1156
Chromatic photographs on textiles of the 1850s, but the
Prussian blue images were alkali-bleached to iron(III)
hydroxide, which then served as a mordant to bind
vegetable dyestuffs.
The Palladium print is a close analogue of the plati-
num printing process, and came to be much-used from
1916, when its invention was credited to William Willis,
whose Platinotype Company marketed a commercial
Palladiotype paper. There is, however, evidence that
prints in palladium were made and exhibited as early
as 1856 by Charles Burnett of Edinburgh, as described
below under uranium processes.
Several siderotype processes yielded an image of
silver, by the iron(II) photoproduct reducing silver
nitrate. This theme originated in 1842 with Sir John
Herschel’s Argentotype. His sensitized paper was coated
with ammonium iron(III) citrate, and the development
bath contained silver nitrate. In 1843 Herschel also de-
vised his curious Breath print process, using a mixture
of ferrotartaric acid and silver nitrate. No image was
visible after exposure, but it sprang into existence as
soon as the paper was breathed upon, because exhaled
moisture promoted the development reaction. Towards
the close of the nineteenth century, several derivatives
of Herschel’s argentotype appeared: variously dubbed
Brownprint (patented by H. Shawcross in 1889), Se-
piatype (Sharp and Hitchmough Company 1891), and
Vandyke (Arndt and Troost 1895), they mixed ammo-
nium iron(III) citrate with silver nitrate, adding tartaric
acid to inhibit precipitation of silver citrate; images
printed-out in shades of brown, were washed in water,
and fi xed in dilute sodium thiosulphate. A closely-
related process, but employing iron(III) oxalate, was
the Kallitype, so-named and patented by W.W.J. Nicol
in 1889, 1890, and 1891, although this well-explored
formulation had been anticipated as early as 1844 by
Robert Hunt in an unnamed process. Kallitype needs
an alkaline-buffered developer to avoid re-dissolving
the silver image. Owing to diffi culties in fi xing and
clearing, iron-based silver prints were generally prone
to deterioration, and acquired a poor reputation for
permanence. Despite wide publicity of these processes
in the nineteenth-century photographic literature, sur-
prisingly few historic specimens have been positively
identifi ed in present-day collections.
The most striking of Herschel’s 1842 discoveries
was Chrysotype, which provided deep purple images in
nanoparticle gold (a pigment known to ceramicists as
the Purple of Cassius). For chemical reasons, Herschel’s
procedure required the gold salt to be in a developing
bath or wash; such profl igacy inhibited his pursuit of
the process, but a number (ca. 20) of his specimens have
survived perfectly to the present. Later attempts to re-in-
vent the gold process (Robert Hunt 1844, Alfred Jarman
1897) re-named it Aurotype. By the end of the nineteenth
century, the employment of gold as a printing medium
had been completely discounted, although it continued
to be much used for toning silver images.
Between 1842 an 1844, Herschel was also striving
to perfect an iron-based process providing an image in
metallic mercury; he found indications that both nega-
tive- and positive-working versions might be possible.
This may account for Talbot’s suggestion of the name
Amphitype for this process (not to be confused with the
1849 silver Amphitype of Blanquart-Evrard), replacing
Herschel’s original name of Celanotype (also spelt Ke-
laenotype). According to Herschel, his mercury photo-
graphs were the most exquisite imaginable. The process
was doomed to failure, however, because mercury metal
is volatile, and in the space of a few days or weeks such
images simply evaporate. Specimens authenticated by
Herschel’s own annotations exist today as stained scraps
of paper, without any discernable images.
It has long been known that iron(III) salts react with
gallic or tannic acid to produce intense black pigments;
iron-gall ink has been the chief writing medium since
medieval times. Procedures for making photographic
images in this substance devised by Colas (1883),
Alphonse Poitevin (Ferrogallic process 1859), and R.
Nakahara (1894), were all positive-working and chiefl y
used for copying line drawings and text, rather than
continuous-tone pictorial purposes.
The Dusting-on or Powder process of Henri Garnier
and Alphonse Salmon (1858) made use of the hygro-
scopic property of iron(III) citrate to remain tacky, es-
pecially in sensitizers containing sugar or honey; it only
dries and hardens where exposed to light. A positive-
working image could therefore be obtained by dusting
over the exposed paper with a powdered pigment, which
adhered selectively to the shadow areas.
Johann Obernetter’s ferrocupric process of 1864 in-
volved a roundabout chemical procedure to yield an im-
age in the stable pigment, Hatchett’s brown, copper(II)
ferrocyanide. In Thomas Phipson’s little-known process
of 1861, ammonium iron(III) oxalate was the sensitizer;
it was reduced by light to an iron(II) salt, which reduced
potassium permanganate solution to the insoluble,
brownish-black manganese dioxide.Uranium processes
In this minor category, the oxidation-reduction chem-
istry is analogous to the previous iron case. Invented
by Charles Burnett over 1855 to 1857, the uranium
printing processes were capable of yielding images in
stable substances, just like the siderotype processes.
Under the action of light, and in the presence of organic
matter, a uranium(VI) salt was reduced to uranium(IV),
which in turn reduced a noble metal salt to the metal. By
this means, Burnett made the fi rst palladium prints in