certain purposes, one agent may be preferred over
another. The temperature of the solution affects the
activity of some agents much more than others.
Hydroquinone, Metol, paraphenylenediamine, and
phenidone are some of the more commonly used
developing agents.
PRESERVATIVE
All developing agents in an alkaline state are
affected by oxygen. When the developing agent
combines with the oxygen in the air, the efficiency of
the developing agent is reduced. When elements
combine with free oxygen from the air or water,
oxidation occurs. Therefore, a preservative is added to
developing solutions to prevent excessive oxidation.
The preservative prolongs the useful life of the
developing solution and prevents stains caused by the
formation of colored oxidation products.
The preservative is a chemical with a great
attraction for free oxygen and combines with it when
mixed into a solution. A large amount of free oxygen is
in the water used for solutions. When the free oxygen is
left in the water, it oxidizes most of the developing agent
and produces stains before the metallic silver image is
developed completely. By adding enough preservative,
you may remove practically all of the free oxygen from
the solution, the developing agent works as intended,
and no stains are produced. Sodium sulfite is the
preservative most commonly used in developing
solutions.
ACCELERATOR
All developing agents (not developing solutions)
are either neutral or slightly acid. Most developing
agents must be in an alkaline state to be effective
reducing agents, thus solutions require an alkali to
activate the developing agent. A developing solution
contains an accelerator so the solution becomes alkaline.
The accelerator serves two functions. First, it speeds
up the swelling of the gelatin in the emulsion. This
swelling permits the solution to penetrate the entire
emulsion more quickly. The effect of this action is
physical. The second action is completely chemical. As
the silver halide salts in the latent image are reduced by
the developing agent, the halide elements freed from the
silver are absorbed by the accelerator and combined into
neutral salts. This prevents harmful effects in the
solution.
Because the accelerator is a determining factor in
the activity of a developing solution, it affects the degree
of graininess produced in the negative. This graininess
is dependent upon the clumping action of the silver
grains during the development process. The more active
the developer, the higher the pH, and the greater the
clumping action. Therefore, the milder or less alkaline
developers yield finer grain. Common accelerators used
in developing solutions are sodium carbonate and
potassium carbonate.
Without an accelerator, there is little or no action.
With some accelerators there is too much action. By the
addition of a bromide restrainer, you may slow down the
action of the developing solution to a controllable
degree.
RESTRAINER
Without a restrainer most developing solutions act
too rapidly and develop unexposed silver halides near
the surface of the emulsion. This causes chemical fog,
developing streaks, and an image with low contrast.
During development, some restrainer is released from
the silver and has a restraining action on the reducing
agent during development. However, its action is not
enough to prevent fog. When a restrainer is added,
development time is prolonged and fog is minimized.
Contrast is increased because the activity of the
developing agent is cut down in unexposed areas.
However, too much restrainer greatly retards the amount
of development. The chemical most commonly used as
a restrainer is potassium bromide.
TYPES OF BLACK-AND-WHITE
DEVELOPERS
As stated previously, when a photographic emulsion
is exposed to light, there is an invisible change produced
in the minute crystals of silver halides that results in a
latent image. To make the image visible, you must treat
the exposed emulsion in a solution known as a
developer. This solution converts the halides affected by
light to black metallic silver. These black metallic silver
particles form the visible image on the negative.
At the beginning of development, there is little
difference in density between the highlight and shadow
areas of the film. However, during normal development,
this difference increases because the highlight densities
continue to increase after the shadow areas are
completely developed. Development should stop when
the contrast between the shadows and highlights reaches
a desired difference. The activity of the developer, and
to some extent the type of film, primarily determines this
developing action.
10-2

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