LESSON 1
(3) Preservative.
When acid is added to the fixing bath,
decomposition of sodium thiosulfate sets in.
Sodium sulfite, a
preservative, is added and combines with the decomposition (free sulphur) to
form new hypo.
(4) Hardener.
During development, the emulsion becomes soft and
swollen.
A hardening agent is added to prevent frilling (peeling of the
emulsion) and minimize scratching.
Potassium alum is used to harden the
emulsion.
c. Types of fixing baths. There are three major types of fixing baths:
plain, acid, and acid-hardening.
(1) Plain fixing bath. The standard plain fixing bath is seldom used
except when preparing prints for toning. It contains sodium thiosulfate and
water.
(2) Acid fixing bath. A satisfactory acid fixing bath contains hypo-
acetic, acid-sodium sulfite and water. The acid fix is primarily used for
prints.
(3) Acid-hardening fixing bath.
An acid-hardening fixing bath
contains a hardening agent, usually potassium alum, as well as a silver
halide solvent, a neutralizing agent, and preservative. This type of hypo
is suitable for film and if diluted, paper.
8. Washing and drying film. Negatives and prints must be washed to remove
the hypo left in the emulsion after fixing. This is necessary because an
unwashed, or improperly washed, emulsion will stain, change color, and fade.
Therefore, washing photographic emulsions is just as important as any other
part of the development process.
a. Washing time depends largely on water temperature and the amount of
fresh water coming in contact with a negative or print.
Most chemicals
diffuse faster in warm water.
With photographic material, however, the
warmer the water, the more the film gelatin swells.
Water temperature
should be within the range of 65 to 75 degrees Fahrenheit.
b. As the washing proceeds, the hypo remaining in the sensitized
material is halved in equal periods of time.
For example, the average
negative gives up approximately half of its hypo in 15 seconds of direct
contact with running water.
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