Blueprints use the cyanotype process invented by the astronomer John Herschel in 1842. The paper is coated with a solution of two soluble iron(III) salts. The two iron salts do not react with each other in the dark, but when they are exposed to ultraviolet light the iron(III) ammonium citrate becomes an iron(II) salt. The iron(II) ion reacts with the potassium ferricyanide to form an insoluble blue compound, blue iron(III) ferrocyanide, also known as Prussian blue.

Student Sheet

In this practical I will be:

  • Carrying out an experiment to produce Blueprint paper.
  • Producing an image or diagram on my Blueprint paper.
  • Investigating the process of producing Blueprints and the role UV light plays.

Introduction:

While on a school trip, you saw that some renovation work was being carried out by some builders. On a table were the Blueprints for the building. You realise that the shades of white and blue would be perfect for a piece of art you are currently working on. However, before you can use these shades, you need to understand how they are made. You decide to investigate further…   

Equipment:

  • 1 beaker (250 cm3 )
  • 2 beakers (100 cm3)
  • 1 measuring cylinder (100 cm3)
  • 1 glass stirring rod
  • 1 plastic tray
  • 1 wash bottle containing distilled water
  • 20 sheets (or access to) plain A4 paper avoid shiny or very absorbent papers
  • 2 weighing boats (or gallipots)
  • 2 spatulas
  • Potassium hexacyanoferrate(III) – labelled “Substance A – Irritant” (low hazard)
  • Ammonium iron(III) citrate – labelled “Substance B” (low hazard)
  • 1 drying line with 2 bulldog clips (or string and pegs)

Access to:

  • Marker pen
  • A4 paper
  • Digital balance
  • Drying line (string and pegs)
  • Paper towelling
  • Disposable gloves
  • Newspaper (to cover the work area)

Method:

Making the blueprint paper

Wear gloves and goggles. 

  1. Get two 100 cm3 beakers, a measuring cylinder and a stirring rod. Mark one beaker A and the other B.
  2. Weigh 5 g of Substance A into the beaker marked A.
  3. Now weigh 9 g of Substance B into the beaker marked B. 

Use the measuring cylinder to measure 50 cm3 of water and pour the water into beaker A. 

  1. Stir carefully until all the crystals have dissolved.
  2. Now measure out another 50 cm3 of water and pour into beaker B.
  3. Stir carefully with a clean glass rod until all the crystals have dissolved.
  4. Do steps 10–12 in a dark part of the lab. 
  5. Mix the two liquids together, and pour them into a tray. Move the tray gently to get the liquid to cover the base of the tray properly.
  6. Put a piece of white A4 paper into the liquid just long enough to get it damp - not wet! Place a piece of A4 paper onto the liquid in the tray, then lift the paper out of the tray by the two corners nearest to you. Allow the excess solution to drip into the tray before placing it wet side up onto some newspaper on a desk. 

Your paper will turn greenish blue. Hang it up to dry out in a dark part of the laboratory or store it lying flat in a dark drawer. Hang your paper up using the string line and pegs in a darkened area to dry.

  1. The paper must be dry since the experiment will not work with wet or damp paper.
    • Why do you think you have to wear gloves and goggles?
    • What does dissolve mean?
    • Why do you think the mixing has to be carried out in a dark place?
    • Why do think the experiment will not work if the paper is wet?

Making the blueprints

Wear disposable plastic gloves

  1. When dry place your prepared paper under another piece of paper to keep it away from the sun.
  2. Place the package by the window so the light can fall on it.
  3. Remove the protecting piece of paper and place an object on the surface.
  4. Leave it in the light for about 1–5 minutes. Longer exposure leaves a shadow; shorter exposure times produce a sharper image. 
  5. When you think it has gone blue enough, take the object off the paper. The covered parts will still be green.
  6. Wash the paper with water to wash away the green chemicals and leave the blue behind.
  7. Hang your blueprint up to dry out.
  8. Wash your hands carefully.
    • Why does your prepared blueprint paper need to be kept in the dark?
    • Does the paper change colour quickly when it is exposed to the light?
    • What does the washing do to the paper?
    • Why do you have to wash your hands at the end?

Going further:

Try a range of different types of paper to see if the paper type makes a difference to exposure time, depth of exposure, etc.

If you can get some old black and white negatives try using those on the blueprint paper. You will have to experiment with exposure times.

Describe how the blueprint paper is similar and how different it is to photographic developing with a film. Research the chemicals used in photography.

Theory:

Blueprints use the cyanotype process invented by the astronomer John Herschel in 1842. The paper is coated with a solution of two soluble iron(III) salts - potassium hexacyanoferrate(III) (potassium ferricyanide) and iron(III) ammonium citrate.

The two iron salts do not react with each other in the dark, but when they are exposed to ultraviolet light the iron(III) ammonium citrate becomes an iron(II) salt. The iron(II) ion reacts with the potassium ferricyanide to form an insoluble blue compound, blue iron(III) ferrocyanide, also known as Prussian blue.

A blueprint starts out as a black ink sketch on clear plastic or tracing paper. The ink sketch is laid on top of a sheet of blueprint paper and exposed to ultraviolet light or sunlight. Where the light strikes the paper, it turns blue. The black ink prevents the area under the drawing from turning blue. After exposure to UV light, the water-soluble chemicals are washed off the blueprint, leaving a white (or whatever colour the paper is) drawing on a blue background. The resulting blueprint is light-stable and as permanent as the substrate upon which it is printed.

