Kodak manual Introduction, Definitions, D-31June

Introduction

Photographic film remains an important documentary material. The increasing quantity and value of microfilm records used in financial institutions, libraries, government offices, and industrial firms have focused attention on the care of such records to make certain that they last as long as possible.

The distinction between photographic film records intended for storage and those intended for use has not always been clear. Use or work copies are the predominant photographic records found in libraries or record centers and are subject

to much handling due to their value as quick references. However, because of this handling, they are subject to dirt, abrasion, fingerprints, contamination with foreign materials, and exposure to excessive light, temperatures, and harmful atmospheric pollutants. As a result, these copies in daily use cannot be considered suitable for long-term preservation. For long-term storage, it is imperative to prepare duplicate copies that meet certain criteria, such as proper filming, duplicating, processing, minimum handling, controlled environment, and storage.

In general, the care needed for storing photographic records is similar to that for storing written paper records, although there are some requirements peculiar to the storage of photographic film.

The permanence of photographic records depends on the chemical stability of the film, how the film is processed, and the conditions under which the processed film records are stored. The stability of the film layers is determined in manufacture and processing, while storage is controlled by the user. This pamphlet discusses the composition and properties of black-and-white silver-gelatin, thermally processed silver (TPS), diazo, and vesicular films as they relate to film permanence. It also describes the essential requirements of good processing and storage practices.

NOTE:Refer to the latest revision of each ANSI or ISO Standard specified.

Definitions

To help understand storage requirements, the composition and structure of microfilm are described and definitions of commonly used terms are given.1

Structure of a typical black-and-white negative film.

Acetate film—(acetate base) safety film with a base composed principally of cellulose acetate or triacetate.

Antihalation undercoat—separate layer of light-absorbing dye located between the film emulsion and the base to suppress light reflection. During processing of this film, the dye layer becomes transparent.

Base or support—a flexible plastic material that is coated with a thin, light-sensitive, image- forming layer. The thickness of the base varies with different film types.

Dimensional stability—ability of photographic materials to maintain their original size and shape during and after processing and also under various conditions of temperature and humidity.

Emulsion—the image-forming layer. For unprocessed black-and-white silver-gelatin films, it is composed primarily of minute silver halide crystals suspended in gelatin. Exposure to light in a camera or printer causes no visible effect, but there is an invisible change which produces a “latent image.” To obtain a visible, usable image, the exposed material must be chemically processed.

For diazo and vesicular films, the sensitized layers are composed of light-sensitive diazonium salts. To obtain a visible, usable image with these films, the exposed material is heat-processed. Diazo films are typically heat-processed in the presence of ammonia.

For TPS films, the image-forming layer is typically silver halide and silver salts suspended in a polymeric binding.

D-31 June 2002 1