Colour reversal process contrasted with B&W reversal - seeking clarity

[pdeeh]

I'm just seeking to clarify my understanding of the differences and similarities between black & white and colour reversal processes, from a purely theoretical point of view.

I'm thinking of modern E6 processes, not any of the other older ones, and certainly not Kodachrome!

Using AgX as shorthand for "Silver halides" just for ease of typing, and omitting some necessary but not sufficient steps for simplicity's sake. I'm hoping some chemists will read this, but I also hope they understand I'm not one and will forgive me howlers.

My understanding of the B&W reversal process goes as follows:

• 1st Developer: Reduces latent image (on AgX) to metallic Silver negative image. Leaves nearly all unexposed and some partially-exposed halides untouched
• Bleach (non-rehalogenating) : Removes all metallic Silver (has to be removed at this stage or 2nd Dev will simply "fill in the blanks" and no image will be left after all stages are complete)
• Fog: Forms (positive) latent image on remaining AgX
• 2nd Developer: Reduces latent image on AgX to metallic Silver positive image
• Fix: Removes remaining AgX.

My understanding of the colour reversal process goes as follows

• 1st Developer: Reduces latent image (on AgX) to metallic Silver negative image. Leaves nearly all unexposed and some partially-exposed AgX untouched
• Fog: Forms (positive) latent image on remaining AgX
• 2nd Developer: Reduces latent image on AgX to metallic Silver positive image, and oxidation products of this reaction creates colour dye image thereby
• Bleach (rehalogenating): Converts metallic Silver back to AgX, leaving colour dye images intact.
• Fix: Removes remaining AgX, leaving colour dye images intact

Correct so far?


I'm assuming that the metallic silver negative image from 1st development is entirely redundant once developed, and so can be rehal-bleached and then fixed out at the end of the process without worrying about it further.

Now, just as a matter of interest (no suggestion that anyone would ever want to do this), in the colour reversal process I can't see a reason why - after the 1st developer - the metallic silver could not be removed immediately with a non-rehal bleach, before proceeding with the remaining stages as they stand.

Again, to emphasise, this is purely a matter of my checking my understanding, not because I think the latter would be a clever thing to do.

 

[Daire Quinlan]

You probably could do, but it would be an entirely redundant step I guess. You're going to bleach and fix ANYWAY, because you have to get rid of the AGx and silver from the colour development step, so it makes sense to get rid of the silver from the first developer step at the same time.

 

[Ian Grant]

Some of the early colour reversal process did use a bleach stage after first development, but these were additive processes so essentially B&W emulsions, there's no need or point at all with Subtractive reversal processing as all the silver has to be removed anyway during the final Bleach/Fixer stages.

Ian

 

[Steve Roberts]

Having spent two hours yesterday afternoon reversal processing a roll of FP4, I am impressed that you can put the process so succinctly in a few lines!
Best wishes,
Steve

 

[pdeeh]

it's a relatively succinct process if you miss out all the clearing baths and various washes

 

[Photo Engineer]

That was my reaction! No clearing baths and washes!

Your comments are entirely correct in all aspects.

PE

 

[pdeeh]

Thanks PE

 

[Athiril]

Now, just as a matter of interest (no suggestion that anyone would ever want to do this), in the colour reversal process I can't see a reason why - after the 1st developer - the metallic silver could not be removed immediately with a non-rehal bleach, before proceeding with the remaining stages as they stand.

I've done this before. It's useful for rehal processing... that is, after colour developing and bleaching... washing then colour developing again (then rest of process as normal) to increase dye density.

 

[pdeeh]

Thanks for the additional info. I think I (vaguely) remember reading about this technique before - is it one used in astrophotography?

 

[Athiril]

Thanks for the additional info. I think I (vaguely) remember reading about this technique before - is it one used in astrophotography?

It's a bit convoluted/tedious to do in E-6.

In C-41, it's simpler, you develop, fix, wash, bleach, re-develop, bleach and fix. You can increase density of the already developed negative.

You basically need only the image left over in silver, so you can bleach it back for re-development, hence in E-6, b&w bleach becomes necessary.


Be careful about using bisulphate in the bleach though. I did that recently, and it was a partial rehalogenating bleach (which means it isnt good for its job as as a B&W type reversal bleach). I used some pool/spa pH down, which is bisulphate, it must have had some chloride or bromide in it I'm guessing.