534
fi xer had to be more vigorous; an acidifi ed fi xer of am-
monium thiosulphate was eventually substituted. When
used with alkaline developers, this required an interme-
diate acidifi ed ‘stop bath.’ Organic developers (such as
pyrogallol, hydroquinone, and eikonogen) required an
acidic clearing agent to eliminate fogging. In 1889, Al-
exander Lainer discovered that sodium bisulphite could
be added to sulphurous acid for a one-stop fi xing bath.
Non-silver processes (such as cyanotype and platinum
prints) require different fi xing or clearing baths: cyano-
types are cleared in plain water, and platinum in a weak
solution of hydrochloric acid.
The fi xing of a silver image necessitates dissolution
of the unexposed silver halides, which remain photo-
sensitive after development. To stabilise the image, a
chemical fi xer is used to convert the halides into soluble
silver compounds that can be removed by washing.
Washing may not clear all residual silver complexes, or
remaining fi xer: the former may be stabilised by toning,
while the latter requires a hypo clearing bath.
By 1855, it was recognised that silver photographs
had poor archival permanence, and in May of that
year both the Photographic Society of London and the
Société française de photographie set up committees
to investigate the fading of positive photographs. Both
efforts had been inspired by Louis-Alphonse Davanne
and Jules Girard, whose initial analysis was presented
to the Société in March and April 1855. Contributions
were also made by Baron Humbert de Molard, Eugène
Durieu, Thomas Hardwich, Thomas Malone, and
George Shadbolt. It was discovered that residual fi xer
carries sulphur compounds, which convert the metallic
silver of the image to silver sulphide. Silver sulphide is
yellowish in colour and has half the density of metallic
silver, producing irreversible yellowing and fading in
the image, particularly in the highlights and mid-tones.
In 1866, John Spiller reported that in albumen prints,
residual sulphur compounds in the egg-white coating
exacerbated deterioration.
Sulphiding could be prevented by gold-toning, as dis-
cussed below, but also crucial was the proper removal of
the fi xing compounds, achieved by prolonged washing
in clean water and the use of clearing agents. Many hypo
clearing agents were suggested, with the most effective
being a weak solution of sodium sulphate, or ammonia
combined with hydrogen peroxide. Silver prints on pa-
per typically require about an hour to wash out the fi xer,
although in 1855, Thomas Sutton recommended several
hours for paper negatives, and up to twenty-four hours
for positive prints. Numerous chemical tests for hypo
were proposed, with potassium permanganate, mercuric
chloride, and silver nitrate being the most accurate.
In 1840, Hippolyte Fizeau discovered that treatment
of a silver photograph in a solution of gold chloride
would partially replace and partly plate the silver par-
ticles, producing a stable silver-gold alloy. Toning often
preceded fi xing, but Fizeau combined both, using gold
chloride and hypo in a ratio of one to three-hundred.
Both alkaline and acid toning baths were used, and in the
1890s, combined toning-fi xing baths were introduced
for self-toning silver gelatin and collodion papers, with
the addition of potassium and ammonium thiocyanate,
or the substitution of lead acetate for the gold chloride.
More recently, it has been established that there is a
protective effect to residual silver sulphide, which is
actually more stable than pure metallic silver.
The mechanics of fi xing, processing, and washing
infl uenced the design of studio and fi eld darkrooms.
However, the processing environment and problems of
ventilation, consistent temperature and humidity, water
supply, and contamination were largely unaddressed
until the late 1880s, when industrial photographic print-
ing works established large-scale, rapid processing and
washing facilities.
Hope Kingsley
See also: Wedgwood, Thomas; Davy, Sir Humphry;
Niépce, Joseph Nicéphore; Talbot, William Henry
Fox; Daguerre, Louis-Jacques-Mandé; Scheele, Carl
Wilhelm; Herschel, Sir John Frederick William;
Reade, Joseph Bancroft; Calotype and Talbotype;
Wet Collodion Negative; Gelatin Silver Print; Dry
Plate Negatives: Gelatine; Cyanotype; Platinum Print;
Davanne, Louis-Alphonse; Humbert de Molard,
Baron Louis-Adolphe; Durieu, Jean-Louis-Marie-
Eugène; Malone, Thomas Augustine; Shadbolt,
George; Spiller, John; and Fizeau, Louis Armand
Hippolyte.Further Reading
Davanne, Alphonse, and Girard, Jules, “Mémoire analytique sur
la formation, le fi xage et l’alteration des épreuves positives
en photographie [Analytic thesis on the formation, fi xation,
and alteration of positive photographic prints],” Bulletin de
la Société française de photographie, vol. 1 (19 October
1855), 286–96.
Eaton, George T., “History of Processing and Image Stability,”
Pioneers of Photography: Their Achievements in Science and
Technology, ed. Eugene Ostroff, Springfi eld, VA: The Society
for Imaging Science and Technology, 1987.
Eder, Josef Maria, History of Photography, trans. Edward Eps-
tean, New York: Columbia University, 1945.
Glafkidès, Pierre, Photographic Chemistry, trans. Keith Hornsby,
vol. 1, London: Fountain Press, 1958.
Hunt, Robert, Photography: A Treatise on the Chemical changes
produced by Solar Radiation and the Production of Pictures
from Nature, by the daguerreotype, Calotype, and other Pho-
tographic Processes, London: John Joseph Griffi n, 1851.
Ostroff, Eugene, “Anatomy of Photographic Darkrooms,” Pio-
neers of Photography: Their Achievements in Science and
Technology, op. cit.
Spiller, John, “Researches on the Hyposulphites and other Fix-
ing Agents,” Photographic News, vol. 12, no. 489 (1868),
29–31.