[pure-silver] Re: Chemical Fogging in Reversal Processing


Hi everyone,

For reference, the toning re-developer formula that I use is composed of 2 g thiourea and 100 g sodium carbonate in 1 L of water. When fresh, this stuff will "tone" silver halide almost instantaneously. As the thiourea concentration drops with repeated use, its action becomes slower.

I find that this re-developer gives pleasing sepia tones with conventional films (Pan F and FP4 are the ones I have used). With TMX the results were a sickly yellow colour -- not very pleasing to look at. The difference is almost certainly a result of the different grain size and crystal habit in the "new technology" film.

The end result with thiourea is the same as when sodium sulfide is used, but the thiourea is odourless, which is a major advantage. The chemical "driving force" for the reaction is the production of urea, which is a very stable compound. A similar procedure is used to make sulfur compounds in organic chemistry.

Jordan


Richard Knoppow wrote: 

----- Original Message ----- From: "John Banister" <jbanister@xxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Wednesday, August 16, 2006 7:37 AM
Subject: [pure-silver] Re: Chemical Fogging in Reversal Processing


> Thiourea is probably being used as a redeveloper in > place of
sulfide. The
> result is a sepia image of sulfide. Because either > Thiourea or
Sulfide works
> directly on the halide it does not need a fogging > exposure or
chemical fogging.
> It also eliminates the need for a final fixing step > because all of
the halide
> is converted to sulfide whereas there is always a small > residual of halide
> which is not made developable by flashing. I am > uncertain about
the effect of a
> chemical fogging agent but would guess its the same as > the use of
Sulfide, that
> is, no final fixing step is necessary.
>
> --
> Richard Knoppow
> dickburk@xxxxxxxxxxxxx
> Los Angeles, CA, USA

Is this situation with Thiourea/Sulfide unique, or do you think it's more
generally true that if a toned result is desired from a B&W reversal image that
the fogging - second development steps can be bypassed or would likely be
easy to incorporate in the toning process?
Normal two step sepia toning, for instance of a print, is done by first bleaching the metallic silver image to some form of Silver Halide, most usually Silver Bromide. The halide is then treated in a solution of Sodium Sulfide or Thiourea. When using sulfide or thiourea is used for "redevelopment" in reversal processing the image is already composed of a halide since the bleach used for reversing _removes_ the metallic image silver and leaves the undeveloped halide the film was composed of orinally. The second development can be done with a normal, rather active, developer, after treating the remaining halide with light or a fogging agent, to make it developable, or, it can be directly developed with the sulfide solution resulting in an image composed of Silver Sulfide. The Sulfide process is simpler since it eliminates both the flashing step and a final fixing step, which regular development requires. Provided the Sepia colored image is acceptable the sulfide process works fine and the image is very permanent in the same way as a Sepia toned silver image.
Either Sulfide or Thiourea can be used for redevelopment. In toners the Sulfide solution is just Sulfide and water and the same solution is used for reversal development. In toners using Thiourea redevelopment the redevelopment solution usually has some alkali in it to activate the Thiourea. The color of the Sepia image will depend on the nature of the original halide and, when using Thiourea the formula for the redeveloper. I have never seen a Thiourea formula for redeveloping in reversal development but probably any toner formula would work. Defender Photo (later owned by Dupont) published a series of formulae for toning Varigam paper with Thiourea. The redevelopers contain either Sodium Hydroxide or Sodium Cabonate, these formulas will probably work for reversal redevelopment.
One key to reversal is the presence of a halide solvent in the first developer. Some formulas used Hypo but the solvent mostly used was Sodium Thiocyanate. The first devloper must be a very active developer capable of producing high contrast and also developing fast enough to prevent too much solvent action. The solvent is needed because some halide exists in the original emulsion which is so insensitive to light that it is never exposed, no matter how much exposure is given in the camera. This halide remains to be developed in the reversal bath and causes veiling of the highlights. The solvent is present in an amount just sufficient to eliminate the veiling. The amount of solvent varies with the emulsion being developed. When using old formulas for modern film the solvent content, film speed, devleopment time, etc., must be determined by experimante
When the concentration of solvent is correct the film speed for reversal purposes is close to the speed for use as a negative. The original exposure and the first development are the only really critical parts of reversal. Bleaching and second development is carried out to completion. The flashing exposure must be sufficient to make all the halide remaining after the bleaching step redevelopable but not so much as to drive the emulsion into reversal or to generate photolytic silver. This is a pretty big window of exposure. The second developer is, again, an active developer with low solvent content. Any solvent in this developer may dissolve some of the halide needed for the final image and cause low density of the shadows. A good print developer or a high contrast developer like Kodak D-19 will work for both steps.
When a flashing exposure and conventional development is used for reversal there will always be some residue of undeveloped halide that survives the entire the reversal process. This halide must be removed by a final fixing step or it will eventually attack the image in the same fashion as in an incompletely fixed print or film. After fixing the film should be washed as usual. Again, the fixing step is not needed when sulfide is used to produce the final image because no halide will remain.
I have some formulas for reversal of Agfa motion picture films of the 1940's and Kodak publishes a set of formulas for their current B&W reversal films. I will post these if required.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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Jordan Wosnick
jwosnick@xxxxxxxxxxx
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