curious about how the C-41 process works

Discussion in 'Color: Film, Paper, and Chemistry' started by jbl, Nov 28, 2011.

  1. jbl

    jbl Member

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    I've been wondering about the following, more just for my own understanding than anything else.

    In color negative processing, the bleach removes the left over silver from the different layers of the film, just leaving the dyes in place. Why, then, do we need to fix? Isn't the silver already gone? I get why we need to fix in the B&W case, but I was thinking about what happens in bleach bypass processing when the B&W image stays behind.

    Thanks for the help, I don't have a processing problem that I'm trying to address and I'm not planning to go without fixer, I just want to understand how it works.

    -jbl
     
  2. mts

    mts Subscriber

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    Bleach doesn't remove silver. Color bleach does actually fix some unexposed silver halide and can interact with dyes but its main purpose is to convert the exposed silver portions from metallic to halide form. Fixer dissolves silver halide but not metallic silver. If you skip the bleach you will have very dense unprintable negatives. If the bleach is exhausted or is not allowed to work to completion then you will have grainy rather muddy negatives that print with reduced color saturation.

    The combination bath known as blix is notorious for not working to completion leaving poor quality negatives. Most custom workers prefer separate bleach and fixer baths for that reason and because the individual baths have longer working life. But you must include a good rinse following the stop bath and after the bleach bath.
     
  3. jbl

    jbl Member

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    So the bleach converts the silver so that the fixer can't dissolve it?

    I've noticed when I am washing between the bleach and the fixer that if I look at the film, it's magenta in color, why does this happen?

    Also, I'm kind of wondering why you can turn the lights on after the bleach even though you haven't fixed yet.

    -jbl
     
  4. Bob-D659

    Bob-D659 Member

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    No, the bleach converts the silver to a halide salt the fixer can dissolve and remove from the film.
     
  5. polyglot

    polyglot Member

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    The whole process is like this:
    * exposure converts silver halide to silver halide in a higher energy state
    * development converts the activated halide to metallic silver; in C41 the couplers also produce dyes in the same step
    * C41 bleach converts metallic silver to silver halide
    * fixer removes silver halide but not metallic silver

    If you consider normal B&W, there is obviously no bleach because the metallic silver is the image. With C41, the dye is the image so the bleach step is required to enable the fixer to remove the developed silver. If you don't fix, you end up with an emulsion full of silver halide that would slowly print-out, covering up the image.

    If you bypass (or reduce) the bleach, you end up with the dye image superimposed on a (partial) silver image, which makes for a very thick negative with low saturation; it's effectively the subtractive product of a colour image and a B&W image. Google for "bleach bypass" sometime. The bleach also seems responsible for stripping back some of the dye in the film base; without bleaching you get a thick, dark film-base.

    Once the bleach has run, you're not re-developing (the halide being activated is irrelevant to the fixer), so a bit of light doesn't matter. If you leave it out in the light and unfixed long-term though, then all of halide will print-out (undo the bleaching), i.e. develop itself physically into a solid layer of silver.

    No idea what colour it looks like prior to bleaching; my film stays in the drum until I'm done with the whole process. I find that wet C41 (Kodak, at least) is brown on one side and purple (possibly fluorescent, like some developing agents) on the other with a bit of milkiness (purple side probably); the purple fades as it dries and you end up with the uniform burnt orange mask.
     
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  6. jbl

    jbl Member

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    That helps a lot, thanks!

    -jbl
     
  7. polyglot

    polyglot Member

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    I've added this topic to my analogue FAQ, albeit with more of a "how to" flavour.
     
  8. Photo Engineer

    Photo Engineer Subscriber

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    All current color films contain dyes and couplers encased in solvent droplets. When the film is wet, the droplets are more opaque than when dry and the film assumes a reddish color when viewed from the emulsion side and a cyanish color when viewed from the base. When dry, this effect vanishes and you get clear color images.

    Color papers used to be notorious for this and could not be evaluated until dry, but current papers have pretty much solved the problem and are able to be reasonably well judged wet.

    PE
     
  9. postalman

    postalman Member

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    Can you explain this part? It seems contradictory to me

     
  10. Photo Engineer

    Photo Engineer Subscriber

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    Develop, Stop (optional), Bleach, wash, Fix, wash, Stabilzer or Final Rinse. DRY!

    What could be simpler. No wash after the Stabilzer or Final Rinse as it is now called.

    PE
     
  11. nathan96

    nathan96 Member

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    The spirals are washed not the film (spirals are unloaded)

    If that's what you mean?
     
  12. postalman

    postalman Member

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    Ahhh. That makes sense now.
     
  13. polyglot

    polyglot Member

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    Yes, you wash the stab off the spirals to prevent pollution of later runs, but you do not wash the film after stabiliser. I've updated it to make it a little clearer.
     
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