Ammonium or potassium dichromate dissolved in water sensitizes gelatin by
depositing a hexavalent
chromium ion Cr(VI) next to the long irregularly shaped polypeptide chains
of amino acids that constitute the gelatin. When this photosensitive gelatin
is exposed to actinic radiation a light quantum is absorbed and either
directly or indirectly reduces the Cr(VI) to a trivalent Cr(III) ion and
chromic acid (H2CrO4) or chromium oxide (Cr2O3). The trivalent chromic ion
Cr(III) is thought to form a cross-link bond between neighbouring gelatin
molecules. A small amount of water needs to be present for this reaction to
occur. Heat accelerates it, even in the absence of light (hence the "dark
reaction").
Unexposed Cr(VI) is later washed out during development and has no effect on
the gelatin, but the remaining Cr(III) has a differential hardening effect
directly related to the sensitized gelatine's exposure to UV light. This
increases the rigidity of the gelatine, raises its melting point, makes it
less liable to swell during development, and most important for the process
of development, makes it less soluble.
Development is largely a physical process whereby unexposed and therefore
soluble gelatin is washed away with warm water. The result is a variation in
thickness. Residual chemical compounds such as chromic acid and unexposed
Cr(VI) are removed.
Although there may not be a clear consensus on all the details, it seems the
reaction is basically the same in all dichromated colloids - gelatine, gum,
albumen, etc.
There is only one chemical reaction occuring - the brown print-out image
Pete thinks is a separate image forming mechanism is probably a combination
of green chromium oxide and residual unreacted orange dichromate. Hence the
colours he's encountered on clearing.
A brown print-out is indeed the image forming mechanism in Mungo Ponton's
original process, but most other dichromated colloid processes (excluding
collotype, bromoil and dusting-on processes BUT including direct carbon
processes) rely more heavily on differential hardening and the resulting
differences in thicknesses in the final layer of dichromated colloid to form
the final image. Direct carbon processes such as gum and Fresson differ from
carbon transfer only because of physical factors.
If I understand their differences correctly Pete relies on "the stain or
chrome oxidation layer" and a carefully regulated pigment to colloid ratio
"to achieve a reasonable tonal range gum print in one exposure"; Terry
prefers to clear all coloured residual chromium compounds leaving only
pigments and to build up an image with multiple printing. Pete's approach is
a little unusual but both Terry and Pete are both using exactly the same
chemical reaction.
Hope that clears the water somewhat,
Philip Jackson