This came up on the list once before and Mike Ware asked if anyone had any
experience with dichromated colloids for in-camera use. Having recently had
occasion to mix up some dichromated gelatine I poured a little extra on
paper, dried it, and exposed it. Conclusion: It is possible to form a very
low contrast image, if you have enough patience. Three mostly sunny days in
a f11 Agfa Clack resulted in an underexposed image of the Hills hoist in my
backyard; two sunny days in a f4 Zeiss Nettar produced an overexposed view
out my front window every bit as hard to interpret as Niepce's famous view
from his window in Gras!
This immediately raises the question of what sort of optical system Leonardo
might have used. I strongly suspect camera obscuras in his day simply
weren't up to forming life size images with sufficient illumination at the
focal plane. Pinhole cameras might be worth considering with other emulsions
but with the dark reaction occuring in dichromated colloids after about a
week they're probably out of the question.
It's important to remember that the invention of photography in the early
19th century depended heavily on recent chemical advances. I don't think
Niepce regarded his famous eight hour exposure as very practical - he
subsequently switched to printing-out processes for reproducing engravings.
And Wedgwood and Davy's "Method of Copying Paintings Upon Glass [i.e.
cliche verre], and of Making Profiles [photograms], by the Agency of Light
Upon Nitrate of Silver" (1802) failed not just for want of a fixer: "The
images formed by means of a camera obscura, have been found too faint to
produce, in any moderate time, an effect upon the nitrate of silver." Just
what Wedgwood considered a "moderate time" could be determined by
experiment, but in practical terms the invention of photography had to await
the discovery of other compounds of silver. There were major improvements in
cameras too - apparently Talbot's camera had an effective aperture of around
f3, far better than what Wedgwood probably used.
On the other hand, if I remember correctly the colour of the shroud seems
vaguely right and dichromated albumen seems so simple and intuitive that you
wouldn't have to be Leonardo to arrive at a workable process: grind up some
chromite ore; mix with albumen (this was, after all, standard procedure in
the days before paint came in tubes), apply to cloth, expose, and rinse out
in water. There's a bit of a question mark over exposure, as I indicated
above, and there may be other obstacles. Did chromite reach Italy in
Leonardo's time? Presumably it could have been imported from Turkey? Is it
the right valency if you just grind it up? Anybody trying this (Dan
Shapiro?) would be well advised to protect themselves from the dust. Or do
you have to roast it first to oxidize it? I'll leave these questions for the
list's resident chemists.
Foto temporarily yours,
Philip Jackson
pjackson@nla.gov.au
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