B&W Reversal - Proper agitation using hypo to clear highlights

Discussion in 'B&W: Film, Paper, Chemistry' started by amuderick, Jun 29, 2008.

  1. amuderick

    amuderick Member

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    I am dipping my film in a solution of 30g/L sodium thiosulfate to 'clear' my highlights after bleaching. My gut tells me that I need to use vigorous/continuous agitation to maximize my eventual Dmin while preserving as much Dmax as possible. The idea is to check the film every 15-30 seconds and once the specular highlights are as clear as the film base, remove it and wash it off.

    When I let the film sit in the hypo undisturbed, it seems to dissolve the areas with the most remaining silver halide (unexposed) more preferentially than it clears areas with less (i.e. highlights). The result is a muddy, low contrast image.

    Thoughts? Is there any chemistry to support this or am I imagining this problem? Does the agitation amount/type/frequency really matter here?
     
  2. PHOTOTONE

    PHOTOTONE Member

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    "HYPO" should not dissolve the black "developed" areas of any film..reversal or negative. If it appears to, then, in the case of your reversal processing, you are evaluating your image with a combination of undeveloped remaining silver and developed silver producing an image with more "black" look than obtained by the developed silver alone. Thus your reversal development is not vigorous enough to produce a good black. By just fixing to clear the highlights, you are not fully fixing your film, leaving unfixed silver salts in the "black" areas of the image that can and will deteriorate the image over time. Of course I know that "hypo" will indeed attack the developed portions of a film, but the time for that to happen is well in excess of the time needed for fully "fixing" the negative or positive. There should be a wide margin of time between fixing, and the start of image dissolving.

    I don't know what your process is, but I ask you: Are you fully washing out the bleach from the film before you go into the fix? If there is residual bleach in the film, it will over time convert the developed silver back to a salt, which the fixer will remove, thus giving you low contrast.

    Just as a point of conversation..in color E-6 processing we wish to remove ALL the silver from the film, leaving only a dye image, and the bleach is followed by a fix without an intermediate rinse. Hence the minor amount of bleach carryover does no harm. This is not the case with b/w reversal processing, where reduced silver IS the image.
     
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  3. amuderick

    amuderick Member

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    I do first develop, wash, bleach, clear, wash and then get to the hypo. When I reach the hypo stage all metallic silver has been removed. The latent image is visible as cream on clear. There is no metallic silver on the film. So, when I speak of 'clearing the highlights', I mean removing undeveloped silver halide such that during second development there is no silver in that region and the clear film base will shine through after second development is finished.

    So...

    I do my first development to completion using a 1st developer that contains no silver solvent. After bleaching, I want to do a hypo stage (as indicated above). At some point in the hypo, I will reach 'just right'...where I've removed enough halide so that the highlights are completely clear (no 2nd development will occur in that area) but I haven't removed any more halide than necessary.

    If I leave the film in the hypo for too short a time, the highlights will retain some silver halide and, after 2nd development, be darker than the film base reducing Dmin. If I leave the film in the hypo for too long a time, not only will the image appear 'overexposed', but even the deepest shadows will have a very low Dmax (muddy, low contrast).

    Returning to my original question, if I let my film sit in the hypo will it dissolve silver halide equally, or, more preferentially in areas with higher density than lower density? Does my agitation method matter?
     
  4. Photo Engineer

    Photo Engineer Subscriber

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    Getting good clear areas is a function of the developers not the bleach and fix.

    Lack of a silver solvent may not force development to completion and may leave gray whites. You are trying to jump into the deep end without knowing how deep it is. :D

    PE
     
  5. amuderick

    amuderick Member

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    I have no problem jumping into the deep. This project is about learning film chemistry as much as it is about the photos produced. I will experiment with adding hypo to the first development stage in various amounts to 'push' that process to completion and eliminate this as a separate step. Unless you have major objections, I'd like to use either HC-110 or Rodinal for my first developer. Can you recommend an amount of sodium thiosulfate to add to a 500ml mix of Dilution B? Right now I am running the first developer to 2x recommended time in an attempt to reach completion. With the silver solvent, should I reduce that or won't it matter?
     
  6. Ian Grant

    Ian Grant Subscriber

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    Why don't you go to the Ilford website, they have an excellent PDF file on Reversal processing.

    Ian
     
  7. amuderick

    amuderick Member

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    I have referenced that file previously, thanks.
     
  8. Lowell Huff

    Lowell Huff Inactive

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    The secret to great imaging in reversal processing is the First developer. It must be a "long scale" continous tone, compensating developer. The second key to reversal processing is a chromate-sulfuric acid bleach.
     
  9. Photo Engineer

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    Ok then, when you get to the hypo step in a reversal B&W process, there should be NO silver halide left in the film. If any area is milky or cloudy, something is wrong with the process. Here are the steps and appaeances:

    1. First developer - strong black negative silver image and cloudy milky positive silver halide.
    2. wash
    3. Bleach - clear negative image and cloudy silver halide positive image. The negative silver is now in the bleach as silver sulfate.
    4. wash
    5. Clear - not too much change here unless bleach is retained. If so, the color vanishes.
    6. Wash - reversal exposure
    7. Developer - strong black positive image
    8. wash
    9. Fix - not too much change here
    10. wash
    11. photo flo

    I hope this helps and I hope I got this right as a generic process.

