1077
paper.” For the next ten years he used it to make his
“photogenic drawings,” by exposure to sunlight in
contact with fl at, semi-opaque objects, such as leaves,
lace, clichés-verres, or printed pages—a type of image
now called a photogram. In August 1835, using the same
sensitized paper, Talbot succeeded in making the fi rst
camera photographs in silver, which he referred to as
“Views taken with the camera obscura. The pictures ...
represent the scene reversed with respect to right and
left, and also with respect to light and shade.” It was
not until 1840 that Sir John Herschel proposed the noun
“negative” for such a photograph having a reversed tonal
scale, but neither he nor Talbot ever used the expression
“photogenic drawing negative,” which has been coined
in recent times to distinguish a camera negative recorded
on photogenic drawing paper.
Talbot choose the fi nest rag paper available—cus-
tomarily, the gelatin-sized writing-paper from John
Whatman’s Turkey Mill in Maidstone, Kent. He im-
mersed each sheet for a few minutes in a dilute (ca.
one per cent) solution of sodium chloride (common
table salt), then, after blotting the sheet dry, he brushed
one side of it with a strong (ca. 18 per cent) solution
of silver nitrate. The result was to precipitate silver
chloride within the fi bres of the paper, according to the
chemical reaction:
ILLUSTRATION
AgNO 3 + NaCl —> AgCl + NaNO 3
silver + sodium —> silver + sodium
nitrate chloride chloride nitrate
The high concentration of silver nitrate ensured that
an excess of this substance was retained within the
paper—a condition that Talbot had found essential for
light-sensitivity. The paper was usually dried in front
of a fi re. Upon exposure to sunlight for a few minutes,
it turned a rich purplish-black, due to the formation of
metallic silver in a fi nely-divided state, according to the
photochemical reaction:
ILLUSTRATION
light + AgCl —> Ag +^1 / 2 Cl 2
UV + silver —> silver + chlorine
light chloride metal gas
The chlorine evolved was rapidly absorbed by other
constituents of the sensitized paper, otherwise it would
have reversed the reaction. This is the silver chloride
printing-out process, in which the silver image is formed
entirely by the action of light; consequently, it has very
low sensitivity compared with development processes.
In contrast to the ease of making photograms with
“photogenic drawing paper,” it proved barely sensitive
enough to yield negatives in a camera obscura, which
had to be small, with a lens of large aperture to maxi-
mise the brightness of the image. Even so, exposures of
about one hour were required, and the subjects had to
be brightly sun-lit. Talbot’s earliest known photogenic
drawing negative—of the sky seen through a latticed
window at Lacock Abbey—is dated August 1835. This,
and other photogenic drawings, were fi rst exhibited to
the public at the Royal Institution on 25 January 1839,
when Michael Faraday announced Talbot’s invention.
After exposure, photogenic drawing paper remains
sensitive to light owing to the unchanged silver chloride.
In February 1835, Talbot discovered that the obliteration
of his images could be prevented by treatment with a
saturated (32 per cent) solution of sodium chloride, or
a solution of potassium iodide; the former rendered the
silver chloride much less light-sensitive, and the latter
converted it to inert silver iodide, so that his photographs
could conveniently be viewed in daylight. These were
the fi rst fi xing processes: chloride-fi xed specimens were
often reddish-brown in the shadows and tended rapidly
to acquire a characteristic pale lilac “veil” over their
highlights, which Talbot found quite attractive; iodide-
fi xed images showed primrose-yellow highlights due
to the colour of silver iodide. Both types of photograph
remain somewhat light-sensitive, however, and cannot
be exhibited without risk of perceptible damage. It has
been estimated that a light exposure of only 3–4 hours
under the most stringent gallery illumination of 50 lux,
may cause a just-noticeable change in a halide-fi xed
photogenic drawing.
In January 1839, Sir John Herschel discovered the
more effective “hypo” method of fi xing—or “washing
out” as he more accurately described it—using sodium
thiosulphate (then known as “hyposulphite of soda”) to
dissolve out the residual silver chloride entirely. Photo-
genic drawings fi xed by this means are stable to light,
but if the sodium thiosulphate itself has not been fully
washed out of the paper, they may fade severely owing
to the slow conversion of image silver into yellowish
silver sulphide:
ILLUSTRATION
2Ag + Na 2 S 2 O 3 —> Ag 2 S + Na 2 SO 3
silver + sodium —> silver + sodium
metal thiosulphate sulphide sulphite
In March 1839, Talbot made the fi rst use of silver
bromide in photography—an important innovation for
which he does not receive full credit. His “common
photogenic paper” was treated with a ten per cent solu-
tion of potassium bromide to convert the silver chloride
into silver bromide, and then coated with excess silver
nitrate, to yield a more sensitive paper. Talbot privately
called this his “Waterloo paper,” and employed it with
some success in his cameras until the use of photogenic
drawing paper for negative-making was totally eclipsed,