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72-73.
CYANOTYPE
During the brilliant summer of 1842, Sir John Herschel
was experimenting with highly-coloured, light-sensitive
chemicals, when in June, following a suggestion by Dr
Alfred Smee, he chanced upon a novel process for print-
ing photographs in Prussian blue—a pigment already
familiar to painters for over a century. Herschel called
his process Cyanotype, which was negative-working in
its simplest form. He also devised a positive-working
version, but this presented diffi culties that were partially
solved in 1877 by Henri Pellet. Herschel published his
processes in 1842, without restraint by patent, like all
his photographic inventions. Cyanotype did not begin to
enjoy wide use, however, until 1872, the year following
Herschel’s death, when it was taken up commercially by
Marion and Company of Paris, who bestowed upon it the
proprietary name of “Ferro-prussiate,” and marketed a
paper chiefl y for copying plans in drawing offi ces. Thus
the word “blueprint” entered our language, to describe
the fi rst reprographic process, with its advantages of low
material cost compared with silver photography, and a
simplicity of processing that required nothing but water.
The manufacture of blueprint paper grew rapidly into a
profi table industry, becoming the dominant process of
reprography for the next 80 years.
Cyanotype is just one of several iron-based processes
for positive printing invented by Herschel in 1842. The
key to them all is the light-sensitive substance, ammo-
nium ferric citrate, which was then being promoted by
pharmacists as an iron tonic. This salt is mixed, in ca. 20
per cent aqueous solution, with ca.16 per cent potas-
sium ferricyanide, to make the sensitizer for coating
plain paper. On exposure to sunlight, the ferric salt is
reduced to the ferrous state, which then combines with
the ferricyanide to yield Prussian blue (also known as
Turnbull’s blue)—which is not ferrous ferricyanide, as
was long-believed, but ferric ferrocyanide. Ammonium
ferric citrate is an ill-defi ned substance, with no precise
formula, and Prussian blue also varies in composition,
so the following chemical equations representing these
two reactions are necessarily approximate:
Illustration
UV light + 2Fe3+ + C 6 H 6 O 7 2- → 2Fe2+ + C 5 H 6 O 5 + CO 2
UV light + ferric + citrate → ferrous + acetone + carbon dioxide
ions ions ions dicarboxylic acid
Fe2+ + K 3 [Fe(CN) 6 ] → KFe[Fe(CN) 6 ] + 2K+
ferrous + potassium → ferric ferrocyanide + potassium
ions ferricyanide (Prussian blue) ionsAs the exposure proceeds, the image prints out in
Prussian blue, but dense regions of the tonal scale re-
verse to pale grey (overexposure causing the formation
of Prussian white, ferrous ferrocyanide, K 2 Fe[Fe(CN) 6 ]),
which gives the provisional image a solarized appear-
ance. However, re-oxidation by air restores the shadow
densities fully within a few hours. Because the unex-
posed sensitizer and reaction products are very soluble,
the cyanotype needs nothing more than a wash in water
to complete its processing—a fi xing procedure that
Herschel regarded as ideal. Cyanotypes have a perfectly
matte surface, with no binder layer; the monochrome
blue may vary in hue, and there are numerous methods
of toning the image to violet, green, brown, red, or even
black, as discovered by John Mercer in the 1850s. Like
all iron-based processes, cyanotype has a very low sen-
sitivity to light and can only be used for making contact
prints and photograms.
Herschel used his invention to copy steel engrav-
ings and scientifi c notes, but it was taken up chiefl y by
botanical illustrators for making photograms of plant
specimens; the fi rst and most celebrated practitioner of
“autobotanography” was Anna Atkins, a friend of the
Herschel family, one of the fi rst women photographers,
and the author of the world’s fi rst book illustrated with
photographs, British Algae: Cyanotype Impressions.
Over the period 1843–61, Anna Atkins, in collaboration
with her close friend Anne Dixon, hand-printed several
elegant cyanotype albums of botanical and textile speci-
mens, which have now become treasures of the early
photographic canon.
As John Mercer discovered in the 1850s, the cyano-
type process is also particularly well-suited for printing
photographs onto cotton textiles: the sensitizer is cheap
and non-toxic, processing is easy, and the absence of
any binding agent ensures that the fabric remains uns-
tiffened, and can drape in a natural fashion. In the 1890s
a company existed in North America to print clients’
photographs as cyanotypes onto pillow covers, quilts,
and other soft furnishings.
Regarding the pictorial use of cyanotype, many
connoisseurs experienced aesthetic diffi culty with its
ineluctably blue images. In Britain, Peter Henry Emer-
son set the tone with his acerbic dictum: “No one but a
vandal would print a landscape in red, or in cyanotype.”
The English photographic establishment deemed cya-
notypes to be inferior, and unworthy of acquisition or
exhibition. The process was largely confi ned to proof-
ing the domestic negatives of hobbyist photographers.
As a consequence, 19th-century pictorial cyanotypes