Air-Britain Colour Image Archive

Bristol 175 Britannia 324 G-ARKA, British Eagle, Heathrow, date unk ab-ixn (Eric W Sawyer/CS20139)

Conservation of Films and Slides

by Peter W Dance

[originally published in Air-Britain Digest, Spring 1998]

Modern photographic films are made of a cellulose acetate base medium, coated with an emulsion of several materials, usually on the "reverse" side. The emulsion is a multi-layered coating of gelatin that carries within it colour dyes or silver compounds. Gelatin is a protein derived from collagen, and is normally stable in clean, dry, cool conditions. However, warmth and moisture causes acid formation, swelling, migration of emulsion constituents, and absorption of contaminants. Warmth and moisture can encourage both latent and fresh fungus spores to grow and feed on both the film base and the organic ingredients of the emulsion. The pigment dyes or silver compounds vary in stability according to the presence of light, moisture, temperature, gases and chemical residues, and they are also vulnerable to biological attack. The effects can be fogging, colour fading, colour casts, emulsion loss (holes) and destructive fungus growth, and all these can result from unfavourable conditions before or after film processing.

For most cleaning and restoration procedures, some of the best tools are brushes at least 6 mm wide, made with sable or squirrel hair. Soft cloths and cotton buds can also be used with care, but they can be extremely harmful when they collect and retain abrasive dust and grit.

To remove finger prints or similar deposits on the "top" side of a film, first remove all dust and loose material with a brush alone or with dry blown air. Then, apply a single wipe with a moist pad or freshly scrubbed soft finger tip. Water with very mild detergent can be used, but the whole film will then need to be rinsed and held clamped flat to avoid distortion due to migration and absorption of moisture.

Fungus attacks and growths are most often found on the emulsion side, but are sometimes found on the top side also. Fungus typically starts from a pinpoint mark, growing and developing to appear like a cross between cracked ice and a spider with many legs. Fungus may have been activated before receipt and storage in good conditions, so you should periodically review films that may have been subjected to fingermarks, exposure to tropical climate or other risk of contamination by live fungus spores. Agitation with solvents may not always remove the effects completely, but such a procedure may at least arrest or slow further growth.

Intentional cleaning of the emulsion side of a film is highly risky, and if you have any doubts, don't even attempt it. The emulsion side is noticeably softer and generally more vulnerable than the "top" side. Careful cleaning might be effective in removing residual chemicals, ingrained dust or fungus growth. Specialty film cleaning products are available that combine volatile (evaporative) solvents with anti-static additives. After cleaning, film may need to be clamped flat for an hour or more to dry, to stabilise and thus avoid permanent distortion. If emulsion material has been lost to fungus or other attack, there will remain "valleys" where colour or light transmission varies. Touch-in dyes and scratch filling coatings, requiring considerable skills, can subsequently be used to try and match surrounding areas.

When planning storage methods and materials, bear in mind that emulsions are vulnerable to the invisible effects of airborne contaminants. Many common materials emit volatile chemical constituents, and films should be isolated from ozone, engine emissions, PVC, polyethylene, and newly painted rooms and containers. Remember, if you can detect a chemical smell, there is some gas or vapour present.

Mounting films between thin glass sheets can protect against dust and direct attacks, plus minimal protection from ultra-violet light, but the glass surfaces and the film must be clinically clean and dust free to avoid long term damage. Also, the resistance to moisture and fungal damage may be lower, due to capillary action. Visible rainbow effects, often called Newton rings, are indicators of trapped moisture.

Close proximity to domestic fabrics, and human or animal living conditions, may allow insects easy access to your films. Carpet beetles and silverfish sometimes attack films or lay eggs in crevices between films and mounts.

Kodak recommendation for 10 year stable storage, is to store colour slides below 10 degC, black & white negatives below 25 degC. For "extended-term storage", maintain colour slides below 2 degC, black & white negatives below 21 degC. In all cases, recommended relative humidity range is 20-30 per cent, so you must eliminate all moisture if kept in cold storage. Darkness is best, and ultra violet light from fluorescent tubes or sunlight particularly causes fogging, fading and colour casts.

Much of the above data is the result of recent experience in reviewing, restoring and archiving colour slides held by the Air-Britain Colour Slide Library. We always welcome slide donations and bequests, and are happy to discuss special arrangements or conditions. P W Dance, Air-Britain Colour Image Archive, 9A Connaught Road, Teddington, Middx TW11 0PS.

Conservation - 2007 Appendix

Sadly, the oft suggested practice of optical digital scanning is not the panacea that many people assume. It cannot preserve all molecular data in negatives or prints, and in due time current methods will be superseded by technologies that will offer better results. It also takes a lot of preparation, scans often repeated until the best is achieved, then much de-spotting and other editing, finally saving as a very large high definition TIF image file, backed up to additional storage.

The whole of our archive would take at least 10 man years to scan, with large investment. Having done that, new technologies will require the whole process to be repeated. Meanwhile, another 10 years worth of donations will have arrived, and here we go round the mulberry bush.