ing the calotype process in 1841. In this ‘‘salted
paper’’ process, the paper is first coated with a
solution of silver chloride or ammonium chloride
combined with a sizing agent such as arrowroot,
starch, or gelatin. The paper was then sensitized
with a coat of silver nitrate and allowed to dry.
After exposure through a negative to sunlight, the
print was washed in water to remove the unexposed
silver nitrate. The image was gold-toned then fixed
in sodium thiosulfate and washed again to remove
the excess fixer. The final prints ranged in color
from reddish brown or, if gold-toned, anywhere
from reddish to purplish brown. The color de-
pended on the sizing, trace materials found in the
paper, and length of time in the toning bath. The
calotype process is similar to the Van Dyke brown
and kallitype processes.
The inexpensive alternative to a daguerreotype
followed on the heels of Frederick Scott Archer’s
published results on the use of collodion to make
glass plate negatives in 1851. The tintype, a direct
descendant of the collodion process, was invented
by an American, Hannibal L. Smith in 1856. It was
simply a thin metal plate, lacquered black then
coated with the collodion and potassium iodide
emulsion and sensitized with silver nitrate. The
image was developed in pyrogallic acid and fixed
in sodium thiosulfate.
While silver was making headway into the main-
stream of photographic processes, other light-sen-
sitive materials were also being explored, including
ferric (iron) salts, a process to become known as
cyanotype. In 1842, Sir John Herschel, renowned
British astronomer and the first to note the impor-
tance of sodium thiosulfate as a fixer, discovered
that ferric ammonium citrate or ferrous chloride
combined with potassium ferricyanide and coated
on a sheet of paper, placed under a negative and
exposed to sunlight, then washed out in water,
produced a blue positive image. The process was
easy to combine with other non-silver processes
and the relative simplicity of the cyanotype (or
blueprint) process ensured its popularity for gen-
erations to come. Engineers and draftsmen con-
tinue to use it for copying line drawings. Herschel
also noted that ferrous salts could reduce silver to
its metallic state, leaving the door open for the
development of the kallitype in 1899, an iron print-
ing process that allows the development of an
image similar to platinotypes, but which is much
less expensive and can be used on fabrics.
The cyanotype process may have been an impor-
tant and true alternative to silver, and far less
susceptible to the problems of impermanence in
the silver-based processes, but it remained relent-
lessly blue. The discovery of the pigmented pro-
cesses in the 1850s seemed like the answer.
These pigmented processes were the result of the
research of three men: Mongo Ponton, who discov-
ered the light sensitivity of bichromates; William
Henry Fox Talbot, who noted that soluble colloids
such as gum arabic and gelatin mixed with bichro-
mate lose their solubility; and Alphonse Louis Poi-
tevin, who added pigment to the colloid.
The carbon process, in which carbon black is
suspended in potassium dichromate and a colloid
such as gelatin, albumen, or gum arabic, and
brushed on the paper, dried, and exposed to sun-
light through a negative, proved to be the most
popular process of its day. Later fine-tuned as the
carbon transfer process, it involved exposing the
negative to the colloidal film mounted on thin tis-
sue, stripping the tissue away, and transferring the
final image to a paper backing. The dry transfer
process, developed in 1935 as a means to make
permanent color prints, grew out of the carbon
transfer process.
Gum bichromate came along a few years later, in
the 1860s. Watercolor pigments were added to the
gum arabic and ammonium dichromate solution
and exposed to sunlight through a negative. (Alter-
nately, starch and gelatin could be used as an emul-
sion, and potassium or sodium bichromate could
be used to fix the light.) Gum bichromate became a
popular process among the Pictorialists in the
1890s and has continued to attract photographers
and printmakers with its versatility of palette and
its ability to combine well with other processes.
Another version of the gum bichromate process is
the casein bichromate process. In this process
casein—milk solids dissolved in ammonia—is
used instead of gum arabic.
Bromoil, developed in 1907 by C. Welbourne
Piper and E.J. Wall, is another of the bichromate
pigment processes. In this case, the gelatin-bro-
mide print is treated with a bichromate solution,
thus bleaching the dark silver image and chemi-
cally altering the gelatin. An oil pigment could thus
be absorbed into the print in proportion to the
original silver image. In an earlier version of this,
the bichromated gelatin is hardened through expo-
sure to sunlight through a negative. The print is
soaked in water and oil-based pigments are
brushed or rolled on, penetrating the print more
or less, depending on the amount of water the
emulsion has absorbed. It was a frequent choice
among the Pictorialists. The Fresson Process, a
patented secret process developed by Theodore–
Henri Fresson, is a version of the bromoil process
still in use today.NON-SILVER PROCESSES