Kodak D-31 - Manual (Page 14)

D-31 June 2002 11
Processing for Permanence
Silver-Gelatin Films
Processing is one of the most important
factors affecting the potential permanence of
photographic records. The removal of residual
processing chemicals, the exclusion of dirt
contamination, and uniform drying are factors
normally controlled by proper processing.
Processed film, for long-term storage or
permanent records, must meet the requirements
of ANSI/PIMA IT9.1-1992 (R1996) or ISO 18901.
2
The ability to meet most of these requirements is
controlled in film manufacture, but films must be
fixed and washed adequately, whether given
conventional or full-reversal processing, to meet
residual thiosulfate requirements. Films
processed without fixation, such as in halide-
reversal processes and certain reversal
processes with only redevelopment, may give a
“visible, usable image,” but do not come under the
scope of the long-term storage standards.
If fixing and washing are inadequate, thiosulfates
or silver salts, or both, will be retained by the film.
These can break down, especially under poor
storage conditions, to produce yellow stain in
clear areas and fading in areas containing
image silver.
Fixing Baths
In the fixing step, undeveloped silver-halide
crystals in the emulsion are converted to soluble
silver compounds which can be washed away with
water. The chemicals most commonly used for
fixing are sodium or ammonium thiosulfate
(commonly called “hypo”). The fixing bath may
also contain other chemicals to maintain a
desirable pH, provide hardening, stabilize the
solution, protect the image from microscopic
blemishes, etc.
For maximum washing efficiency, a non-hardening
fixer followed by a non-oxidizing washing aid,
such as Microfilm Clearing Bath and Replenisher
should be used. (Call Solutek at 1-617-445-5335
to order CAT No. 414-24). Hypo eliminators
containing oxidizing agents, such as peroxide,
should be avoided. Oxidizing agents may promote
image deterioration.
To make sure that there is enough time for the
fixing reaction to be completed, specific
recommendations for each type of film should
be followed. Undeveloped silver halides are
dissolved by the fixer as complex ions of silver
and thiosulfate. As the silver content of the fixer
increases with processing, the silver-thiosulfate
complex becomes increasingly difficult to wash
from the film. This can be avoided by frequent
replacement or proper replenishment of the fixing
bath or by in-line recirculating electrolytic recovery
of silver from the fixer.
Recovery of silver from fixer is advisable for
reasons of economics, conservation and ecology.
There are three principal methods of silver
recovery:
18 metallic replacement, electrolytic
and sulfide precipitation. For additional
information, refer to Kodak Publication J-212.
23
Washing
Adequate washing is essential to the permanence
of silver-gelatin microfilm. After all undeveloped
silver halide is converted, the emulsion is still
saturated with fixing bath and some dissolved
silver compounds. If these are not removed by
washing, they will slowly decompose and attack
the image, causing discoloration and fading.
The effect is accelerated greatly by high humidity
and temperature. The smaller the grain size of
the image, the greater this reaction is. Microfilms,
being very fine grained, are very sensitive to
this effect.
For good washing, a rapid flow of fresh water
should be used. The water should be filtered to
remove dirt particles. To improve washing,
countercurrent and spray systems are frequently
used in processing-machine construction.
Washing efficiency decreases rapidly with
decreased temperature and is very low
at temperatures below 16°C (60°F). High
wash-water temperatures produce the most
efficient washing, but emulsion reticulation can
result if the wash-water temperature is too high
and not kept close to that of the other processing
solutions. In some cases, extremely soft water
can also cause reticulation. In the case of
contained chemical kits such as the Kodak
Miniprocessor, the manufacturer's recommended
film processing capacity should not be exceeded.