Chromium intensification: Two questions

Context: doing tests since 2 months to qualify chromium intensification before using it on actual film that need it. The film I need to intensify is Fuji Neopan 400 120. Developed as per Fuji's datasheet, D-76 1+1 9'30" @20°C. Contrast too low for comfortable printing; can't give value because I phtographed scenes, not gray scales. Test are made with Fuji Neopan 400 135, same devt. Contrast (well, slope, do NOT want to argue about the relative merits of the various definitions) is ~0.45. After some initial trouble from using "household" quality HCl, I have decent intensification. Concentrations in working solution: 1% dichromate and 0.25% HCl (from maximum strength HCl 37% w/w). Re-development in HC-110 dil.E; also tried FX-37 (similar result), Rodinal, Dektol 1+3. BUT, when plotting D(after) versus D(before), the slope, initially >1 (intensification!) tapers off. Process can't boost high densities (in relative value) as well as low densities.


Question 1. Anyone here has personal experience with chromium intensification? Pre-soak time? Time in intensifier: just the time to bleach, or twice that, or what? Agitation? Wash time after bleach? Developer; which? Use normal time for that film, or develop to "gamma-infinity" (how long?). Any sensitometry like I show?

I have the impression that the problem does not lie in the re-development but in sub-proportional deposition of chromium. To prove it, I should measure densities before re-development. TBC.

Question 2. More out of curiosity, a question to qualified chemists, a few of which seem to be around. The oxidizer is Cr+6/Cr+3, that has a potential larger than Ag+/Ag. So, in the presence of Cl- ions, we have re-halogenation. But, how does the chromium get to Cr0, supposedly the cause of intensification? The potential of Cr+3/Cr0 is less than Ag+/Ag. And, (related to first question) what would influence how many chromium moles end up as Cr0 per mole undergoing Cr+6->Cr+3?

By advance, thank you for your time.

According to Grant Haist (Modern Photographic Processing) Chromium intensifier works by bleaching the metallic silver image back to silver chloride and a chromium compound (which was not further described) is added to it. It is this additional chromium compound that intensifies the image. When the bleached image is redeveloped it consists of the metallic silver plus the chromium.

Bernard,

While we're on the topic of bleach-redevelop intensification, let me make a suggestion.

Perhaps you could try bleaching and redeveloping with a staining developer such as PMK. I've had pretty good luck intensifying weak negatives this way.

Basic process is:
15g potassium ferricyanide
15g potassium bromide
1 liter of water

Presoak the neg, then bleach until the image is completely gone or until only a very faint residual image remains. Rinse the bleach off then redevelop in the staining developer. As mentioned, I've had good luck with PMK. Some maintain you can repeat this process to build up even more density. I've never repeated it more than once. I get more shadow detail as well; a nice side effect.

The resulting negative is the silver image plus the extra stain. Since stain formation is proportional to silver density, you may have better luck intensifying the highlights.

Best,

Doremus

Doremus, Gerald,

Thank you for your input. So, the "chromium compound" shall remain a mystery of photgraphic chemistry. As for staining developer, I had that on my list in case regular chromium + developer does not meet my goals. But I had in mind rather Pyrocat HD, and still bleaching in dichromate. According to some accounts (tbc) Pyrocat HD has a less pronounced compensating behavior than PMK. But since I have no personal experience I'm not going to argue on their relative merits. When I'm through, I'll post an account of the results.