Black and White Photography

Chemistry of Silver Halides

• Almost all photographic films and printing papers are based on the interaction of light and silver halides
  • AgCl, AgBr and AgI (AgF is not used)
  • We'll generally talk about silver chloride which is the most light sensitive of the three
  • Commercial products often mix Chlorides, Bromides and Iodides to achieve desired properties
    • for example, it is sometime important that the material be made less sensitive to light so it can be handled safely in faint light. Replacing the silver chloride with iodide is an easy way to reduce the sensitivity
  • The silver halides are insoluble in water and almost all the chemistry occurs on the face of the silver halide crystal.

• The key reaction is simply
  • AgCl + light -----> Ag (metal) + Cl (lost)
    • After prolonged exposure to light, some / most / all of the Ag+ ions become Ag (metal)
    • Silver halides are white or colorless
    • Silver metal is black

• One of the earliest forms of Photography (Fox-Talbot, 1835) involved paper sensitized with Silver Chloride

• His early work involved prints of leaves, ferns and other objects placed directly on the paper and exposing the combination to light. He referred to these as photogenic drawings.
• By 1835 he had succeeded in taking a picture with a camera. The exposure time was nearly 8 hours.
  • it takes so long because each silver atoms requires the absorption of one photon of light.

• In lab, we will make a light sensitive paper by pouring a little 0.3M Silver Nitrate solution on the surface, waiting for it to be absorbed, and allowing it to dry
• We then dip the paper in a 1M solution of Sodium Chloride
  • The AgCl forms and it is an insoluble precipitate
  • Some of the AgCl is trapped by the paper fibers
  • We simply let the paper dry in a dark location.
    • The material is much more sensitive when dry and it's also a lot neater.
• We will use this as a Print Out Material -- that is, we will expose it long enough to form the final image
  • We will cover the paper with something that creates a pattern
    • coins, keys, leaves, paper cutouts or transparencies (negatives)
    • if necessary, use a glass cover to flatten the covering
  • Expose this to bright light
    • a 150 watt lamp at 1 ft will probably take about 45 minutes to show an image
    • placing this in direct sunlight for 15-30 minutes is also probably satisfactory if we have a sunny day
    • in practice we will use a light box equipped with ultraviolet lamps; a 5 minute exposure is adequate.
  • Remove the objects and inspect the paper
    • you should see a dark color (black, brown or purple tint) where light reached the paper. The color is due to tiny particles of silver metal.
    • the shielded areas will remain white and they remain light sensitive (silver chloride)

• The early photographers were unable to preserve their images. The remaining silver halide would eventually turn black and the entire picture became black. You could preserve an image in a dark cabinet long enough to show your friends, but every time you showed off your work, it would get darker.
  • One solution is to retitle the picture "ghosts hunting for their sheets at midnight" or some such nonsense.

Fixing The Image

• the solution: get rid of the remaining AgCl before it can darken
  • You already know how to do this
    • AgCl will dissolve in 6M ammonia
      • if you rinse the paper in 6M ammonia, no AgCl will remain
      • ammonia fumes are not a good idea in an enclosed darkroom
    • photographers prefer to use the thiosulfate ion, S₂O₃2-.
      • the older name is hyposulfite ion and photographers often call it "hypo"
      • the other name is the Fixer, because it fixes (makes permanent) the picture
  • you should take your image (see above) and immerse it in a sodium thiosulfate bath for about 5 minutes
  • then wash the image in gentle stream running water for about 15-30 minutes to remove the last trace of the fixer.
    • you can cheat and cut the rinse to about 5 minutes
    • your print will begin to yellow in a few months as traces of thiosulfate decompose.
      • you really didn't plan to keep this for years, did you?

Negatives and Prints

• This is a photographic negative (color reversal)
  • Wherever the image was bright or well lighted we form black silver
  • Wherever the image was black or dark we end up with white paper
• If the paper is thin enough to be translucent we can use the photogenic drawing as an image and make a new print
  • this new print is now a photographic positive
  • we can make hundreds of copies (prints) from the original negative

The Cyanotype or Blue Print another print out material used for over 100 years as the way to copy documents

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