    PE
     
  10. Jordan

    Jordan Member

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    Aaron has done a lot of reading on the reversal process and is referencing a modification of it that someone proposed on another forum a year or two ago. I wrote a brief blog post on my website (http://www.photosensitive.ca/wp/archives/68) for anyone interested. Not many people know about this technique, which is why I think everyone's getting confused here.

    The idea behind this modification is that instead of adding a thiocyanate or thiosulfate salt to the first developer as a silver solvent, you instead use a strong, non-solvent first developer and then incorporate a plain hypo bath after the bleach step to eliminate enough silver halide from the latent positive image to "clear the highlights". The net effect is the same as the traditional order for reversal processing, but because the highlights are cleared after bleaching, you can do it more-or-less by inspection.

    Following the hypo bath, the film is rinsed, re-exposed and re-developed (or treated with alkaline thiourea for sepia-coloured slides), rinsed again, fixed, washed and dried as usual.

    I've tried this method, and it works as advertised. It takes a bit of practice, but the results look nice. I used 3% sodium thiosulfate to do the reduction. I think the key is to be consistent with your agitation and thiosulfate concentration and then experiment with times (doing some clip tests would be good.) I would work on that this summer, but with a four-week-old baby in the house it's hard to find a solid block of time for those kinds of experiments...
     
  11. Photo Engineer

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    Jordan;

    I'm somewhat aware of this process but had pretty much dismissed it, if you will pardon the expression. You see, the silver halide solvent in a separate bath can also IMHO, bleach out the detail in both highlights and shadows if you miss the sweet spot even slightly, and the lack of solvent in the developer profoundly affects grain and sharpness that you would otherwise be entitled to.

    I was not aware that this is what was meant, and did not know that it was still in use due to the difficulty of hitting the sweet spot on a long roll of film. It probably works best with sheet films done one at a time.

    If you can get it to work, more power to you, but I would not recommend it myself.

    Sorry for the misunderstanding.

    PE
     
  12. Jordan

    Jordan Member

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    PE,

    I agree that there is necessarily a sweet spot and that it's easy to miss. (B&W reversal is all about sweet spots, which is why I don't play around with it as much as I used to.) I first encountered this method in a forum post about two years ago, where the poster appears to have stumbled on the idea himself. Is there some earlier patent or scholarly literature on this approach?
     
  13. Photo Engineer

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    I would have to review my library and Haist in particular. IDK. I know that Grant and I talked about this and other items like this. I also worked in the same office as some of the E6 team and heard their comments. I also am 'heir' to one of the rare chemicals used in the first developer for use as a silver halide solvent. I am trying to get a reasonable source for the Formulary to sell, but this chemical is VERY expensive. It gives a super boost to B&W reversal processes but Kodak will only sell it in $25,000 drums.

    PE
     
  14. amuderick

    amuderick Member

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    PE, I agree with you about the difficulty of hitting the sweet spot, especially on a whole roll at once. That has been a real part of the frustration with this process. How does integrating the silver solvent into the developer make the process less tricky? Does the developer moderate the silver solvent or have some kind of compensating effect?
     
  15. Photo Engineer

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    The silver solvent in the developer makes it more like a self limiting situation.

    PE
     
  16. amuderick

    amuderick Member

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    That is good news. Can you elucidate the mechanism of this for my chemistry-oriented brain? Thanks ;-)
     
  17. Photo Engineer

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    Well, two or more things go on when silver halide solvents are in the developer. The first thing is that with proper formulation fog goes up which in a sense is increasing the development until all silver is used up and this clears the highlights in the reversed image.

    The second thing is that the silver ion concentration in the developer goes up and it can be reprecipitated at other points increasing image density and contrast.

    The silver ion has to come from somewhere, and this is done by dissolving crystals making grain finer.

    This is all done at the expense of sharpness as it dissolves some finer grains and also can mute the edge effects. This is visually offset by the increased contrast. It is also the source of the other reference to fine grain is achieved at the expense of sharpness. The reversal process is an intermediate position that appears to contradict this statement as it does improve grain, but sharpness appears better due to the high contrast of the visual image.

    Microdol X is an example of a fine grain developer that uses a silver halide solvent.

    PE
     
  18. Jordan

    Jordan Member

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    PE, interesting to think of fog being a way to clear highlights (by consuming unexposed AgX). Most reversal first-developers are D19-based (plus thiocyanate) and I don't think of this as a "foggy" developer. Or is it?
     
  19. Photo Engineer

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    Actually, the developer may or may not be "foggy". The intent is to force all silver to develop in negative Dmax areas, even so called "dead grains". If any are left, then they surely develop after reexposure and second development. So, a forceful developer with silver halide solvent is often foggy when used as a reversal first developer. The fogging action is a byproduct of its intense activity.

    If you look at E6 film after first development, the film is visually foggy. This is due to the fact that the first developer is "foggy" with a silver halide solvent and high activity, and also the fact that the emulsions are relatively over sensitized with sulfur + gold to give high speed and this leads to a bit of fog. They don't care about the negative fog, as it vanishes in the reversal process and it helps clean out the dmin by leading to full development of all negative grains in negatie dmax areas.

    OTOH, I look at silver halide solvents as being natural fogging agents. Ammonia in a developer can cause strong fog, thiourea is a fogging agent and etc... Used properly they can yield a monobath or Microdol X and improper use can lead to dichroic fog or overall ordinary fog.

    PE