What's it all about?
A blueprinting mixture is very easy to set up. Many plain papers will successfully take up this mixture. Objects put onto the dried blueprinting paper will block sunlight from getting to the reactant mixture - these bits stay unchanged. Where the sunlight 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.
Lots of things can be blueprinted, such as pens, rulers and protractors, with great results. You can experiment with cut-out messages and pictures, and even get different shades using masks and different exposure times. It works even on an overcast day in Yorkshire!
There are two parts to the blueprinting activity:
They can be carried out on separate days if required.
Put your gloves and eye protection on. Then get two small beakers:
- weigh 5 g of Substance A into one beaker
- weigh 9 g of Substance B into the other beaker.
Use the measuring cylinder to add 50 cm3 of water to each beaker. Stir carefully with the stirring rod until all the crystals in each beaker have dissolved.
Do the next bit in a dark part of the lab. Mix the two solutions (A and B) together, and pour them into a tray. Lay a piece of white A4 paper onto the surface of the liquid just long enough to get it damp – not wet! Your paper will turn greenish blue. Hang it up to dry out.
Carry your dry blueprint paper and object over to the window. Place your object on top of the paper, and leave it in the sunlight:
- the bits covered by your object will stay green
- the exposed bits will turn blue.
When you think it has gone blue enough, take the object off the paper. Wash the blueprint paper carefully with tap water. This removes the green colour but leaves the blue colour behind. You will see white where the green has been washed away. Leave your blueprint to dry out.
A technical drawing blueprint
Making the blueprint paper
This stage should be done away from direct sunlight, ideally in a semi-dark lab. Two solutions need to be made:
- substance A: 5 g of potassium hexacyanoferrate(III) dissolved in 50 cm3 of water
- substance B: 9 g of ammonium iron(III) citrate dissolved in 50 cm3 of water.
The two solutions are named as A and B just to make the method easier for the students to follow.
The two solutions should be mixed together and poured into a tray. Plain A4-sized paper is then floated on the surface. A wide range of paper types seem to work, but avoid very shiny paper or very coarse paper. It works best if the tray is gently swirled to cover all the underside of the paper, without prodding it with fingers and pens! The paper should then be hung up to dry in a dark part of the lab, and then left flat in a drawer or a lightproof box.
Making a blueprint
When the students are ready, they should arrange their objects or paper cut-outs on the blueprint paper. The paper must be dry – it doesn't work if the paper is damp.
The assembly can then be left in sunlight on a bench to develop. It helps if a sheet of glass is placed onto to keep it all flat. After the blue colour develops, the paper is then washed with clean tap water to remove the undeveloped green colour, and then left to dry. In good light, the blueprint develops within a minute or two to give a sharp image. On dull days, the exposure time needed is much longer, and produces a shadowy effect due to the movement of the sunlight.
Per group of students:
2 × 150 cm3 beakers
1 × 100 cm3 measuring cylinder
1 × glass rod
1 × plastic tray
1 × wash bottle containing distilled water
approx. 20 sheets plain A4 paper (or a size appropriate to the tray)
In the lab:
digital balance, precise to ±1 g or ±0.1 g
potassium hexacyanoferrate(III) – labelled as 'Substance A – Irritant'
ammonium iron(III) citrate – labelled as 'Substance B'
drying line with bulldog clips
Irritant to the skin and eyes.
Stains skin and clothes. Wear protective clothing (e.g. a lab coat), gloves and eye protection. Protect surfaces using old newspapers or similar.