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  1. #1
    Photo Engineer's Avatar
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    Blixes for color films?

    I have been asked this question or seen comments about this more than any other in threads related to color photography, so here goes.

    Lets first cover color negative films. During development, silver forms and the oxidized color developer (Dox) begins to form a cloud of organic dye around the silver grain as the Dox diffuses away. This cloud gets denser and denser as more silver is developed and the dye cloud becomes more impenetrable and effectively encapsulates the silver. Ok, now keep this thought in mind then. A dye cloud is an oily cloud that tightly wraps the silver metal image. At the same time, DIR (Developer Inhibitor Release) fragments are released (as are DIAR fragments - and you don't need to know more) and these are very powerful development restrainers meant to inhibit silver development. They adhere to any silver or silver halide surface.

    Now lets look at reversal films. Here you have negative silver and then the positive silver begins to develop in the color developer. As it does, it is encapsulated as described above. In mid scales of reversal images, this can be a positive and a negative silver image that becomes encapsulated with both silver images becoming entwined with the dye particles. This is tough stuff!

    Now, you have a blix. To work, a blix has to mix an oxidant with a reductant. A good one might be Potassium Ferricyanide and Hypo. Well, this is a Farmer's reducer and lasts about long enough for a few deep breaths and it is gone. Oxidants and reductants don't like to mix. So, you are left with weak oxidants and hypo. One example is Ferric EDTA and Hypo. This is what is used for color papers, but just cannot cut it with film, and so Kodak and Fuji have decided to yield to the chemistry demands of the system and separate a moderate strength bleach from a good fix for good shelf life, low pollution and relatively low cost.

    The chemistry also includes some proprietary conditioners to help jog the silver and dye clouds around, and remove DIR fragments to help bleaching and fixing.

    So, in the end, most blixes for film tend to leave silver in areas of dye Dmax (negative and positive) and in mid scale (reversal) and to some extent in Dmin (reversal).

    This tends to decrease color saturation by making the image more neutral, it tends to change the grain and sharpness and it does it so subtly that most people don't notice it.

    Blixes can also introduce a yellowish bias due to the retention of Iron salts or by the introduction of fine silver sulfide particles which you sure don't want.

    So, these are the facts about film blixes. You can use them, but they are generally too weak to work and they tend to hide the very problems they create unless you analyze the film for retained silver content as a function of density. I have done this. I have seen the silver retention and have then obtained spectrophotometric curves of the dyes to show that the colors are degraded. Many people have this in their blixed films and don't know it unless they can compare results side-by-side with properly bleached and fixed films.

    Some blixes are better than others. For example, powdered blixes are not very effective as they contain too little ammonium ion and too much sodium ion to work well. They don't bleach or fix well. Liquid kits can work to an extent but exhaust too quickly and can vary from film to film.

    So, user beware. This is one thing about color processing that I wish to make you all aware of.

    BTW, blixes work well with color papers. These are low silver AgCl emulsions with little iodide and no DIR couplers. The emulsion layers are thin and blix easily.

    PE

  2. #2
    Mick Fagan's Avatar
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    Very interesting, thank you for this.

    In the mid eighties when I was doing my first home brew C41, I looked at a blix product from Unicolor for prints and thought about the same for film.

    I asked the Kodak rep at work who referred me to a lab fellow. Basically he told me the same thing as you just have. More interestingly, he had some sample prints from film that had been processed with a blix and ones using bleach and fix. There wasn't much in it, but it was possible with the naked eye to see some very subtle differences, none of which I can remember. It was enough to ensure that I have never used a blix for films.

    Never knew about the apparent suitability of blix, for prints though.

    Mick.

  3. #3

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    Quote Originally Posted by Photo Engineer View Post
    I have been asked this question or seen comments about this more than any other in threads related to color photography, so here goes.

    (cut)
    So, user beware. This is one thing about color processing that I wish to make you all aware of.

    PE
    Thank you, thank you, thank you!



 

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