also affects the reflective layers of optical discs, except for those made of gold.
3.1.4 Dimensional influences. Humidity has also an influence on the dimensions of materials used as components of audiovisual carriers. For CA tapes, the humidity related expansion coefficient is quoted to be 15 times higher than that for polyester based tapes.16 A considerable dimension change must also be taken into account for several materials used for instantaneous discs, such as cardboard, gelatine, or the information carrying lacquer.
3.1.5 Indirect influence through bio-degradation. Water causes bio-degradation, specifically mould (fungus growth), which happens at prolonged exposure to relative humidities (RH) of 70% and higher. Fungi of various kinds are present everywhere in the world and affect nearly all audiovisual carriers. Fungi “eat” the surface of analogue mechanical carriers, which leads to excessive surface noise—a particular problem with wax cylinders. Fungi grow on magnetic tape pigment layers, which renders replay difficult or impossible. Fungi are also known to affect CDs, rendering them unplayable. Chemical prevention of fungus must be seen as a last resort. Unfavourable chemical interactions, particularly with the variety of magnetic pigment binder formulations that exist, can never be excluded. Chemical treatment may also endanger the health of archive staff.
Because of its potential for unfavourably influencing carriers, both directly and indirectly, fungus growth must be prevented by keeping relative humidity low. Any direct contact with water, even when permissible in principle, must be kept as short as possible.
3.1.6 Humidity/temperature interrelation. It must be noted that relative humidity and temperature are interrelated (for more details see 3.2.3).
