B&W reversal with post-bleach reduction

This is old news, but worth putting on Photosensitive for the benefit of those who end up here after Google searches for B&W slide information.

The standard chemical process for B&W reversal (slides) involves a first developer containing a thiosulfate or thiocyanate salt, followed by a permanganate or dichromate bleach, re-exposure and second development (or treatment with a sulfiding toner like alkaline thiourea), fixing, and washing. The purpose of the thiosulfate or thiocyanate salt is to act as a silver solvent — an agent that removes a small amount of undevelopable silver halide from the film during development. If these salts are omitted, too much silver halide is retained in the film, and on re-exposure and second development the film gets an overall gray haze, even in bright highlight areas (which should be clear). This is why people say the purpose of the silver solvent is to ‘clear the highlights’.

Turns out there’s another way to do it. Some time ago, Julio of Montevideo, Uruguay posted his own B&W reversal process on the Nelsonfoto forums. He avoids mixing the silver solvent with the first developer, instead including it in a separate step, after the bleach stage. The advantage is that you can “clear the highlights by inspection” on the pure silver halide positive image (which appears creamy-yellow on the bleached film and can be handled in room light). It takes time to nail the process, but also takes some of the guesswork out of reversal processing. I tried it and it works (though I got streaky results due to under-agitation).

Anyone interested in this stuff should also check out Jens Osbahr’s reversal process (PDF).

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