Teacher and Technician Sheet

In this practical students will:

  • Produce Blueprint paper.
  • Create an image or diagram on Blueprint paper.
  • Investigate the process of producing Blueprints and the role UV light plays.

Introduction: 

(The topic could start with a group discussion during which teachers introduce the following ideas, especially the words in bold.)

A blueprint is an old term used for a reproduction of a technical drawing of an object such as an architectural or engineering design. They were made by a contact process using light-sensitive sheets. It was important because it allowed the rapid and accurate reproduction of design documents. It was called a blueprint because of the light lines on a blue background, forming a negative of the original. 

Paper was frequently used but for more durable prints linen was sometimes used. Sadly, over time the linen prints would shrink slightly, so later imitation vellum and polyester film were used instead. Nowadays drawings are produced on computer, printed, and then photocopied.

These blueprint papers have absorbed certain chemicals that are changed when visible light or ultraviolet (UV) light falls on them. Hence objects put onto the dried blueprinting paper will block visible or UV light from getting to the chemicals and those areas, untouched by the visible or UV light, stay unchanged.

Where the visible or UV light can get to the paper, an intense blue colour develops. The blue colour will not wash out of the paper, but the greenish colour left under the object will. This leaves a white image of the object on a blue background. It is possible to investigate the effects of differing exposure times, screening with certain materials.

(To make the process easier for the students and safer the two solutions can be made up in the dark and stored in dark bottles.)

(This practical can be done with pupils working as individuals or in groups of two. Groups of two allows for good discussion between the pupils. Teachers can use the questions set as the stimulus for discussion and the answers can be used as a group report, article, presentation, poster or talk.)

Curriculum range:

Suitable for middle school or lower secondary students; it links with:

  • ask questions and develop a line of enquiry based on observations of the real world, alongside prior knowledge and experience; 
  • use appropriate techniques, apparatus, and materials during fieldwork and laboratory work, paying attention to health and safety; 
  • make and record observations and measurements using a range of methods for different investigations; and evaluate the reliability of methods and suggest possible improvements; 
  • present observations and data using appropriate methods, including tables and graphs; 
  • interpret observations and data, including identifying patterns and using observations, measurements and data to draw conclusions; 
  • present reasoned explanations, including explaining data in relation to predictions and hypotheses; 
  • the concept of a pure substance; 
  • mixtures, including dissolving. 

Hazard warnings: 

Potassium hexacyanoferrate(III) – Skin/eye irritant, (Cat 2)  Respiratory irritant (STOT SE3) Ammonium iron(III) citrate –  Skin/eye irritant, (Cat 2)  

Good practice would require exposure to be kept to a minimum and suitable gloves be used by the students. Students with impaired respiratory function may incur further disability if excessive concentrations of particulate are inhaled so good ventilation is required. 

In addition, contact with strong acids causes the release of highly toxic hydrogen cyanide. This is not likely to be an issue but care should be taken on disposal to ensure that the drain/sink does not have acid already present.

Ammonium iron(III) citrate is slightly hazardous as an irritant through skin or eye contact.

Wear safety glasses. Wear disposable gloves.

Equipment:

For a group of students:

  • 1 beaker (250 cm3)
  • 2 beakers (100 cm3)
  • 1 measuring cylinder (100 cm3)
  • 2 glass stirring rods
  • 1 plastic tray
  • 1 wash bottle containing distilled water
  • 20 sheets (or access to) plain A4 paper (avoid shiny or very absorbent papers)
  • 2 weighing boats (or gallipots)
  • 2 spatulas
  • 10 g potassium hexacyanoferrate(III) – labelled “Substance A – Irritant”
  • 15 g ammonium iron(III) citrate – labelled “Substance B – Irritant”
  • 1 drying line with 2 bulldog clips (or string and pegs)

Access to:

  • Marker pen
  • A4 paper
  • 1 digital balance
  • Drying line (string and pegs)
  • Paper towelling
  • Disposable gloves
  • Newspaper (to cover the work area)

Technical notes:

If available use a fume cupboard to hang the string lines up in ready to peg the paper to dry and close any blinds near it.

It is possible to dry the prepared sheets more quickly by using radiators and/or hairdryers if available, but otherwise this practical would have to be carried out over two lessons to allow for drying time.

The amount of pages that can be hung out to dry is limited by the amount of space available.

Laminating sheets can be drawn on and placed onto the prepared sheets before placing in bright light to leave an imprint on the paper.

An alternative is to get pupils to use image editing software to produce a negative of their choice that can then be printed out on transparency film.

The paper may stain yellow and dry yellow, but it will still change colour when exposed to bright light and develop a blueprint when washed with water.

Results:

This practical works well in normal daylight with the internal lights switched off. 

The amount of space to dry the papers directs how many sheets can be used in a practical.

Good results can be obtained using ordinary A4 paper and using laminating sheets to draw on.

Any shadows will also be processed on the paper so try to place the paper where it is in direct light and is laid flat.

The amount of chemicals used and solution produced could be halved and still cover about 10 sides of A4.

The hazards are minimal assuming the expected level of behaviour from students.

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