intense and prolonged in order to create grains of
metallic silver that are large enough to form a
visible image without chemical amplification.
It is obvious, by comparing the developed out
process and printing out process, that the develop-
ment process is a major technological advance that
greatly improved photographic sensitivity. Together
with the various sensitizing techniques introduced to
silver-gelatin emulsions and improved optics, in-
creased overall photographic sensitivity in the twen-
tieth century enabled the photographing of a far
wider range of subjects, such as fast moving objects
and dimly lit scenes.
History of Developing Processes
Development is an electrochemical reduction process,
where a silver ion gains an electron from a reducing
agent, to deposit metallic silver selectively on exposed
crystals having a latent image center. Historically, the
silver ions were supplied from the developer solution,
as in William Henry Fox Talbot’s developer for the
calotype. Calotype developer consisted of a solution
of gallic acid fortified with silver nitrate. Silver ions
are reduced by the gallic acid, and the metallic silver
was deposited on the latent image to form a visible
image. This type of developer is called ‘‘solution
physical developer,’’ which is very slow working and
gives slow photographic speed, and therefore is rarely
used in current practice. In modern developers, silver
ions are supplied from within the silver halide crys-
tals. This type of developer is called ‘‘direct develo-
per’’ or ‘‘chemical developer.’’ These two modes of
development are not mutually exclusive, and in fact,
solution physical development occurs to limited
extents in many modern developer solutions, which
predominantly use chemical development. Readers
are warned that the terms ‘‘physical,’’ ‘‘direct,’’ and
‘‘chemical’’ are misleading.
Pyrogallol developer, introduced for the wet col-
lodion process in the 1850s, had been used as a part
of physical developer in acidic solutions, but it also
started the era of direct development when J. Bur-
gess recommended the alkaline developer pyrogal-
lol in ammonia in 1973. This was soon adapted to
development of silver gelatin dry plates, thus enter-
ing the years of chemical development.
The decade of the 1880s saw the first surge of
intense searching for new developing compounds,
with many compounds being introduced as develop-
ing agents. Developing agents are chemical re-
ductants, but useful developers must have certain
electrochemical properties as well so that the devel-
oper can act selectively on exposed crystals. In 1880,
Abney introduced hydroquinone developer, and
Eder and Toth catechol developer. They were fol-
lowed byp-aminophenol and p-phenylenediamine
by Andersen in 1888, Eikonogen (1-amino-2-
naphthol-6-sulfonic acid) by Andersen in 1889, Ami-
dol (2,4-diaminophenol) by Andersen and Bogisch
in 1891, Glycin (N-methyl-p-hydroxyphenylglycine)
by Bogisch and Metol (N-methyl-p-aminophenol)
by Bogisch and Hauff, both in 1891. These develop-
ing agents gave images of varying speeds and con-
trasts. The way images emerged in the developing
plates also varied. In hydroquinone developers, the
highlights emerged first and grew in density, while
midtone and shadow areas after considerable lag
slowly appeared. On the other hand, in Metol devel-
opers, shadows and highlights appeared almost
simultaneously and grew in density together. Since
various developing agents worked differently, many
solutions were formulated with different agents for
diverse applications.
As early as 1889, a developer solution containing
two developing agents was formulated. It is quoted
by Levenson (1991) that Henry J. Newton reported
that he had added hydroquinone to an Eikonogen
developer to boost the highlight density. His devel-
oper offered the best of both agents. Shortly after,
developers of Metol-hydroquinone (MQ) combina-
tion were formulated, and it was noted that the
MQ developer was more active than the sum of
Metol and hydroquinone activities when used
alone. There were several speculative explanations
and partial proofs for this ‘‘superadditive’’ phe-
nomenon, but convincing proof had to wait for
Levenson’s work in the late 1940s, indicating that
the image is developed by Metol, and the reacted
Metol is regenerated to become active again by
hydroquinone, demonstrating the ‘‘regeneration
theory’’ of superadditive development. However,
without waiting for the science, various empirical
data were gathered and practical MQ developers
were made. One of the most important MQ devel-
opers is the Eastman fine grain (D–76) developer
by Capstaff in 1926, which remains one of the most
important black-and-white film developers today.
The MQ combination was found to be very effec-
tive, versatile, and capable of producing high image
quality, so that most other agents were made obso-
lete or spared for special purpose applications, with
an exception described next.
Another wave of new developing agents came
after the discovery of Phenidone (1-phenyl-3-pyra-
zolidone) by Kendal in 1940. Various chemical
derivatives of Phenidone, as well as other com-
pounds containing a similar heterocyclic ring struc-
ture were studied. Phenidone is much more potent
than Metol and more rapid in developing imagesDEVELOPING PROCESSES