Silver Photography continued

The Daguerreotype and Photographic Developers

• About the same time as the Englishman Fox-Talbot was creating his images (1835), an alternative form of photography emerged from France.
  • this was the Daguerreotype, names after Louis Jacques Mande Daguerre
    • Daguerre was a successful commercial artist in Paris in 1825; he created and operated a giant illusionistic theater called the Diorama. He was actively looking at ways to create detailed scenes without the labor of hand painting.
    • Joseph-Nicephore Niépce had developed primitive methods of creating lithographic plates using light. His 1826 view from his window (8 hr exposure) is recognized as the worlds oldest existing photograph. He sold his interests to Daguerre.
  • Daguerre combined some of Niepce's work with that of Becquerel (silver plates) and developed a new technique that he named after himself. (He was a tireless promoter of his own interests.)

a web site devoted to Daguerreotype in many forms

how to make your own Daguerreotype

• The light sensitive plate was based on a layer of silver bromide and iodide
  • A copper sheet was electroplated with silver and polished
  • The silver surface was exposed for several hours to bromine and iodine vapors
    • A thin film of silver halide formed on the surface
  • This sensitized plate was placed in a camera and exposed
    • As before, exposure times of hours were required
    • This was 1830's so there were no bright sources of light other than sunlight
• Before the method became useful it was necessary to reduce the exposure times to a few minutes.
• After all, you can't expect people or animals to hold a pose for several hours unless they are dead
  • That's not a trivial observation. Most commercial photographers (even up to 1900) derived a major portion of their business from photographing people who died. This was an important remembrance for the bereaved. Remember that people had no photographs of their children or parents to refer to and this last remembrance became an important component of the funeral ritual.

Development of the Image

• Daguerre accidentally discovered that a hopelessly underexposed picture had become a respectable image when taken from a drawer.
  • By a process of elimination he realized that the mercury metal in the drawer was responsible
  • Faint images could be developed by exposure to mercury vapor
  • Now, exposures of a few minutes were enough
    • No image was observable on the plate
    • But exposure to mercury vapor would cause the image to appear and intensity
  • About the same time, Fox-Talbot discovered that adding Gallic Acid to his paper had a similar effect in reducing the exposure time.
• We now recognize these as chemicals that can reduce Ag+ to Ag metal.
  • There are many reducing agents available, but a really good reducing agent would be a disaster
    • if we converted all Ag+ to Ag, we get an all black image
    • what we need is a mediocre reducing agent
      • one that normally several hours to convert most of the Ag+ to Ag
  • a good catalyst can speed up the reaction (say to 4 minutes)
    • A great catalyst is silver metal
      • every AgCl crystal that's seen a little light has a built in catalyst
        • it quickly turns black in the developer
      • those AgCl crystals in the dark areas have little or no catalyst
        • they hardly change in the first five minutes
    • we stop the developer after 5 minutes
      • in practice, crystals that absorbed as few as 50 photons will probably be turned completely black
      • each photon now ends up producing millions or billions of silver atoms
      • short exposure times were possible.
        • for Daguerre, this was 2-5 minutes (and photographic studios included neck clamps to help people hold the pose long enough without moving.)
        • with improvements to photographic films, exposure times are now around 1/100 second.

• there's another important discovery that leads to shorter exposure times
  • Silver Chloride is white, meaning it doesn't absorb visible light
    • Actually it's very slightly yellow meaning it absorbs a little bit of violet light
      • What it really reacts with is the UV light in sunlight
      • so 99% of the visible light is of no value
    • it also means that almost all other colors are perceived as black
• The trick is to add a dye that absorbs light and transfers the energy to the AgCl crystal
  • now the film is sensitive to light over most of the visible spectra
  • it is also probably hundreds of times more sensitive to light

Limitations of the Daguerreotype

• One serious limitation was that the camera produced a single picture
  • There was no practical way of making copies of a Daguerreotype
  • Some cameras used multiple lenses and made 2 or 4 copies of the scene
  • It's also worth noting that most early photographs were either very small or the photographer was required to work with a massive camera. The finished picture could never be bigger than the body of the camera.
• The Daguerreotype was really a negative with black silver particles on a shiny silver plate
  • the plates were usually mounted in a special holder with a black velvet facing
  • at the right angle, the silver particles scattered light and looked lighter (gray really)
  • the shiny silver reflected the black velvet and looked darker
  • at the correct angle the image becomes a positive image (really black and gray.)
• The process was really tough on photographers
  • the continuous exposure to mercury vapor in an enclosed area undermined the health of most early photographers.
• Exposures were still relatively long and features (like eyes and ears ) are often blurred

the cyanotype

making prints; enlargements

the chemistry of making positives directly

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