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  1. #1
    BetterSense's Avatar
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    Why does unexposed film get fogged with age?

    I have heard and experienced that old film becomes fogged with age. I have heard that it is caused by cosmic rays. What kind of cosmic rays? Does anyone actually know? Could they be stopped by lead?

    I have also heard that freezing unexposed film will prevent fogging. However, using film for photography at freezing temperatures doesn't seem to effect the film's speed or sensitivity, so why should putting it in the freezer slow down fogging?
    f/22 and be there.

  2. #2

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    Oxidation of the silver in the emulsion. Molecular activity slows as temperature is decreased.

    True or false?
    "Lo único de lo que el mundo no se cansará nunca es de exageración." Salvador Dalí

  3. #3
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    Heat fogs film. Radiation will fog film. Heat fogs faster than radiation. Fast films fog faster from both sources than slow film. Freezing slows down heat death of film. Heat death is a continuation of the sulfur + gold finish placed on film to speed it up photographically. (in simple terms)

    Film changes at any temperature at a finite rate, but more slowly at low temperature. Temperature does not materially affect speed, but rather the rate of change of speed.

    I hope this answers your questions.

    PE

  4. #4
    Photo Engineer's Avatar
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    Quote Originally Posted by DannL View Post
    Oxidation of the silver in the emulsion. Molecular activity slows as temperature is decreased.

    True or false?
    Oxidation = false.
    Siver in emulsion = false.

    PE

  5. #5

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    Quote Originally Posted by Photo Engineer View Post
    Oxidation = false.
    Siver in emulsion = false.

    PE
    Oh well, I got that info from a publication describing a kodak film. It sounded good at the time. I should have known better.
    "Lo único de lo que el mundo no se cansará nunca es de exageración." Salvador Dalí

  6. #6
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    Quote Originally Posted by BetterSense View Post
    I have heard and experienced that old film becomes fogged with age. I have heard that it is caused by cosmic rays. What kind of cosmic rays? Does anyone actually know? Could they be stopped by lead?

    I have also heard that freezing unexposed film will prevent fogging. However, using film for photography at freezing temperatures doesn't seem to effect the film's speed or sensitivity, so why should putting it in the freezer slow down fogging?
    Kodak had an article about accumulation of cosmic rays. Cosmic rays penetrate freezers, so while freezing might slow color shift in films, it will not do much to slow accumulation of cosmic radiation.

    Cosmic rays are resultant from our own sun and other stars. Cosmic rays originate outside our solar system. They consist of ionizing particles such as electrons, and atomic nuclei and non-ionizing particles such as gamma rays (photons), and neutrinos. Nuclei from every element and nearly every isotope are found.
    The most common of these are hydrogen (protons), and helium nuclei. Those nuclei formed in stars, such as carbon through iron, are the next most abundant.

    As for lead, I just found this description in a NASA publication about the methods used to shield a detector from cosmic rays,
    "Passive shielding is used to stop low-energy cosmic rays before they hit the CZT detectors. EXIST uses layered sheets of lead (Pb), tin (Sn) and copper (Cu). This layered approach is called "Graded-Z" shielding, which refers to the order and atomic number (Z) of those metals (Z equals 82 for lead, 50 for tin, and 29 for copper). Materials with larger Z have greater stopping power, and lead is used for the outermost layer. When a low-energy cosmic ray hits the lead it will be absorbed and ionize a lead atom which then emits an X-ray at the "characteristic energy" of lead, 88 keV, in a random direction. Lead does not absorb its own X-rays very well, so to prevent any of those X-rays from getting through the shielding, a layer of tin comes next. After absorbing the 88-keV X-ray, the tin may then emit a 29-keV X-ray. A layer of copper comes last. The few copper X-rays that reach the CZT detector are too low in energy (9 keV) to cause a problem. "

    So simple lead alone is insufficient, as it is necessary to shield from the secondary radiation byproducts that result from the cosmic rays striking the lead, too!

  7. #7

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    Kryptonite is the real killer tho'!

  8. #8
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    Wow. What interesting arcana! A trove.

    I love this place!
    Michael Sebastian
    Website | Blog

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    Quote Originally Posted by wiltw View Post
    Cosmic rays are resultant from our own sun and other stars. Cosmic rays originate outside our solar system.
    Except, of course, the big part of it produced by "our own sun".

  10. #10

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    Cosmic rays, and stray radiation added to the incredible effect of Grandma's Gefilte Fish...sorry, I just couldn't resist...

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