B&W Reversal - Proper agitation using hypo to clear highlights
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?
"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.
Last edited by PHOTOTONE; 06-29-2008 at 06:09 PM. Click to view previous post history.
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.
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?
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.
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?
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Why don't you go to the Ilford website, they have an excellent PDF file on Reversal processing.
I have referenced that file previously, thanks.
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.
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.
3. Bleach - clear negative image and cloudy silver halide positive image. The negative silver is now in the bleach as silver sulfate.
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
9. Fix - not too much change here
11. photo flo
I hope this helps and I hope I got this right as a generic process.
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...