1157
1856, and obtained fi ne images in gold and silver. The
uranium salts were washed out in the wet processing, so
the fi nished Uranium prints, as they were inaccurately
called, should contain no uranium, only the precious
metal, and are therefore not radioactive. However, this
is not true of Burnett’s other uranium printing process of
1855, sometimes called Uranotype, as a close analogue
of the Cyanotype: the uranium(IV) photoproduct reacted
with potassium ferricyanide to yield uranyl ferrocyanide,
a Bartolozzi-red pigment (also formed in the “uranium
toning” of silver or platinum prints). Such photographs
will be radioactive. In 1858, Niépce de Saint Victor also
published, and sought patent rights for uranium printing
processes essentially identical to those published by
Burnett a year earlier. Unsurprisingly, this gave rise to
some highly acrimonious exchanges in the photographic
periodicals of the day.
Dichromate Processes
The orange-coloured, water-soluble dichromates of
sodium, potassium, and ammonium were called “bi-
chromates” in the nineteenth century—a nomenclature
now disapproved as chemically misleading, but which
persists in photohistorical usages. Under the action of
light, dichromates are reduced by organic matter to
chromium(III) salts, which have the ability to harden
organic colloids, such as gum or gelatin, as explained
under light-sensitive chemicals. If a pigment is incor-
porated in the sensitized layer, it will be retained where
light has fallen and rendered the colloid layer insoluble,
but may be washed off the paper where the colloid re-
mains soluble—in the unexposed regions—thus provid-
ing a negative-working photographic process.
The major nineteenth century dichromate processes
may be distinguished by the colloid: Carbon printing
uses gelatin, Gum Bichromate employs gum Arabic
(gum Acacia), but in the Fresson process the identity of
the colloid still remains a proprietary secret. Dichromate
processes are also of great importance in the preparation
of plates for photomechanical printing processes.
Mungo Ponton was the discoverer of light-sensitivity
in dichromated paper in 1839; he noted that exposure
caused a colour change from yellow to brown, and the
former could be washed out, leaving a negative-work-
ing image in white on greenish-brown. Henry Talbot
experimented with dichromated gelatin in 1852 with
a view to using it for photomechanical printing, but
Alphonse Poitevin is generally acknowledged as the
major promoter of the photographic pigment printing
processes in 1855. John Pouncy obtained a patent for the
gum process in 1858. Some named minor variations on
the major dichromate processes will now be outlined.
The direct carbon process of 1878 due to Frédéric
Artigue, also known as Charbon-Velours, was improved
by his son, Victor in 1893, and was the forerunner of the
Fresson process; the image was developed by a mildly
abrasive suspension of sawdust in water. Autotype was
the name adopted by the Autotype Fine Art Company, set
up in the 1870s, for its carbon transfer tissues, marketed
in a variety of pigment colours. Lambert-type (1875) was
a carbon transfer from the surface of collodionised glass,
so produced a print surface of notable brilliance. The
Photo-aquatint was a re-naming of the gum bichromate
process in 1894, when it was popularised by the skilled
exponents, Alfred Maskell, Robert Demachy, and Alain
Rouillé-Ladevèse.
Dusting-on or Powder processes are also possible
with dichromated colloids. In preference to iron salts,
Garnier and Salmon turned to dichromated gum and
sugar in 1859, relying on this hygroscopic colloid to
remain ‘tacky’ in the absence of light. Pigment was
dusted onto the exposed surface, as described before,
to yield a positive-working image. It is alleged that
the funerary ashes of cremated loved-ones could thus
be used to constitute their own portraits! Alessandro
Sobacchi’s Anthracotype of 1879 was also a dust-on
process, using graphite powder as the pigment. There
is also an Ink processes in hardened dichromated col-
loids due to G.W. Perry in 1856 or V.J. Sella’s process
of 1857. Thomas Manly’s Ozotype of 1898 included
manganese(II) salts in the dichromate sensitizer, and
essentially produced hardening of a separate gelatin
layer by diffusion transfer. Manly’s Gum ozotype of
1899 was the analogue using gum Arabic.
A few dichromate processes differ from those above
in not entailing colloid-hardening. The Aniline process
of William Willis senior (1864) relied on the residual
dichromate, after exposure, to exert a powerful oxidising
action on aniline vapour, producing in the unexposed
areas intensely coloured “aniline dyes” of the mauveine
type. It enjoyed some importance as an early positive-
working reprographic process for plans.
The Chromatype process of Robert Hunt (1843) had
several manifestations: he added copper(II) sulphate
to the potassium dichromate to improve its sensitiv-
ity; the image substance formed by development was
an insoluble chromate of a heavy metal such as silver,
mercury or lead, which are all highly coloured—orange
and red. This process is positive-working, commonly
producing a yellow image on an intense red ground of
silver chromate, and, because dichromate solutions tend
to penetrate paper, the image is usually clearly visible
on the verso. Hunt also employed gold chloride to de-
velop a Gold chromatype in which the fi nal image was
deep violet nanoparticle gold. Burnett’s little-known
dichromate-based Cuprotype of 1857 resembles Hunt’s
chromatype in its sensitizer, but the image substance
formed was Hatchett’s brown, copper(II) ferrocyanide,
as in Obernetter’s process.