3.0 The nature of archive materials

The essential chemical and physical structures of most of the materials in archives are broadly understood, but their actual behaviour may be affected by unknown complications associated with the large effects of additives or impurities on the behaviour. Such considerations are particularly important with modern papers and plastics. It is more difficult to predict the behaviour of graphic marks and images because of their great variety and chemical complexity, even greater than their substrates. Any reliance placed on accelerated testing implies several assumptions. In practice there is little opportunity even for extensive experiments. Judgements therefore have to be made from general scientific knowledge of the principles of degradation in conjunction with empirical observations made on archives materials over very long periods of time. An awareness of the degradation processes of polymers is necessary for predictions of the behaviour of most archival materials, which are mainly composed of polymers.

Polymers are simply very long chainlike molecules and form the basis of most record materials, including the fibres in papers, skins, the sizes and coatings and many ink and paint binders. In addition films, tapes, discs and even glass are essentially polymeric. Protein polymers form the fibres of wool, hairs and silks, and collagen which is the basis of leather, parchment and vellum, and the gelatines which are derived from them. The chains may be of great length, with some 10,000 units joined together, yet the whole chain is too small to be seen in an optical microscope. If the chain structure is simple and symmetrical then crystalline regions of tightly packed bundles can form. There will however always be some non-crystalline regions even in a well ordered polymer. The crystalline components confer strength, stiffness and a degree of resistance to chemicals because the permeability, and hence access, is restricted. Crystallinity is then a defence against chemical pollution. Some examples of crystalline polymers found in archives are:

In such substances the chains may through accident, aging or design, become connected onto another by forming new chemical bonds called cross-linking. This usually makes the material harder and brittle. If however some of the chain linkages become broken through chemical attack or from the action of other agencies such as light, heat or radiation, then the material usually becomes weaker and fragile. Both types of degradation make archival materials difficult to handle. They may both occur together.

Amorphous, that is non-crystalline, materials are rigid or rubbery according to their temperature, but they are inevitably permeable to chemicals, especially small molecules such as water, acids, ozone and oxygen. Chemical attack therefore is to be expected in amorphous regions of crystalline polymers as well as in polymers that are entirely amorphous. Quite small amounts of crosslinking or of chainbreaking cause a big change in handling properties, generally for the worse. If only 1 or 2% of the linkages are so changed the effects are noticeable.

By contrast polymers which are highly crosslinked by design are apparently very resistant to chemical attack because they have such a great preponderance of crosslinkages that the loss of a few would be barely detectable. These substances are however rather rigid and somewhat brittle. Such thermoses or 'cured' polymers include:

- vulcanised rubber & gutta percha
- old oil paint and varnishes
- amino-formaldehyde resins (wood glues & paper strengtheners)
- phenol-formaldehyde resins (plastic laminates on wood - plywood)

These useful materials cause problems because their curing reactions may not be completed and their ingredients or byproducts may act as pollutants as they evaporate into the surrounding air.

Glasses are highly crosslinked silica networks with great chemical stability, but they include a variety of metal ions which may diffuse out into condensation droplets and make them very alkaline.

Metals and their alloys can be considered to be the ultimate in crosslinking. Each atom is firmly bonded and attracted to all of its neighbours, thus producing a very tightly packed highly organised crystal structure. This accounts for their generally high densities but also for their impermeability to all gases, which makes even thin films of metal a perfect barrier to pollutants so long as the metal does not corrode away.