Cyanotype or Blue prints
- 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
samples of modern cycanotypes
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.
travel on to the wonderful world of photoresists and the rest of the lab work
return to Chemistry 128 web site
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