**************Dichromate becomes decomposed even by uv light only if organic
substances are present. During the reaction, which is accelerated in the uv part
of the spectrum, oxygen is set free. It has been shown that this oxygen oxidizes
the gelatin. However the oxidation products are not responsible for the
gelatins tanning, because with the exception of the chrome content, tanned and
untanned gelatin have the same composition. To give a reaction sheme for the
reduction is difficult, as during the reduction of dichromate several easily
decomposed compounds develop. Basically the following reactions are considered:
2 K2Cr2O7 -> h*v -> 2 K2Cr2O4+ CrO2 +O2 (The h*v should be over the arrow)
another aproach is::
6 h*v + 3 K2Cr2O7 + 6 H -> (CrO2)CrO4 + 6 KOH
It is not clear yet, which compound has the tanning effect. On one hand the
Chromichromate with the formula Cr2O3*CrO3 or CrO2 is considered, but on the
other hand the reaction product CrO2 or (CrO)2CrO4 is discussed.
Newer research shows, that stable Cr(III)-compounds are creating a crosslinking
of the gelatin. The formation of Cr (III)-compounds has bee described as
follows:
K2CrO4 -> H2O -> HCrO4- -> Cr(V)-Ion -> Cr(III)-compound (The H2O should be over
the arrow)
*********************
Juergen quotes the following books: Walther, W.: Fotografische Verfahren mit
Silberhalogeniden, VEB Fotokinoverlag, Leipzig 1983, p. 61 ff; Watter, O.: Die
Lichtgerbung, VEB Wilhelm Knapp Verlag, Halle 1953, p. 1-61.
As I am not a chemist, I absolutely don't understand what I just have written
;-) but it sounds well, doesn't it? I'll go to my darkroom now and do some
carbon printing. And I'll keep wondering why it works...
Klaus