• This is one of the oldest photographic processes
  • For reasons we'll see it almost never is used inside a camera
  • It was actually developed as the earliest "photocopy" machine
    • It was the most important way of copying large documents for about 100 years
    • this is the "blueprint" of the engineering and the construction trades
    • this is the origin of phrases like "blueprint for success"
      • literally a printed version of the plans

details on the process

• Chemically this involves the reaction between Fe(III) salts and light
  • some are easily reduced to Fe(II) on exposure to light
  • in blue prints we use Ferric Ammonium Citrate as the Fe(III) source
  • We also add Ferricyanide ion
    • this ion does not react with Fe(III) ion
    • it does however, react with Fe(II)
      • it forms an intense blue compound, insoluble in water
      • this only occurs in areas exposed to light

• Your green crystals from Lab #2 are light sensitive
  • the green crystals turn yellow on prolonged light exposure
  • the Fe(III) is reduced to Fe(II)
  • one test for Fe(II) is its reaction with ferricyanide ion to form a intensely blue salt
• Dissolve a small amount of green crystal in water and split it between two small test tubes
  • keep one in the dark
  • place the other in sunlight on the window sill or in front of a bright light
  • then put the two tubes together and add a drop of Potassium Ferricyanide solution
    • the unexposed tube should show only a slight color change
    • the exposed tube should show strong blue colors

• In blue prints, we start with a solution of Ferric Ammonium Citrate and Potassium Ferricyanide
  • We brush this on pieces of paper and let it dry in a dark location
    • it takes several hours to be dry enough to use (we'll give you dried material)
    • (This has a yellow color, telling us it absorbed blue light)
  • We now cover the paper with the object to be copied
    • a photographic negative or transparency
    • a black and white drawing on translucent paper
    • or simply place opaque object like coins, keys and paper cutouts
  • Now expose the sandwich to bright light
    • exposure is about 20-30 minutes of bright sunlight
      • (only the deep blue and UV light is effective)
    • in lab, we will use a bank of UV lamps and cut the exposure to five minutes
  • When we examine the result, we see a faint blue/green/yellow image on the paper
    • (some Fe(III) has been reduced but most has not yet found Ferricyanide ions)
  • Placing the print in water has two effects
    • It quickly lets Fe(II) find ferricyande and the full picture forms in a few seconds
    • It dissolve the unreacted materials
      • This is important since otherwise room light would eventually turn everything blue and the image would be lost.
    • An optional treatment with hydrogen Peroxide can make the image a deeper blue color.

• This is a very slow process-- we need a five minute exposure under bright lights
• the reason is simple--
  • one photon of light can only produce one molecule of the blue material.
  • it's even worse than that since maybe 1% of the light is actually absorbed and causes a reaction
    • so we either need an extremely intense light source or we need to be very patient.

• It's hopeless putting this material into a camera
  • the amount of light that reaches film is much lower than the amount in sunlight
  • we might get a useful image after 8 hrs of expose to a brightly lighted scene with very big lens
  • note also, the camera would need to be as big as the finished product

• Still, this is a reasonable way to copy large drawings
  • the image needs to be very clear (high contrast)
  • draftsmen did this with India ink (opaque black ink) on thin translucent drawing paper
    • usually they drew on more substantial paper
    • then they traced the picture onto the thin paper
  • then a large light box is used to copy the image onto blue print material
    • the result is a paper that is almost all blue (a blue print)
    • the lines appear as white areas

• The method was developed by the astronomer John Herschel about 1835.
  • He wanted a method to copy detailed astronomical data tables
    • The current method of setting type and printing was always full of type setting errors
  • He used this technique to copy directly though the paper, making multiple accurate copies
    • (He didn't have other choices like an office copier or a trip to Kinko's)
• Herschel is an extremely important person in the early development of photography
  • there is evidence that he showed a photograph (of a telescope) to friends in 1825.
    • this could be the oldest true photograph, but it does not survive
    • no one knew how to fix the image then
  • he simultaneously developed silver chloride on paper photography, but withdrew his claims in favor of Fox-Talbot
  • around 1840 he coined the words photography, photograph and photographing
    • he later coined the word "snapshot" around 1860
    • he tirelessly promoted photographs and had a great vision of how the field would eventually develop.

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