Direct Positive Paper - How Does It Work?

Discussion in 'B&W: Film, Paper, Chemistry' started by Vlad Soare, Oct 27, 2010.

  1. Vlad Soare

    Vlad Soare Member

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    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? :confused:

    Thanks.
     
  2. AgX

    AgX Member

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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. Vlad Soare

    Vlad Soare Member

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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. AgX

    AgX Member

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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. holmburgers

    holmburgers Member

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    Maybe you could PM Simon (the Ilford rep) and see what he says.
     
  6. holmburgers

    holmburgers Member

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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?
     
  7. RalphLambrecht

    RalphLambrecht Subscriber

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    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 a moderator: Feb 4, 2011
  8. holmburgers

    holmburgers Member

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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.
     
  9. Photo Engineer

    Photo Engineer Subscriber

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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. holmburgers

    holmburgers Member

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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?
     
  11. Photo Engineer

    Photo Engineer Subscriber

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    Not that I know of. It is one of those things that slipped through the cracks.

    PE
     
  12. Joe VanCleave

    Joe VanCleave Member

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    I've seen a video on how photo booths work, they use a dip-and-dunk mechanism that rotates over an assortment of about eight different chemical pots, which implies to me that it's a reversal chemistry process, not just develop/stop/fix/rinse. I recall some chemical bright green in color, IIRC.

    I've toyed with the Efke direct positive paper in LF cameras. It requires a significant amount of preflash exposure to get an adequate tonal range; and even then, I couldn't get acceptably deep shadows while still maintaining good midtones and highlights; and even minor adjustments to the preflash exposure (i.e. over a 25-35 second range) produced dramatically different results, as if there's some steep cliff of sensitivity in that range that's hard to nail down exactly. I ended up not using the paper any more, because I found a whole pack could easily be used up just doing "tests."

    ~Joe
     
  13. Hexavalent

    Hexavalent Subscriber

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    Are these covered in Workshop Level XCIV ? :laugh:
     
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  15. Photo Engineer

    Photo Engineer Subscriber

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    Thats MMMLXVII. Too complex.

    Joe, there are two types of photo booths. One uses "regular" paper and the other uses direct reversal. I have seen both.

    PE
     
  16. Ray Rogers

    Ray Rogers Member

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    I've made one of these;
    it worked, but the fog was... like a hurricane!
    I wouldn't mind having another go at it someday though.

    It would be nice to work on a group project....
     
  17. holmburgers

    holmburgers Member

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    How feasible would a panchromatic direct-reversal emulsion be? :wink:
     
  18. Photo Engineer

    Photo Engineer Subscriber

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    Easy, once the emulsion is made.

    PE
     
  19. Hexavalent

    Hexavalent Subscriber

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    Can we see some?
     
  20. Ray Rogers

    Ray Rogers Member

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    It was a Dark and Foggy Night...

    Probably not.

    At least not unless I decide to repeat that experiement.
    It was after all, a long time ago, but I will see if I still have anything from that time.

    You will not be impressed, though.

    Once I saw how bad the fog was, I stopped experimenting.
    It eats up somewhat more silver than the other emulsions I was making at the time and frankly, I was just curious.

    I probably still have the quick and dirty test I did
    before I put the emulsion away and forgot about it.

    For the proof of concept,
    I just put a dodge tool on a test sheet
    and exposed it to light.
    One exposure - & no test strips at all IIRC.

    (This was a silhouette exposure so no midtones;
    I did this because I knew there was a problem with low contrast/high fog )

    The result I had desired was a black dodge tool on white paper,
    but what came up in the developer was
    a dark grey dodge tool on light grey paper.

    After putting it on the drying rack,
    I went to sleep and tried never to think about it again.

    (I was discouraged and, not knowing what was causing the fog,
    didn't want to waste any more time on it)

    That was 1993 (?) or so... maybe now I should have another go at it?
    The main thing is still not having a clue as to where to start making adjustments at.

    I need to know more about DR theory before I approach it again.

    I guess the first question is, Why ARE they so foggy anyway?

    :sad:
     
  21. Photo Engineer

    Photo Engineer Subscriber

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    If you don't do it right, the negative and positive images "leak" between the "layers" and you see low discrimination!

    PE
     
  22. Hexavalent

    Hexavalent Subscriber

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    When you say "layers", is that layers of emulsion, or 'layers' of the grains?
     
  23. Photo Engineer

    Photo Engineer Subscriber

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    Ian;

    I use the word layers loosely as noted by the quotes in my post. When you "convert" an emulsion, you create a core of one salt, a hazy vague layer of the mix and a topping of the other salt. So, this is not a true core shell, but a conversion (suspecting what Ray did, ie, to make a Reversal F emulsion).

    Now, I must go further and state that all emulsions give a reversal and negative image. This is why we have solarization. It is the job of the photo engineer to separate these two phenomena in speed so that they don't overlap. If they overlap by a great deal, you see it as fog, and if they are separated just right you see either a negative OR a positive. If they are separated by a great deal, you see solarization in the negative image. Again, a gross simplification.

    PE
     
  24. Ray Rogers

    Ray Rogers Member

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    Aye, but helpful!

    In theory, in the case where they overlap,
    could a solution be approached by changing the speed of EITHER layer?
    (In the right direction, of course)
    Or, do I need to focus on the core?

    :confused:
     
  25. holmburgers

    holmburgers Member

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    I dont' understand the physio-chemical basis of solarization. Could that be elaborated upon?
     
  26. Ray Rogers

    Ray Rogers Member

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    Chris, Do your teachers like you?