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Cyanotypes: Taking Pictures with the Sun Mark as Favorite (0 Favorites)

LESSON PLAN in Observations, Chemical Change, Redox Reaction, Chemical Change, Activation Energy, Oxidation Number, Electron Transfer. Last updated September 20, 2024.

Summary

In this lesson, students will read about the chemistry behind two photographic methods, including cyanotypes, and then prepare their own sun-sensitive cyanotype paper from two different types of paper. They will place items of their choosing on the prepared papers, place them in the sunlight, and develop and compare the images. Finally, students will be asked to think about different variables they could test with adjustments to the original procedures.

Grade Level

High School

NGSS Alignment

This lesson will help prepare your students to meet the following scientific and engineering practices:

  • Scientific and Engineering Practices:
    • Asking Questions and Defining Problems
    • Planning and Carrying Out Investigations
    • Obtaining, Evaluating, and Communicating Information

Objectives

By the end of this lesson, students should be able to:

  • Explain the basic redox chemistry behind two photographic methods.
  • Use a chemical reaction to create a cyanotype.
  • Compare cyanotypes produced on different paper types.

Chemistry Topics

This lesson supports students’ understanding of:

  • Chemical changes
  • Oxidation-Reduction Reactions
  • Activation energy

Time

Teacher Preparation: 30 minutes
Lesson: 90 minutes

Materials (per lab group)

  • Iron(III) Ammonium citrate
  • Potassium ferricyanide
  • Distilled water
  • Electronic balance and weigh boats
  • 2 100-mL beakers
  • 2 100-mL graduated cylinders
  • Shallow plastic dishes (like these or something similar) that are larger than the paper
  • Several types of white paper (construction paper, printer paper, cardstock, etc.) cut into quarters (4.25” x 5.5” rectangles) and 15 cm filter paper cut to the same size
  • Crucible tongs
  • Cardboard cut to approximately the same size as the paper
  • Thumbtacks or paperclips (for securing paper to cardboard)
  • Small objects to create images on the cyanotype paper – students can bring in objects from nature (ex: leaves/flowers), lab equipment, school supplies, items from home, etc.

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

  • This lesson was created as part of the 2024 National Chemistry Week (NCW) Lesson Plan Contest. NCW is celebrated the week of October 20–26, 2024 with the theme, “Picture Perfect Chemistry.” Through this theme, students, teachers, and all those interested are encouraged to explore the chemistry of photography and imaging.
  • In this lesson, students will make two solutions, mix them, and use the mixture to coat two types of paper. When the paper is exposed to sunlight, a reaction takes place that turns the exposed paper blue, creating a basic type of photograph called a cyanotype. This lesson plan will reinforce students’ understanding of chemical changes, redox reactions, and activation energy.
  • After students read the background and answer the prelab questions, it may be helpful to review the answers as a class to ensure students understand the chemistry concepts.
  • Be sure to use plastic containers to mix the solutions and coat the paper. It’s tempting to use foil pans but results will not be as good. Plastic containers of nearly any size will do, but your container should be larger than the paper you will use.
  • To save class time, cut the paper samples and cardboard prior to the lab. An easy source of cardboard is old boxes cut to the approximate size of the paper. Students will place their treated paper on cardboard, fasten it to the cardboard with thumbtacks or paperclips, and then place objects. Students will need another piece of cardboard placed on top so that as they carry the paper and objects outside, the cyanotype is hidden from the sun until exposure should occur. Cardboard can be reused from year to year.
  • Students can collect objects outside, bring objects to class, or choose things from a lab drawer or junk drawer. In the images below, several different types of objects were used: plastic magnetic numbers, round plastic magnets, paperclips, a drinking glass, and a plant. Encourage students to use objects of various sizes, shapes, textures, opacities, etc. and to be creative with their images!
  • You may want to have students finish recording their observations and draw their conclusions on a subsequent day to give the cyanotypes time to dry completely.
  • If time allows, have students complete the first extension activity, which has them carry out the procedures they wrote in analysis question 7 for testing a different variable.
    • For more advanced students, consider providing the basic procedures for preparing the cyanotype paper and then have them select their own variables to evaluate, rather than limiting them to paper type. Other possible variables they could test include length of sun exposure, ratios or concentrations of solutions, using other materials besides paper, etc. (See the answer key for more options.)
  • The following sources provide more information on photography and cyanotypes:
  • Classroom resources from the AACT Library that may be used to further teach this topic include:
  • The images included below provide examples of cyanotypes at various stages in the process:

Figure 1: Coating the paper in the plastic tray

Figure 2: Developing the images outside on a sunny day

Figure 3: Dried cyanotypes on printer paper, cardstock, and filter paper

For the Student

Background

In the age of digital photography, it’s easy to point your phone at a scene and take a picture without giving much thought to how the image is captured. Before digital photography, pictures were produced on paper by using light-sensitive chemicals to create an image. Though digital photography is very popular and can generate many images without expensive and time-consuming development processes, film photography provides a different method of capturing images with distinctive possibilities for creative expression. In this experiment, you will become familiar with the chemistry of photography and take a picture using sunlight and light-sensitive chemicals.

Film photography takes place in a series of steps. First, paper is coated with compounds made of silver and a halogen like chlorine, bromine, or iodine. When exposed to the light, the silver ion in the compound is reduced to silver metal, in a reaction like this:

2 AgCl → 2 Ag + Cl2

Next the image is developed by exposing the film to an organic compound to reduce even more of the silver ions, which makes the image visible to the naked eye. Once it has developed, the paper is rinsed with acetic acid to stop the oxidation-reduction process, preventing the whole paper from darkening. Finally, a process called fixing is used to remove the excess silver compounds and preserve the black-and-white image.

