JoeV said:
Is there any monochrome (B&W) transparency (i.e. slide) film being made these days? I think it was Agfa's Scala that was the last one I knew of, but alas, the Agfa conglomerate has been split up.
So, can reversal processing of B&W negative film be reliably done in a home-based darkroom? I am certainly not a chemist, but have been developing and printing for 20 years; however, reversal processing is entirely new to me.
There is Fomapan R100, which is a B&W 35mm movie film obviously designed for reversal. Foma is also selling a reversing kit for use in the home darkroom. Maybe you can get the Foma reversal kit somewhere, it's quite convenient to use.
I've done reversal in the home darkroom a couple of times. It isn't that complicated, only a bit lengthy. It helps if you have a processor, to keep the temperatures constant.
The basic idea of a reversal process is that your film emulsion consists of silver halide crystals. During exposure, every crystal is either exposed to light or it isn't; there is no in-between. In a standard development process those crystals that were exposed to light are developed to black, while the others are removed during fixing. This is what we don't want; we want those that were
exposed to light to be clear, while the
unexposed ones should be developed to black. So between developing the film and fixing it we have to include a number of extra steps. The process works as follows:
- Develop your film normally using a standard developer, without fixing it. Those silver halide crystals that has been exposed to light will go black. Because we want to reverse the film, this is precisely what we don't want; we want the exposed parts to be clear in the end.
- So now we start the reversal. Put the film into a bleaching bath. By bleaching away the developed silver halide crystals, only the unexposed silver halide crystals will be left.
- Put the film into a clearing bath.
- We now have a film where the parts that were exposed to light are blank, while those that weren't exposed to light have unexposed silver halide crystals. Those are the portions of the film we want to have black in the end, so now we expose those to light by taking the film out of the development tank and putting under a strong light for a couple of minutes. We now have a film where all silver halide crystals that
weren't exposed to light in the first place are ready for development.
- Put the film back into the development tank and develop it normally, using stop and fixer.
You can use basically any developer (well, I guess staining developers would be bad). The bleaching bath consists of potassium permanganate and sulphuric acid, the clearing bath consists of sodium bisulfite; I guess you can find the precise relations online as well as more detailed descriptions of the process.
This process has two problems. The first is that obviously you need a film with a rather clear base, because if your base is grayish or bluish you'll end up with grayish or bluish slides. The second is that the process places a lot of stress on the emulsion, because the pH value of the various baths is wildly different. So in order to keep your emulsion from dissolving, you have to be quite careful that all your baths are at the same temperature. I normally included a short water bath between all the individual steps, because water has a pH value somewhere in the middle and that way the emulsion can adapt step-by-step instead of going from the alkalic developer straight into the acidic bleaching bath. If you take precautions, however, the results are fairly impressive.
Philipp
