Our lab as a Darkroom

• It is not practical for us to develop ordinary photographic film in the chem 128 laboratory
  • we can turn off the lights but we won't have a place that is dark enough to handle the film
    • remember, film reacts to 1/100 sec exposure to ordinary lighting
    • a few seconds in faint light would fog (darken) the entire film
    • it's also not very interesting, since you develop a film in total darkness and there's not that much to see anyway.
• We could expose and develop a special graphics art film
  • this is deliberately made much less sensitive
  • it is deliberately made insensitive to red light so you can watch it develop
  • in this sense, such films are much like photographic plates from 1900

• Instead we will focus on making a print from a negative
  • We will use a small (35mm) black and white negative
  • We will use an enlarger and blow it up to about 3-4" size
  • We will use a commercial printing paper
    • this are coated with a gelatin (or polymer) layer containing silver halide crystals
    • exposure time will be about 15 seconds with white light
    • enlarging paper is generally not sensitive to red or orange light
      • so we can handle it under a red or amber safelight
      • we can actually see what we are doing
  • After exposure we will develop and fix the print

link on black and white prints

• It will be helpful to select negatives with relatively high contrast
  • Strong back regions and relatively clear areas
  • a subtle range of gray will he herder to print well
  • an image with fine details can be harder to print well
  • we won't have the time to get the details well controlled

• There was one other development that made film more sensitive
  • It takes about 50 photons to expose one silver chloride crystal
    • that's enough to make it develop
    • it doesn't depend on how big the crystal is
  • So a large crystal gets 50 photons in a short exposure
    • a small crystal needs either a longer exposure or a brighter light
• Making a film with larger crystals makes it faster (more light sensitive)
  • there is a trick to making the crystals grow to the desired size
• The price it that the final picture is made up of bigger particles (or grains) of silver
  • fast films produce grainer images
  • if you want to make big enlargements, choose a slower film
  • if you try to enlarge your negative too much, the image will show up as dots (grain)

• The graphics art film mentioned early is intended to make very fine detailed images
  • as a result it has tiny crystals
  • hence it is relatively insensitive to light

Reversal Chemistry -- making a film produce a positive image upon developing

Cyanotypes-- blue prints

Photoresists

Chem 128 photography index

chem 128 home site