oper of Capstaff, previously mentioned in relation
to the MQ developer, appeared just at the time this
need was realized. Another important requirement
was a developer suitable for consistent processing
of a large volume of materials, especially in the
processing of motion pictures.
Most amateur processing chemicals are used
only once, but this practice is wasteful; a number
of alternative processing technologies were devel-
oped. While small amounts of developing agents
are consumed in the process, loss of the agent can
easily be supplemented. The main cause of dimin-
ished activity in repeatedly used developer solution
thus is accumulation of a development reaction
byproduct, free bromide ions, and it is the removal
of byproducts that presented a major challenge in
reusable developers. In the early years of motion
picture processing, two-bath development was stu-
died, where the first bath contained developing
agent and the second bath, alkaline buffer with no
developing agent. The idea is that the film emulsion
absorbs the developing agent in the first bath, and
the actual development is activated in the second
bath, where the byproduct is released. Since the
film is not fully developed in the first bath, this
more costly solution can be used repeatedly, while
the second bath, could be inexpensively replaced.
Although the two-bath development technique
yields acceptable results, it provides a limited
range of contrast control. The more preferred
technique is to replenish the working bath of the
developer. The replenishing solution is designed to
maintain a stable pH value and other critical para-
meters of the developer bath as well. The replen-
ishing technique was a major improvement, but
later, minilab machine processing increased the
pressure to further reduce the amount of waste
solution. A later technology takes part of the
working developer bath and removes free bromide
ions from it by an ion exchange resin, and puts the
rejuvenated solution back into the bath, together
with a small quantity of the replenishing solution
to supplement for the spent agents, greatly redu-
cing the amount of waste solution. This was very
important for the convenience of machine opera-
tion as well as the cost of proper disposal of indus-
trial waste solutions.
Stop Bath, Fixing, and Subsequent Handling
Following development, the film or paper is usually
immersed in an acid stop bath to acidify the
adsorbed solution, thereby rapidly arresting the
development reaction. In machine processing, ad-
sorbed developer solution is sometimes removed by
a squeegee. In small scale darkrooms, this process
is sometimes substituted with a plain water rinse.
Despite its name, the stop bath does not literally
end the development process. Its most important
role is to prevent the developer from being carried
over to the fixing bath.
The developed and stopped material is then
fixed. The fixing bath contains thiosulfate, which
dissolves undeveloped silver halide crystals. In
black-and-white processing, acid fixer is tradition-
ally used to minimize the risk of dichroic fog
caused by developer carryover. Alum hardener is
often added to the fixing bath to minimize the risk
of emulsion damage while handling the wet mate-
rial. However, modern films and papers have gen-
erally much more durable coating layers than the
time these acid hardening fixers were designed,
and therefore hardener is optional and often un-
necessary in manual processing. Some fixing so-
lutions are based on sodium thiosulfate and
others, ammonium thiosulfate, or a combination
of sodium thiosulfate and ammonium chloride.
Those that contain an ammonium salt are called
‘‘rapid fixer’’ because they fix materials faster than
traditional sodium salt. (Thiosulfates, especially
sodium thiosulfate is often called ‘‘hypo’’ but this
is a misnomer.)
Each time it is employed, the fixing reaction uses
up a small amount of thiosulfate and releases sil-
ver-thiosulfate complex ions. The solution becomes
exhausted when an excessive amount of silver accu-
mulates in the solution. As in the case of enabling
developer to be reused, it is the removal of the
silver that is the focus of regenerating fixing bath.
There are several practical methods in use for pro-
cessing on various scales, but because silver is
rather precious, methods that maximize its recov-
ery and minimize the volume of waste solution are
in widespread use in commercial photofinishing.
After successful fixing of the photographic
image, the material must be thoroughly washed
because high levels of residual thiosulfate is detri-
mental to the life of the photograph. The washing
time necessary to achieve a level of residual thio-
sulfate that enables very long life expectancy varies
widely, depending on the material, the fixer for-
mula, and the washing water. Impermeable support
materials such as polyester film and resin-coated
paper can be sufficiently washed in a few minutes
of running water. However, traditional fiber-based
prints adsorb thiosulfate ions rather stubbornly in
both their fibers and sizing materials, and adequate
washing requires much longer time. Therefore,
fiber-based prints are usually treated with a wash-
ing aid, which is a buffered sulfite solution, whichDEVELOPING PROCESSES