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  1. #1
    Vlad Soare's Avatar
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    Direct Positive Paper - How Does It Work?

    Hi guys,

    How does a direct positive paper work? How does a regular developer create metallic silver in unexposed areas, while leaving the silver halides unaffected in exposed areas?

    Thanks.

  2. #2
    AgX
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    I don't know about that paper. but one can manipulate on the ability of the halide to establish interior as well exterior nulclei. Combined with a well chosen developer as well as with fogging action during development one can achieve image reversal within one single bath.

  3. #3
    Vlad Soare's Avatar
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    That sounds very interesting. Do you by any chance know where I could find more specific info on this subject? I'd like to know more about it.
    Thank you.

  4. #4
    AgX
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    There are no good texts on direct-reversal (non-diffusion) processes in the accessable books.
    Out of memory I would advise the Mees&James and the multivolume L.P.Clercq where you find at least some hints.

  5. #5
    holmburgers's Avatar
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    Maybe you could PM Simon (the Ilford rep) and see what he says.

  6. #6
    holmburgers's Avatar
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    This question has not been addressed properly. I demand an answer!

    Haha, j/k, but wouldn't you like to know??

    I read somewhere that some reversal papers are brought just to the brink of solarization, and then the exposure kicks it over the edge and voila, reversal. But that kinda left me scratching my head...

    Why does solarization reversal occur?
    If you are the big tree, we are the small axe

  7. #7
    RalphLambrecht's Avatar
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    Quote Originally Posted by Vlad Soare View Post
    That sounds very interesting. Do you by any chance know where I could find more specific info on this subject? I'd like to know more about it.
    Thank you.
    Grant Haist's book 'Modern Photographic Processing' Vol.2 has a 60-page section called 'Reversal Processing of B&W Materials'. It starts with the following text:

    'By a modification of processing, called reversal processing, a normal emulsion can be made to give a positive instead of a negative result.'

    A brief summary would be:

    1. 1st development
    2. bleaching
    3. clearing
    4. 2nd development
    5. fixing

    It is an excellent book, but unfortunately, it explains positive B&W film but not how this positive paper works!
    Last edited by RalphLambrecht; 02-04-2011 at 04:13 PM. Click to view previous post history.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  8. #8
    holmburgers's Avatar
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    Thanks Ralph, but indeed, these direct reversal papers work differently! They create a positive image in a normal developing sequence.

    It's possible that photobooth paper is of this style, though that's open for speculation too I believe. The dip & dunk system of those could be the reversal processing like you've mentioned, or direct+ paper.
    If you are the big tree, we are the small axe

  9. #9
    Photo Engineer's Avatar
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    There are at least two types of direct reversal emulsions. The process is Develop, Stop or Rinse, and Fix using normal chemistry.

    The two types in common use are Reversal F and Reversal P which differ considerably. These emulsions are VERY complex and beyond the scope of any discussion so far on APUG, but to make it very simple, A reversal F emulsion is usually say a pure chloride cube which is then treated with bromide and made into an octahedron kind of shape. The cur of this, the chloride, may be fogged, sensitized some way or just left as is before shelling. This process is called conversion and does not make a true core shell emulsion.

    That final emulsion then is sensitized as normal but uses a special chemical called a nucleating agent added to the mix.

    At exposure, exposed areas will not develop, but unexposed areas will develop thus giving us a positive image directly with a normal developer.

    There are two disadvantages. One is that the nucleating agent can build up in the developer changing both direct reversal results with time, and also it can affect negative emulsions so the developer is best used one-shot or only for direct reversal emulsions with lower capacity. The second disadvantage is that these materials have a shorter tone scale and higher fog than normal materials. This results in re-reversal or combined negative and positive images in the same frame. This is rare, but very ungood when it takes place.

    Reversal F was used in Kodak PR-10 instant films and in Ektaflex R material.

    PE

  10. #10
    holmburgers's Avatar
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    Wow, thank you very much Ron. This is really good information!

    Is this covered in any of the popular literature on photographic chemistry?
    If you are the big tree, we are the small axe

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