>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
Thanks for sharing this experience, Philip, it saves me exhausting my
patience: the sun is not a conspicuous feature in the UK meteorological
scene at present!
>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.
Rough calculations suggest a plausible lens for a 1:1 whole body image
would be of focal length about 2000mm and aperture f/100; this gives an
illuminance at the focal plane requiring an exposure of around 1000 hours
for a sensitizer with a quantum efficiency of unity. A teeny bit slow, I'd
have thought.
>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.
Good point. The thermal reaction will tend to overtake the photochemical one.
>It's important to remember that the invention of photography in the early
>19th century depended heavily on recent chemical advances.
and some very shrewd exploitation of serendipitous observations.
>I don't think Niepce regarded his famous eight hour exposure as very practical
According to the recent researches of Jean-Louis Marignier at CNRS, Orsay,
Niepce's exposure must have been a considerable multiple of this - the
'famous eight hour exposure' is actually erroneous photohistory - five days
exposure, or so, now seems much more probable.
>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?
No. Chromite is FeCr2O4 - a spinel containing chromium(III): to be any use
photochemically it would have to be oxidised - either by air in molten
caustic alkali, or by potassium nitrate - to give a chromate(VI). A bit
implausible, but not impossible for the alchemists of the day. There is a
less abundant ore with the right valency (oxidation state VI): crocoite,
PbCrO4, a nice yellow pigment; but it's too insoluble to work. It would
have to be converted to a soluble chromate first - again, not impossible,
but there's no evidence this was done before the eighteenth century.
Sorry to be so eggsacting.
Mike