Dichromate vs Tanning Developers
Originally Posted by holmburgers , 10-29-10
So my question is, is gelatin hardened in a tanning developer fundamentally different than gelatin hardened by K-dichromate?
and answered by holmburgers, 11-01-10
Seek, and you shall find....
J.S. Friedman, History of Color Photography, pg. 443
"...tanned gelatin has properties identical with light-exposed dichromated gelatin."
This is not completely accurate. While the chemical cross linking may (no one really is sure) be "identical" the results are not. Gelatin hardened by tanning developers (think carbro, dye-transfer) has a fixed end point or "boundary" beyond which no more can be washed away without physically abrading the emulsion. Dichromate hardened gelatin (think carbon,Fresson) has a very soft boundary between the soluble and insoluble that easily washes away and is difficult to control. This difference in repeatable results, most evident in highlight detail and neutral color balance, is why it is so difficult to make two continuous tone color carbons look exactly alike.
If a Cr3 hardened gelatin emulsion is used to make matrices for tricolor dye transfer , the unpredictable results would require that the printing of each color be individually and laboriously determined by trial and error. Silver-reactive tanning developers, by comparison, provide the controlled and repeatable results which are necessary for a viable color print process. Technicolor, a contact printing process with no need for the enlarger-speed silver based matrices of dye-transfer, switched from dichromate to silver for this reason.