I'm going to chime in and suggest bleach/redevelop as well. The trick is to use a staining developer like PMK or Pyrocat, etc. (any pyrogallol or catechol staining developer will work). Selenium intensification works, but will only get you 1/2 to maybe 1 paper grade difference if you're lucky. Then the other methods won't work...
I've had very good luck salvaging underdeveloped negs with bleach/redevelop. The process seems to add quite a bit of shadow detail that I though would go lost as well.
Here's my method, adapted from my personal darkroom guidebook:
"This method adds contrast to negatives that need more density, whether developed in a staining developer or not to begin with. This is a two-step process: first, bleach the negative to completion in a rehalogenating bleach, then redevelop it in a staining developer. All the steps can be carried out with lights on.
1. Presoak the neg for about 3 minutes.
2. Bleach the neg in a rehalogenating bleach until the image is completely gone and then a bit to be safe (formulae below). If you've used a staining developer to start with, a very faint residual stain image will remain.
3. Rinse the bleach off (I use the water-soak tray) then redevelop in the staining developer of your choice (I use PMK). Time is not critical, but it is important to develop the negative to completion, so leave it in the developer a few minutes longer than the recommended normal to be safe."
This process increases contrast by the amount of stain deposited in addition to the original, redeveloped silver image. As mentioned above, it can be repeated, although I have never had to.
There seems to be a number of ferricyanide/bromide formulas that range from 15-50 grams/l of the two bleaching ingredients with or without other ingredients. Some use the Bleach A from a sepia toner. Strength of the bleach just affects bleaching speed. I had good results with:
Potassium ferricyanide 20g (200ml 10% solution)
Potassium bromide 7g (200ml 3.5% solution)
Water to make 1 liter
If I'm just doing one or two 4x5 negs, I usually divide this recipe by four and just make 250ml.
I keep 10% ferri and 3.5% bromide solutions handy for this and other uses (SLIMT, etc.), but you can make up what you need from scratch easily enough. Don't toss the bleach after use; save it and use it for the next time you need bleach/redevelop. If not contaminated, it keeps a long time.
Best,
Doremus
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