Conservation techniques for photographic materials

Glass Negatives

Many of the measures involved in the care and storage of glass negatives rely on a common-sense approach bolstered by specialist supplies. Simple methods can be adopted to limit human error and minimise the effects of the environment on the images. These will prevent damage to photographs at fairly low cost. Remember: the least sophisticated solutions are often the most effective.

Specialist considerations

There are four areas which merit particular attention when devising a strategy for preserving glass negatives:

1. The general environment under which the negatives are stored
2. The cabinets or containers within which the negatives are stored
3. The housings for each negative
4. Storage technique

Consideration of these requirements will inevitably depend upon location, size and importance of collection and, of course, availability of financial resources.

The environment

The general storage/display environment is the most important consideration in any photographic archive, and the most cost-effective investment.

The bulk of glass plate negatives tend to contain gelatin within the emulsion layer. A relative humidity (RH) of above 70% combined with a temperature of above 22oC will hasten fungal and mould growth on the emulsion. The resultant spores then consume the gelatin, a nutrient, and the image-layer begins to disappear. The levels which are recommended are between 35 and 45% RH and 16–18°C, but achieving a stable environment may be more important than trying to reach absolute levels.

The primary action must therefore be to assess the environment within the storeroom to provide a profile of periodic fluctuation. (A local museum may be able to lend or hire measuring instruments to help with this exercise).

After evaluation the climate in the storeroom can be adjusted to create an appropriate environment. RH may be controlled by using small amounts of anhydrous silica gel, which will absorb water from the atmosphere; temperature can be crudely controlled by fan or heater. At the more expensive end of the scale, sophisticated equipment such as a dehumidifier and thermostatic heating may be employed. As a minimum guide, however, the store room should offer stable, dry, cool storage.

Cabinets

Shelves and cabinets should be made of stainless steel, stove-enamelled metal or anodised aluminium, not materials which may dissipate harmful chemicals (e.g. wood exudes lignin and oils). They should be flame-resistant and well able to bear the heavy loads of photographic materials.

Drawers and shelves should offer a snug fit and a degree of ventilation. Too large a drawer may cause damage to the edges of the image; too small a drawer may cause the images to be insecure within causing damage to the plates.

Machine-tool cabinets, such as those manufactured by Polstore, are one solution. These are capable of containing about 5000 quarter plate glass negatives per cabinet. They offer high density storage; good load-bearing; are extremely easy to manipulate; and they are versatile, being able to accommodate different sizes of drawer and divider. Each drawer can be altered to suit its contents.

Metal plates can then be inserted at frequent intervals between negatives to give plenty of support to the glass. The drawers may be lined with archival board to cushion the drawer and to minimise any damage from bolts or internal movement.

Housings

Paper enclosures are preferable for glass images. Most paper enclosures are translucent and, for general examination, the glass image may usually be viewed on a light-box without having to remove it from its housing.

The paper should be buffered to a neutral pH7–8. Avoid neutralised pulp boards and papers which are not satisfactory over a long timespan. Glassine, PVC, woodpulp derivatives or coloured enclosures should never be used as a storage medium, and do not use polythene which, if sealed too tightly or under poor environmental control, forms an adverse microclimate which can lead to sweating and condensation.

Care should be taken over the adhesives used in the fabrication of paper enclosures. Spirit-based glues can migrate through the paper onto the image and cause irreparable damage. If adhesive–sealed paper envelopes are used, ensure that the seams run down the sides of the enclosure. A much safer form of enclosure is the cruciform variety which, when folded, forms a secure unit around the image without the need for glue.

Storage techniques

The recommended storage technique for glass negatives is to store them on vertically on edge along their longest side, well-supported and individually wrapped in acid-, sulphur- and lignin-free unbuffered enclosures within a neutral storage cabinet.

Different institutions have resolved this problem in different ways according to the quantity and range of sizes and processes within their collections, levels of access, and according to their budget.

A system in which negatives are themselves the primary image, and are used more heavily, is inadvisable for glass but may be inevitable: in this case, consideration should be given to smaller negative boxes. These allow the withdrawal of small groups of material for researchers, rather than giving access to the whole collection or cabinet.

Damaged plates

Broken or damaged plates should be sandwiched between two pieces of glass or acid-, sulphur- and lignin-free board until resources have become available to employ a specialised conservator. These should not be stacked and should, wherever possible, lay flat in a drawer marked ‘damaged material’, indicating that care should be taken when opening.

Colour transparencies

The recommended storage for colour material is very cool and dry: humidity of 35% is recommended and temperatures of 5° or less are suggested, but this is impractical if the slides are in use. You should try to achieve a cool, clean dry and well-ventilated storage area.

Fungal growth may occur when storage conditions are generally moist (relative humidity 70% or above). Either the general environment within which the slides are stored could cause this or, if the slides are covered, the existence of pockets of moist air trapped between the film and cover glass.

We would not recommend any fungicide or cleaner as we have no facilities for testing such products and they are generally harmful to the emulsions and dyes. However, an established photographic conservator in Canada, Siegfried Rempel, has recommended that Kodak Film Cleaner may be used for the removal of fungal growth (ref: The Care of Photographs, 1987, pp83-85).

Lantern slides

Lantern slides, if in good condition, are comparatively stable images. When assessing the slides for treatment, you should:

1. Check that all binding papers are complete, and not torn or damaged;
2. Check for cracked or broken glass;
3. Ensure that each image is protected by a cover glass.

Where binding papers are damaged or glasses are cracked or broken, dirt and pollution may enter the slide and moisture may form from differential temperatures and humidity. The slides should be separated, cleaned, and stored in a stable environment before re-sealing.

No cleaning should be undertaken on the photographic emulsion, except indirectly with a puffer brush. Other surfaces may be cleaned carefully with a slightly moist lens tissue or cotton bud, and allowed to dry fully.

The slide may be re-sealed by making a pocket with 1mm Perspex cut to the size of the slide, using Melanex strip to bind and clear archival tape to seal.

A strong and safe storage container should be provided for the slides, with rigid vertical separators of archival board or metal placed at very frequent intervals (every four or five slides) to stop them pressing against each other and breaking. Consideration should be given to providing a separate viewing system, perhaps using 35mm slide copies, to avoid handling the originals.

Photographic prints

Photographic prints should be copied so that researchers may use copy prints, thereby reducing the handling of the originals. These should then be stored flat in archival print boxes, limiting the number of prints in each box to reduce the pressure on the photographic emulsions.

Each individual print should be window-mounted in archival board, using hinges to secure the print to the backing-board, and the face of the print should be protected with Mylar. This method allows for display or storage of the print. It is described in the book The life of a photograph by L Keefe and D Inch, published by Focal Press (London, 1984). However, as such mounting is expensive, you may wish to cut an archival backing board of a standard size, larger than the print, and fitting the sleeve tightly. The print and backing board may then be inserted into the sleeve.

Albums are not recommended as they encourage handling of the originals, which, unless very securely fixed and protected, are liable to damage. You may wish to compile albums of copy prints.