Ferri Bleach, Thiourea Sepia Toning and Gelatine Vulnerability. What Is Your Experience?

[Letters to Theo]

Hi all.

The title pretty much says it all. For the past few months, I’ve been focusing heavily on toning, but I’m finding it extremely difficult to leave the darkroom with a sepia-toned print that doesn't show even the slightest mark on its surface.

I’m sure you know what I mean. The defects I’m talking about aren’t visible when looking directly at the print, they are surface marks only visible when observed at an angle, where the gelatine looks as if it’s been etched.

There is literally zero margin for error: if a nitrile glove brushes the emulsion while the print is in the tray—at any stage—that’s a defect; if the print rubs against the side of the tray while washing, that’s a defect.

Handling the print by touching the emulsion in any way—whether with gloves or tongs—is simply out of the question.

I’m experiencing this with both Ilford and Foma papers, when using a ferricyanide bleach. I’m sure some of the experts here can explain the physical and chemical reasons behind this.

I personally print my 6×6 negatives in the centre of the sheet with wide borders, as I prefer framing without a mount. Even the smallest mark on the white margin is a dealbreaker for me.

All this is to ask: What is your level of tolerance regarding these defects? Is there a chemical solution to mitigate gelatine vulnerability?

Thanks in advance to anyone who joins the discussion.

 

[awty]

Do you filter your chemistry, so there's no debris.
Do you thoroughly wash your prints after fixing? Any trace of fixer will have a reaction. This especially shows up on fibre prints.
RC paper is easier to keep clean.
Maybe a weaker solution would help.

 

[MattKing]

Any bleach and re-develop process tends to "un-harden" the emulsion. Some papers are much more susceptible then others. "Soft" emulsions are prone to what you are experiencing.
After you finish toning the image, I always recommend fixing it again, in case you haven't fully re-developed the re-halogenated emulsion.
When you fix it, you can either use a fixer with hardener, or you can do what I do. I make use of the part B hardener that comes with the 1 litre bottles of Kodak Rapid fixer - the ones that I intentionally didn't mix in with the fixer. In the case of that hardener, mix it one part concentrate to 13 parts water. I have never seen anything reliable that gives capacity information for the mixed up working strength hardener.
If you use hardener, you will need to wash longer.

 

[argent_negre]

Hi! If you are in Eivissa, I guess the water will be very hard. Since thiourea toning is very is very basic (it requires a lot of hydroxide), calcium from the water diposits in the print in the washing step, after toning. In Barcelona, where the water is also very hard, I do a stop bath step (15grs of citric acid per liter) after the toning and before washing. This fixed my problems with ilford and foma papers.

 

[Letters to Theo]

Thanks all.

I use the classic formula (Anchell’s “Formulary Thiourea Toner”). I’ll try diluting it, the redevelopment is extremely quick so there might be some margin to reduce the concentration.

It seems to me however that the only effective solution is maximum attention and 110% focussing.

15grs of citric acid per liter

This is good advice; water hardness is extremely variable here. Once in a while I get those white deposits — more often actually when Lith printing, Ansco 70 is activated with just a bit less sodium hydroxide per litre.

 

[RalphLambrecht]

Do you filter your chemistry, so there's no debris.
Do you thoroughly wash your prints after fixing? Any trace of fixer will have a reaction. This especially shows up on fibre prints.
RC paper is easier to keep clean.
Maybe a weaker solution would help.

Good point. I suspect what the OP sees are traces of fixer residue. @OP: Is your fixer clear, or do you see a light milkiness?

 

[Letters to Theo]

No, definitely no fixer residues—I’m certain of that. I always use sodium sulfite and then wash my prints for at least thirty minutes. The deposits we were talking about are a different matter, they appear as a fine white powder.

The marks I am referring to, when observed at an angle clearly affect the emulsion, which appears slightly etched. They only show up during sepia-toning sessions. Every time I accidentally brush my glove or anything against the emulsion, I can clearly identify them once the print is dry.

 

[koraks]

I use the classic formula (Anchell’s “Formulary Thiourea Toner”). I’ll try diluting it, the redevelopment is extremely quick so there might be some margin to reduce the concentration.

It's certainly possible that the high pH of the sepia toner is ultimately related to your problem. However, I doubt that diluting the toner will be the solution, for two reasons:
1: Diluting the toner will not drop the pH all that far. It'll still be fairly alkaline.
2: If you dilute the toner, this will make the toning time longer, which means the gelatin is exposed for a longer period to the alkaline environment, giving it more time to soften. So this may in fact turn out to be counterproductive.
The problem with a thiourea toner is that it needs to be very alkaline to produce a satisfying chocolate brown tone. If you were to adjust the thiourea:hydroxide ratio (in order to make it considerably less alkaline), this will dramatically shift the image tone and also final density. At a lower pH, thiourea will become a yellow/tan toner.

A more promising avenue IMO would be to use a hardener after the toner; something like an acidified bath of alum (the common hardener used in conjunction or even as part of paper fixers) should help.
However, I'm kind of puzzled about this problem to begin with, since I've never noticed emulsion damage as a result of toning. My normal workflow (mostly using Foma papers) is to actually squeegee the prints after washing them. That's a heck of a lot more aggressive than just brushing a glove across the surface. Yet, I don't see any effect on the emulsion. So I wonder if perhaps something entirely different is going on.

A picture showing the problem might help.