Kodak D-31 - Manual (Page 4)

D-31 June 2002 1
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.