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  1. #1

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    B&W reversal procedure

    The usual way to do B&W reversal is to first develop the negative image. Then bleach that away.

    Should not some clever chemist be able to devise a more direct method that would just remove the latent image at once instead of developing it?

    I know this may be a naive question. Just can't help asking.

  2. #2

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    How would the chemistry know which latent image to remove ?

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    Well, I said I'm naive.. and not a chemist.
    But how does the developer know which latent image to develop?

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    hi salveslog

    there are, and have been direct positive papers and processes
    ilford made a direct postiive paper ( you expose it, and develop it as a positive, single step as they say )
    they stopped making it for a while and rumor has it ( or maybe i listened to the fleetwood mac too much )
    that it might start up again ? there are direct postiive papers currently being made in europe !
    efke makes a paper, and imago ( maco ) makes some as well ( maybe they are the same ? i have no idea )
    in years gone by there were proprietary developers that would develop, bleach and reverse photo papers that street photographers used
    to make instant street portraits and post cards with ... that process was also called the silver gelatin ferrotype process and besides paper
    they made images on metal (silver gelatin ferrotypes ) ... which were indisnguishable from wet plate tintypes ...


    no need to really be a chemist, you just need direct positive paper

    have fun !
    john

  5. #5
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    How about shooting Agfa Copex and developing in Ultrafin?

  6. #6

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    Quote Originally Posted by SalveSlog View Post
    Well, I said I'm naive.. and not a chemist.
    But how does the developer know which latent image to develop?
    There is only one latent image and that is the negative. I think that this is where your reasoning went astray. The positive image consists of all the unexposed silver halide that is left in the emulsion. So you must first develop the negative image. Once it has been converted to a non-reactive form the positive image can be developed. A simple way to do this is with a fogging developer. This converts all the remaining silver halide to silver. There is no significant difference between film intended for negatives or that same film intended for making positives. The only difference would be in the EI used.

    It might be helpful to read Ilford's directions.

    http://www.ilfordphoto.com/applications/page.asp?n=90
    Last edited by Gerald C Koch; 04-02-2015 at 12:37 PM. Click to view previous post history.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

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    Gerald,
    the latent image is affected by the developer. The developer is a chemical substance. Why can't the latent image (the light-affected silver halide) be removed directly by a chemical substance instead of developed to silver grains first? Can someone explain why this is a chemical impossibility?
    Last edited by SalveSlog; 04-02-2015 at 12:37 PM. Click to view previous post history.

  8. #8

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    Because the latent image substance is indistinguishable chemically from the rest of the emulsion on the film - this is a gross oversimplification as the physics and chemistry involved is remarkably complicated and I don't understand a hundredth part of it, but Gerald does and his explanations are to be trusted
    Last edited by pdeeh; 04-02-2015 at 12:40 PM. Click to view previous post history.

  9. #9

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    I realize I will probably have to give in in the end..
    But how can the developer find the latent image if it is "indistinguishable chemically"?

  10. #10

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    The only way to get rid of the unwanted negative image is to develop it and then bleach it. The bleach converts it to a non-reactive form. Then the remaining silver halide (the positive image) can be developed. Finally the silver compound that is the negative image is removed with fixer. What is left is only a positive image.

    As pdeeh states there is no gross chemical difference between the two images. Physically there IS a difference in some of the crystalline structure of the exposed portion of the emulsion. Exposure to light causes crystal defects that are slightly more reactive to reduction. That is how the developer "finds" them.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

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