A similar process can be used with different chemical compounds to create a blue image on paper called a cyanotype. To create a cyanotype, first solutions of iron(III) ammonium citrate and potassium ferricyanide are combined, and then paper is coated with the combination of the two solutions. When the coated paper is exposed to ultraviolet light, the Fe3+ ion in iron(III) ammonium citrate is reduced to Fe2+. The Fe2+ ion then reacts with potassium ferricyanide, creating a blue product anywhere the paper was exposed to the light. If items are placed on the paper before the paper is exposed to light, the blue color develops around the items, leaving a blue colored print. Once the print is created, the paper is rinsed with water to stop the reaction by removing the remaining reactants. The blue color remains because the blue-colored product is insoluble in water.

Light is an important feature in both photographic methods discussed here. To make chemical reactions take place, a certain amount of energy called the activation energy must be supplied. Often, we think of this energy being absorbed in the form of heat. In photography, light supplies the activation energy for the reaction that produces the image on the paper. When making a cyanotype, the activation energy is supplied by ultraviolet radiation in sunlight.

References:

Prelab Questions

After you have read the background information, answer the following questions:

  1. Determine the oxidation numbers for each element on both sides of the reaction:
    2 AgCl → 2 Ag + Cl2
  2. Which element in the reaction above is being oxidized? Which element is being reduced?
  3. Describe the electron transfer that takes place in the reaction above.
  4. Identify one similarity and one difference between silver-based film photography and making a cyanotype.
  5. What type of paper (printer paper, cardstock, filter paper, construction paper, etc.) do you think will create the clearest image? Justify your choice.

Materials

Iron(III) ammonium citrate
Potassium ferricyanide
Distilled Water
Electronic balance and weigh boats
2 100-mL beakers
2 100-mL graduated cylinders
Shallow plastic dish

Multiple types of white paper
Crucible tongs
4 cardboard sheets
Thumbtacks or paperclips (to secure paper to cardboard)
Objects to photograph

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.

Procedure

  1. Measure 4.00 g of solid iron(III) ammonium citrate on a balance. Place the solid into a 100-mL beaker.
  2. Measure 36.0 mL of distilled water in a graduated cylinder. Add the water to the beaker with the iron(III) ammonium citrate and swirl or stir the contents to dissolve the solid.
  3. Measure 4.00 g of solid potassium ferricyanide on a balance. Place the solid into a separate 100-mL beaker.
  4. Measure 36.0 mL of distilled water in a graduated cylinder. Add the water to the beaker with the potassium ferricyanide and swirl or stir the contents to dissolve the solid.
  5. If possible, dim the lights. In a shallow plastic dish, pour the two solutions together. Gently move the dish or use a stir rod to mix the solutions.
  6. Using crucible tongs, drag a piece of paper through the liquid, thoroughly coating at least one side. Place the paper, wet side up, on top of a sheet of cardboard. Secure it with thumbtacks or paper clips. Record your observations of the treated paper in the Observations table.
  7. Arrange objects on top of the wet paper. Cover the arrangement with a second sheet of cardboard.
  8. Choose a different type of paper and repeat steps 6 and 7.
  9. Wash your hands with soap and water.
  10. Carefully carry the materials outside and lay them on the ground in a sunny spot. Lift the cardboard cover and allow the sun to expose the image for 15 minutes. Record your observations of the paper after 15 minutes of sun exposure in the Observations table.
  11. Replace the cardboard cover and carry the materials inside.
  12. Remove the objects from the paper and then rinse it thoroughly with tap water. Lay the cyanotype on paper towels until it is dry. Record your observations of the rinsed and dried paper in the Observations table.

Observations

Paper Type Observations of the Cyanotype
Initial appearance of treated paper before sun exposure:
Appearance of paper after sun exposure:
Appearance of paper after rinsing and drying:
Initial appearance of treated paper before sun exposure:

Analysis

  1. What evidence in the experiment shows that a chemical reaction has taken place?
  2. Compare the images that you produced on the two different kinds of paper you prepared for your cyanotypes.
  3. What must happen with electrons in order to reduce the Fe3+ ion in iron(III) ammonium citrate to Fe2+?
  4. What supplies the activation energy in the cyanotype reaction? Predict what would happen if you conducted the reaction on a cloudy day. Justify your answer.
  5. The procedures indicate that classroom lights should be dimmed. Propose a reason for that based on what you read in the Background section.
  6. The procedures indicate that the paper should be rinsed after the sun exposure period. This removes excess reactants from the paper, leaving behind just the blue final product. What might happen if this step was skipped?
  7. Identify three other variables besides paper type that could you test in this experiment. Choose one of those variables and write a research question and set of procedures you could use for a future investigation.

Conclusion

Compare and contrast the images you created on the different types of paper. Which type of paper produced the clearest image of your objects? Use your observations to justify your answer.

Extension

  1. Carry out the procedures you wrote in the final analysis question. Record your observations and describe the differences between your various trial conditions.
  2. Cyanotypes produce blue images, but the images can then be treated with other chemicals to recolor them through a process called “toning.” Research this process and describe how it works in a few sentences. Identify three different chemicals or materials that can be used to tone cyanotypes and what color images they produce.