1486
prepared in advance in a shop or laboratory and trans-
ported into the fi eld.
Immediately before the image was to be made, col-
lodion was poured onto a clean glass plate, which was
continuously tilted to produce an even coating. The size
of the plate was dependent upon the required size of the
fi nished print, and plates varied in size from under two
inches square, to mammoth plates, measuring in excess
of 20 × 24 inches. When the collodion had set but not
dried (a matter of seconds), the plate was sensitized by
bathing it in a solution of silver nitrate. During this bath,
the silver nitrate reacted with the potassium iodide in
the collodion to produce light-sensitive fi lm of silver
iodide. This sensitizing process could be carried out
under yellow light.
While the plate was being sensitized, the camera
operator fi nished composing the scene, set up the cam-
era, and focused on the subject. After removal from the
silver nitrate bath, the glass plate, now light-sensitive,
was placed in a light-proof holder and transported to
the camera while still wet. When the subject was ready
and the fi lm holder loaded into the camera, the “dark
slide,” a movable cover on the fi lm holder, was moved
to uncover the plate. The plate was fi nally exposed to
the subject by removing the lens cap; exposure times
ranged from less than one second to several minutes,
depending upon the intensity of the light, and the age and
quality of the collodion. When the proper exposure was
made, the lens cap was replaced, and the “dark slide”
returned to its closed position.
After exposure, the holder containing the plate was
removed from the camera, returned to the darkroom
and immediately developed in a solution of pyrogallic
and acetic acids (a later refi nement of the process used
ferrous sulfate as a developer). The image became vis-
ible within a few seconds as the areas struck by light in
the camera turn to metallic silver. When development
was complete, the developing solution was removed
by a wash of clean water. After fi xing—usually in a
tray of sodium thiosulfate (commonly called sodium
hyposulphate in the nineteenth century)—to remove
the unused silver halides, the plate was no longer sensi-
tive to light, and could be removed from the darkroom
and washed in fresh water. An alcohol lamp was then
used to dry the plate. Once dry, and while still warm,
the plate was coated with a protective varnish made
from gum sandarac, alcohol and oil of lavender. The
glass plate was then a negative, and could be used to
make a wide variety of paper prints.
Bryan Clark Green
See also: Daguerreotype; and Calotype and
Talbotype.
Further Reading
Archer, Frederick Scott, “The Collodion Process on Glass.” The
Chemist (March 1851).
Baldwin, Gordon, Looking at Photographs: A Guide to Technical
Terms. Malibu, CA: The J. Paul Getty Museum, in association
with the British Museum Press, 1991.
Barger, M. Susan (comp.), Bibliography of Photographic Process-
es in Use Before 1880. Rochester, NY: Graphic Arts Research
Center, Rochester Institute of Technology, 1980.
Hardwich, T. Frederick, A Manual of Photographic Chemistry,
Including the Practice of the Collodion Process, 4th ed. New
York: H.H. Snelling, 1858.WET COLLODION POSITIVE
PROCESSES
(Ambrotype, Pannotype, Relievotypes)
The latest fashion in photographic portraiture, the wet
collodion positive process on glass (also known in the
United States as the ambrotype), was introduced in- The images had a warm tone and did not have
the mirror-like refl ection that made daguerreotypes
diffi cult to view. The images were the same standard
sizes as daguerreotypes, and in Western countries and
colonies they were usually presented in a similar man-
ner, with hinged cases, and a glittering brass mat. “The
closed, hinged case introduced the element of surprise, a
sense of drama as one held it in one’s hands, wondering
what was going to be pictured inside. As the case was
opened this sense of theater became part of the viewing
experience.”
The wet collodion positive process was derived from
the collodion negative process described by English
Frederick Scott Archer in1851. In the second edition
of his manual (1854), Archer included a chapter, “The
Whitening of Collodion Pictures as Positives.” In the
United States, the ambrotype process was patented by
James Ambrose Cutting in 1854. Cutting’s patents were
largely ignored and had little effect in the rapid spread
of this process across the United States.
The investment on the part of photographers to adapt
their equipment and studios to the wet collodion posi-
tive process was inconsequential. The glass plates were
the same standard size as the daguerreotypes; cameras
were easily adapted to accommodate the glass plate and
the investment in supplies and equipment was minimal.
The chemicals required in preparing the collodion
emulsion, and assorted paraphernalia including trays,
beakers and funnels were readily available in cities and
port towns.
The preparation and exposure of a wet collodion posi-
tive was a well choreographed dance that required timing
and confi dence in handling chemicals. The photographer
prepared a collodion emulsion by dissolving gun cotton,