How does the negative on old Polaroid peel apart not get exposed when ejected from the camera?

Discussion in 'Film and Processing' started by 10986431, Feb 20, 2020.

  1. Hi all wondering if anyone knew the answer the answer to this here.

    with regards to the old Polaroid black and white peel apart instant film I was wondering how the films negative didn’t get ruined when ejected from the camera. I understand the paper being protected as it’s reversed and covered by the negative and all the gunk but it seems odd the negative doesn’t get ruined when ejected, maybe I’m missing something?
     
  2. The negative portion contains the light-sensitive silver-halide crystals. After exposure the film is pulled through rollers, which spread the developer between the negative and print portion of the film pack. The developer produces the silver image in the negative and then dissolves and transfers the unexposed silver halide to the print (positive) layer of the stack. The print is then chemically exposed (reduced) to produce a positive image. The negative image, being depleted of unexposed silver halide, is relatively stable. After development, the print is treated with a stabilizing solution, which prevents further oxidation or reduction of the image.

    Some film packs have a transparent base for the negative, which can be recovered and stabilized in the same manner as the positive portion. This was popular for roll film cameras with a "Polaroid" back, including Hasselblad. You got a positive print, which could be used to evaluate composition and lighting, and a negative which could be printed conventionally at a later time. In practice, you need about twice the exposure to get a useable negative than a print. You end up with a thin negative or a very light print, depending on your priorities.

    I probably still have several packs of negative/positive Polaroid film for Hasselblad and 4x5. I used it in the early years of the millennium before going strictly digital. The exposure disparity rendered it of little practical use. The sticky stabilizer was a powerful dust magnet in the field, and tended to contaminate any surface you used to support the film for coating.
     
    Last edited: Feb 20, 2020
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  3. I think it is important to note that on most Polaroid film which does not have a recoverable negative, the negative emulsion is coated on a paper backing and not on a cellulose negative, and thus the negative is not transparent. Thus, the negative and positive are both lightproof, as far as the chemical reaction taking place between them is concerned. On most Polaroid materials that I have seen which use a recoverable or transparent negative, there is another opaque layer which gets peeled off the negative in order to lightproof it.

    I should add that all color and some B&W peel-apart materials were coaterless, and didn't need to be treated after the print was peeled.
     
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  4. The back of the negative is black light-proof coated.

    As above, the P/N films are interesting. It seems that the black backing is soluble in,
    I believe, sodium sulfite solution, which also removes the rest of the unneeded chemicals
    from the negative.

    I believe that the above explanation for the process is right. The positive image
    is made from the unexposed silver halide that diffuses out of the negative.

    Color negatives have layers of dye and appropriately color sensitive silver halide layers.
    The dyes diffuse through, and are stopped (in some way) by exposed and developed
    silver halide, but not by undeveloped grains. Ones that are not stopped diffuse into the
    positive print.

    The earlier cameras, before peel-apart pack, used rolls and developed inside
    the camera. At the approrpiate time, the back was opened, and the positive
    peeled off.
     
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  5. In 1977, I had a semiconductor physics lab, in which we made transistors using
    photoresist. (Much bigger than usual transistors.)

    We made the masks with Polaroid 146-L roll film, which makes positive
    transparencies. This is a roll film that develops inside the camera, in the dark,
    and then at the appropriate time you open the camera and peel it off. I believe
    that there is no protective layer, but it is kept in the dark while developing.
    When development is complete, it isn't sensitive anymore.

    We used these for masks on photoresist coated silicon wafer pieces,
    about 1cm square.

    I don't know of an equivalent pack film, though I suppose we could have done
    it with negative film, and make the original patterns negative. I suspect that
    was close to the end of life for 146-L. That was the only time I ever used a
    roll film Polaroid camera.
     
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  6. OK amazing, thanks for the responses on this, I just caught up with the thread after a trip I just got back from.

    Out of interest what is the blackout coating on the back of a polaroid negative? Also being new to film photography and its intricacies in general, what is a negative emulsion and how does it differ to a negative cellulose?

    My BIG question then is with a peel-apart how is the photo taken on the negative side not light protected and then pressed onto the receiving paper and kept lightproof? Amazing isn't it, how this works.
     
  7. For peel-apart black and white, the goo contains a strong developer and sodium hydroxide.
    It is a very fast developer, and also some kind of silver halide solvent.

    At the same time as development, it also dissolves undeveloped silver halide, which
    then diffuses through the developer and reacts with a chemical in the receiving
    (final print) sheet to convert to metallic silver.

    So, no light is used between the negative and positive to make the final print.

    Color prints are more complicated, with diffusing dyes and something to stop them
    from diffusing in the exposed parts of the negative.

    It is supposed to be that Land came up with this after his daughter asked why she
    couldn't (immediately) see the picture he just took.
     
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  8. I suspect that the invention of sheet polarizers had a bigger effect on mankind
    and technology than instant photography. For some years, there were prism shaped
    crystals of birefringent materials that could be used to polarize light. There was
    also herapathite that could polarize, but could only be made in very small crystals.

    Herapathite - Wikipedia

    Land took such crystals, put them in a plastic sheet, and then stretched it.
    The crystals line up in the direction of stretch and make a polarizing sheet.

    Later ones use other iodine treated molecules, but the physics is similar.

    This is the invention that started the Polaroid company, and gave it the name.
     
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