LESSON 1
harden the film emulsion when processing at high temperatures or under
tropical conditions.
(a) If temperatures are above 75 degrees Fahrenheit (24
degrees
celsius), the acid stop bath should contain an antiswelling agent,
such as
sodium sulfate, and a hardening agent, such as chrome-alum. Sodium
sulfate
is recommended for temperatures for temperatures between 75 and 85
degrees
Fahrenheit.
(b) A chrome alum bath should be used for temperatures between 75
and 95 degrees Fahrenheit.
(c) Under ordinary conditions the hardening agent in the fixing
bath is sufficient, therefore a water or acid rinse may be used.
7. Fixing film. After the rinse or acid-stop bath, the film is placed into
an acid-hardening fixing bath, usually called "hypo". The function of the
fixing bath is to make it possible to remove the unexposed, undeveloped
silver halides, which are still sensitive to light.
During fixation, the
undeveloped silver halides are converted to water soluble compounds (silver
salts) that are no longer light sensitive.
a. Fixing time.
The general yardstick for fixing time is twice the
time required for clearing ("clearing" refers to the removal of the silver
salts from the emulsion). However, since most film processing must be done
in total darkness, we cannot tell when the film has cleared.
For most
negative materials a fixing time of 5 minutes is adequate. After repeated
use the hypo is weakened by the dissolved silver halides and clearing time
becomes progressively longer.
b. Hypos contain several ingredients to perform the functions described
above.
(1) Silver halide solvent (fixing agent). Hypos must have a silver
halide solvent, usually sodium thiosulfate (hypo).
This chemical changes
the undeveloped silver halides to water soluble compounds.
(2) Acid and neutralizer. Even though the film has been subjected to
a stop bath, there still remains a considerable amount of developer in the
emulsion.
The acid or neutralizer (acetic acid), completely stops the
development action and prevents stains.
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