FLINN SCIENTIFIC
Morning of Chemistry
March 30, 2012
Indianapolis, IN
A Chemistry and Art
Connection: Cyanotypes!
Background
Although most photography is based on the photochemistry of silver halide salts, the photo-chemical
reduction of the iron in an Fe(III) salt to Fe(II) can be coupled with the non-photochemical reaction of Fe(II)
with the ferricyanide ion as an alternative method to produce an image. This process is known as the
cyanotype or blue print process. You may be familiar with architectural or engineering blue prints.
Technically, these are no longer made by the same procedure, but because of the similarity in color they are
still known as “blue prints”. In the “old days” a draftsman would make a drawing on translucent paper using
India ink. These drawings were copied (in the days before photocopy machines) by placing them directly on
top of a piece of sensitized paper. When light was shone from above, it would penetrate the translucent paper
(except where ink lines were present) and expose the underlying sensitized paper.
The sensitizer consists of a soluble Fe(III) salt, in this case ferric ammonium citrate. When the sensitizer
is exposed to light of suitable energy, the Fe(III) ions are reduced to Fe(II) ions:
Fe3+(aq) + e- + light ----> Fe2+(aq)
Any photochemically produced Fe2+ reacts with a ferricyanide salt, which is also present on the paper.
This reaction:
3 Fe2+(aq) + 2 Fe(CN)63-(aq) -----> Fe3(Fe(CN)6)2(s)
produces an insoluble, dark blue compound, ferric ferricyanide which is also known as Prussian blue.
The cyanotype process was first introduced by John Herschel in 1842, after he first discovered the
photosensitivity of ferric (iron (III) ion) salts. One of the first people to put the cyanotype process to use was
Anna Atkins (1799-1871) who in October 1843, became the first person to produce and photographically
illustrate a book using cyanotypes. Atkins was a botanist and used cyanotypes to print “Cyanotypes of British
and Foreign Flowering Plants and Ferns”.
Objectives
1. Prepare blueprint paper and create a cyanotype.
2. Observe the reduction of iron (III) salts to iron (II) salts.
3. Make some inferences toward the role of UV light on the reduction of the iron (III) ions in
the salts.
Safety
If potassium ferricyanide is heated to decomposition or comes into contact with strong acids, toxic hydrogen
cyanide gas may evolve. The pigment will easily stain skin and clothing and is not easy to remove by washing.
Handle it carefully. Wear chemical splash goggles, chemical-resistant gloves, and a chemical resistant apron.
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A Chemistry and Art Connection: Cyanotypes!
Continued
Materials
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Watercolor paper
400 mL beaker
Graduated cylinder
Foam paint brush
Balance
Glass plate
Water
Ferric ammonium citrate
Potassium ferricyanide (aka hexacyanoferrate) – K3Fe(CN)6
UV light source
Stirring rod
3% hydrogen peroxide
Procedure
1.
Make a 10 % by mass solution of ferric ammonium citrate. In case you aren’t familiar with % by mass
solutions, that is 10 % the ferric ammonium citrate in 90% water. (ie - 1.0 g of ferric ammonium citrate
in 9.0 g (mL) of distilled water. This is best done in a darkened room.)
2.
Make a 10% by mass solution of potassium ferricyanide (K3Fe(CN)6). Again, make this in a darkened
room.
3.
Combine equal volumes of the ferric ammonium citrate and the K3Fe(CN)6 solutions.
Stir. (By the way, you still need to be in a darkened room!).
4.
Carefully coat one side of a piece of watercolor paper with the solution using a foam paintbrush. The
more even the coating, the better your resulting image will be. It is your decision as to whether you
want to leave a white boarder around the edge of the paper or not. Either dry the paper with a hair dryer
or hang the paper to dry. AGAIN, this should all be done in a darkened room. NOTE: Some paper
doesn’t absorb the solution well. Watercolor paper definitely works best but you might get lucky and
find a cheaper paper that also works.
5.
Place the treated paper on a hard surface such as a piece of cardboard. Arrange your object to be
“photographed” on the treated paper. Cover the image with a glass plate and take the paper to your UV
source. This could be sunlight or any other UV light. Expose the paper and object of UV light for
approximately 20 minutes. In really bright sunlight, less time may work fine.
6.
Back in a darkened room, remove the objects. Dip the paper in tub of water for about 3 minutes.
Transfer the paper to a minute of 25% hydrogen peroxide (3%) to 75% water. Rinse your print in this
solution for around a minute then run under water for about 1 minute. This will remove any salts from
the paper that have not dissolved; then let the paper dry.
7.
Viola! You have a cyanotype!
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A Chemistry and Art Connection: Cyanotypes!
Continued
Clean up and Disposal
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific
procedures governing the disposal of laboratory waste. Dispose of any excess pigment according to Flinn
Suggested Disposal Method #14. Dispose of painted samples in the trash according to Flinn Suggested
Disposal Method #26a
Questions to Ask
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When the ferric ammonium citrate is exposed to light, the oxidation state of the iron changes. What is
the new oxidation state of the iron after the chemical reaction has occurred?
What substances were washed away when you rinsed the cyanotypes in water after it had been exposed
to sunlight?
Would the blueprint develop on a cloudy day as well as on a clear day? Explain.
Why would an insufficient washing of the exposed paper result in a deterioration of the image?
35 mm film can sagely be exposed to red light in a darkroom. Do you think the same could be said for
the cyanotype paper.
What is significant about UV light that makes it the appropriate light source for this process?
Other notes
Get online and Google “cyanotype” and be amazed at the info. We are just getting started with both the art
and chemistry side of this process. I, for one, am completely enamored by it!
Get certified in Flinn Safety with our High School and Middle School
Laboratory Safety Courses at www.flinnsci.com.
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