CONSERVATION COLUMN

A fundamental aspect of a photographic conservation program is the duplication of negatives and the copying of prints. A well produced negative will last longer than originals that are unstable or have a history of poor storage. For example, priority should be given to cellulose nitrate and deteriorating acetate negatives. If researchers handle copy prints, this will reduce damage to the originals.

In practice, duplication refers to the production of a negative from an original negative or transparency. Copying is the production of a negative from a print.

SYSTEMS AND FORMATS

Photographic systems are preferred because of their permanence and their superior image quality. A well produced and stored negative can be expected to last 500 years.

Other systems do not have good long term stability or resolution; however, they are suitable for reference purposes. These are photocopying, electronic imaging (note that the system may go obsolete within 10 years), and prints produced by electronic imaging (dye sublimation and ink jet prints have poor stability).

Negatives and transparencies can be duplicated in three ways. First, by the production of an interpositive intermediate, which is used in turn to make a negative. All steps are done by printing the negatives in contact with each other. Second, the original can be contact printed against a direct duplicating film. Third, the negative may be printed and the print may be copied by camera, resulting in a copy negative.

Large and medium file formats are appropriate for preservation copies. The larger the format, the better the quality of the image. The standards for most labs is a 4" x 5" sheet film format. At present, 35 mm is not recommended because of the instability of its cellulose triacetate film base, loss of resolution, and graininess. However, medium format (70 mm and 105 mm) roll film may become popular in archives of the future when polyester based roll film becomes practical.

FILM MATERIALS

Black and white films should have silver images, and their film bases should be made of polyester (tradename Estar). Recent experience and research show that cellulose triacetate, the plastic base of most roll films, is unstable. At present, colour films have a short lifespan, and they should not be used for preservation purposes.

Some direct-duplicating films (such as Kodak S0-339, which replaced previous Kodak 4168 and S0-015) have an extremely fine grain which can be damaged easily by pollutant gases. These films should not be used unless they can be toned.

PROCESSING

Image silver can be degraded by sulphur compounds in the processing chemicals (technically speaking, residual thiosulphates from the fixer and silver thiosulphate complexes from exhausted fixer). The films should be processed with fresh fixer and washed well with hypo clearing agents, salt solutions that hasten the washing away of fixer. Hypo eliminators should not be used as they can oxydize the film.

The film should be processed according to the guidelines in CCI Note 16/6, Processing Contemporary Black and White Photographic Films and Papers. Processed film should be tested for residual thiosulphates, meeting ANSI Standard IT 9.1- 1992. The film should be tested by either the methylene blue or silver sulphide densitometric method (ANSI/NAPM IT 9.17-1993). Methylene blue testing should take place within 2 weeks of processing. The maximum permissible concentration of thiosulphates, for roll and sheet films is 0.014 grams per cubic meter.

The CCI note and current research recommend toning with selenium or polysulphide toner. Toners provide additional protection to film against pollutants and environmental extremes. However, photographers should not use toners unless they have industrial ventilation and personal protective wear.

STORAGE

Negatives should be stored according to ANSI standards. Enclosures should meet ANSI standards IT 9.2-1991. Requirements for storage conditions, facilities, inspection, and handling are given in ANSI IT 9.11-1993. This standard recommends storage in an extended-term environment of 20% to 30% and a temperature not to exceed 21§C. ANSI allows a broader range of relative humidity (to 20%-50%) if the films are stored at cooler temperatures. The relative humidity should not cycle by more than 5% over a 24 hour period.

(A list of citations for further reading is available from the Archives Advisor, Elizabeth Denham)