Preserving microforms

Despite advances in digital technology, Microforms - microfiche and microfilms, are still a useful record format. They are directly readable (with magnification). Microfilms enable, and control access, (limited by the number of copies and access locations). They provide adequate information content where black and white/greyscale images are sufficient.

Microfilming is the production of reduced images of documents by photographic processes. There are three main formats:

  • Microfilm is usually 35 mm or 16 mm non-perforated film which can be presented as strips, rolls or cassettes. Black and white is most common, but colour is possible.
  • Microfiche is sheet film, usually 105 x 148 mm, incorporating multiple images – it is usually monochromatic.
  • Aperture cards, also known as image cards, consist of single 35 mm film frames mounted onto cards. This format is often used for reproducing architectural or engineering drawings.

There are three film composition types generally used for microforms:

  • Silver gelatin or silver halide film is composed of silver salts in a gelatin emulsion on polyester film base. This is preferred as the preservation master because, when correctly processed it has excellent stability and image quality. The image is black and white.
  • Diazo film incorporates light-sensitive diazonium salts in an emulsion. The image is usually deep blue to black and white, with medium to high resolution and available in all formats. The resilient outer surface of this film is less susceptible to scratching than silver halide film, making it very suitable for reference copies. However, it is chemically less stable and not suitable for long-term retention.
  • Vesicular film uses diazonium salts but the image is produced by tiny bubbles formed in a polymer layer when the film is exposed to ultraviolet light. It is popular because it allows the production of quick and easy client copies. The resolution quality is less than diazo and silver halide, the image colour is in the pink to purple range, and all formats are available. It is normally used for reference copies or material for short-term retention, as the inherent instability of this film makes it unsuitable for archival filming.

When film is being produced to create a permanent record, most standards and guidelines recommend that three copies be produced: a preservation master on silver halide film, a duplicating master (also silver halide) from which subsequent copies are made, and a set of as many reference or working copies as required (usually on diazo or vesicular film). The minimum acceptable requirement for archival purposes is one master and one reference copy.

Preservation and storage

As noted above materials are susceptible to deterioration because of their properties and degrade over time. However, by controlling storage environments, we can control how fast it happens. Microforms are produced on photographic film and share the same deterioration characteristics as other photographic media.

  • Microfilming technology predates the 1930s and very old microfilm may be on acetate or nitrate-based film, which has particular preservation problems.
  • Some vesicular films produced in the 1960s and 70s were extremely unstable, resulting in severe image loss and liberating chlorine gas which can cause degradation of other records stored nearby.
  • Diazo and vesicular films can be particularly sensitive to high temperatures, affecting image quality.
  • Film which has been poorly processed or exposed to chemical pollutants in storage often develops a type of staining known as 'redox blemishes'. These are small deposits of metallic silver which interfere with image legibility.
  • Microforms can be damaged by poor handling practices and exposure to dust that can affect image quality and promote mould growth. Like most other photographic images, microforms are also light sensitive.

Correct handling of microforms will aid in preserving them for the long-term:

  • Handle with care.
  • Clean lint-free cotton gloves should be used at all times when handling silver halide film.
  • Silver halide master films should not be used for reference purposes as the film rolling mechanisms on the reader and printer equipment can severely scratch the gelatin emulsion.
  • Regular surveys of the film holdings should be programmed to detect problems, including the development of redox blemishes.
  • Reference films should not be left in viewing equipment as prolonged light exposure will affect image quality.
  • Films and fiche should be returned to their protective packaging immediately after use. Do not leave microform material loose on a work surface.
  • Viewing equipment should be maintained and the work environment clean.

The method of packaging microforms will depend on the exact material and the use for which it is required. Master films should always be stored in archival quality enclosures to ensure their long-term stability. Reference copies may not require such stringent quality controls. The method of packaging will also depend on the specific format.

Microfilm

  • Microfilm should be wound onto inert standard-sized plastic or corrosion-resistant metal reels. The loose end of the film should be secured using a strip of paper that has passed the photographic activity test, (PAT) held in place with cotton tape. Never use rubber bands or sticky tape to secure a film.
  • Each roll of film should be individually enclosed in close- sealing containers made from polypropylene, or cardboard that has passed the photographic activity test, (PAT). The size of the container should fit the size of the film, ie do not put small rolls of film in large containers.
  • Never apply labels directly to the film – mark the outside of the container , preferablywith permanent inks.

Microfiche

  • Master microfiche should be individually enclosed in sleeves or envelopes made of inert plastics such as polyethylene, polypropylene, polyester or Tyvek, or paper that has passed the photographic activity test, (PAT).
  • Reference copies of fiche may be stored, in plastic file boxes or drawers as long as they are protected from dust and light.
  • Do not apply labels directly to fiche or individual enclosures. Each fiche has a header where control information is recorded. However, use a permanent pen and mark the non-image area of the fiche if additional information is required.

Aperture cards

  • Aperture cards are normally only produced for reference purposes and can be stored in clean sturdy containers that exclude light and dust. The master copy would normally be retained as a roll film. However, if master aperture cards are held they should be individually enclosed, in the same way as master microfiche.

The preservation master copy of any fiche or film should be kept in a different location to the duplicating master and reference copies. This should preferably be off site, even in another agency or state. This is a security measure to ensure that at least one copy of the information will always exist, protecting the master against destruction by theft, inadvertent use, or disaster such as flood or fire.

Storage furniture should be made of coated metal. Wood should be avoided, as it can release harmful vapours, can contribute to the spread of fire and may harbour insects. Specially designed storage furniture such as drawers and cabinets can be purchased for microforms allowing more efficient use of space, however, these are only necessary when large quantities of film are held.

Preservation and duplicating masters should be stored in a clean, dark, climate-controlled environment. Black-and-white material should be kept at 8 to 12 °C and 30 to 40 per cent relative humidity, and colour at less than 5 °C and 30 to 40 per cent relative humidity. Temporary records and reference copies can be stored under general archives standards of 18 to 22 °C and 45 to 55 per cent relative humidity.

Microforms degrade more quickly when exposed to ultraviolet (UV) light. Fluorescent tubes which are low in UV should be used wherever possible in storage areas. Ultraviolet light can be easily measured with a light meter, and levels should not exceed 75 µW/lumen. Lights should be turned off whenever possible. Storage areas should not have windows, but if they do they should be covered with curtains or blinds.

Insects and rodents once attracted to a records storage area may start eating the records. To reduce the risk of infestation:

  • Do not eat in storage areas.
  • Keep surfaces (floors, tops of shelves) clean.
  • Bait regularly for rodents and fumigate annually for insects.

It is important to select the correct format and film type to suit the purpose for which the microform is required and the nature of the original record being reproduced. Take care that the filming process does not cause unnecessary damage to fragile or valuable originals. For this reason manual filming is preferable to automated filming.

Production standards are especially critical where microfilm is being produced to replace an original record that is inherently unstable and cannot be preserved. Preservation microfilming production standards encompass quality control issues such as document control, targets, camera settings, and post-production checks on image quality and chemical stability. Silver halide microforms produced and stored according to these standards are predicted to have a life expectancy of 500 to 1000 years, which very few record formats can match. However, films not produced to these standards may be severely degraded after as little as 20 years.

Further advice

Contact the Agency Service Centre.

Copyright National Archives of Australia 2014