Invisible ink, sometimes called ‘sympathetic ink’, is a way of writing secret messages. Spies have used invisible inks to pass on secret messages, as have prisoners and hostages.
Invisible Messages and Heat
Write a message using a cotton bud or a fine paintbrush and lemon juice. Let it dry and then heat the message by putting it on a radiator, carefully holding it next to an incandescent (not low energy) light bulb, baking it in the oven, or ironing it, and wait for the message to appear. Don’t use a halogen light – it gets too hot, and don’t let the paper get too hot and catch light. Try it with grapefruit juice, vinegar, milk or onion juice. Which inks work the best?
The ‘inks’ are acidic and weaken the paper a little, or leave compounds in the paper. The acidic parts of the paper or the compounds left in the paper get hot, burn and change colour (like burned toast).
Invisible Messages and Pencil Lead
Make a strong salt solution (put a bit of hot water into a cup and then stir in as much salt as will dissolve (this is called a saturated solution). Write a message using a cotton bud or a fine paintbrush and the salt solution. Let it dry and then gently shade over the paper with the side of a lead pencil.
As the salt solution dries, it forms salt crystals (see ‘Growing Crystals’) on the surface. These are rough and catch more of the graphite (the black colour) from the pencil lead.
Invisible Messages and Paint
Write a message on a piece of paper using a plain white candle as a pen. Paint over the paper with watercolour paint – the message will appear.
The wax waterproofs the paper so protects it from the paint, which soaks into the paper everywhere else.
Invisible Messages and Indicator Solutions
Make some indicator solution (see ‘Acids and Alkalis: Which Are Which?’) by finely chopping or grating red cabbage and covering it in boiling water, or blending coarsely chopped red cabbage with boiling water. Leave it for ten minutes, and strain it through a coffee filter. Write a message using a cotton bud or a fine paintbrush and lemon juice and vinegar. Let it dry and then paint over the indicator solution, or put it in a plant mister and mist the paper lightly. Try it with a mixture of equal amounts of water and bicarbonate of soda as the ink, and grape juice as the indicator.
Lemon juice and vinegar are acids – the red cabbage indicator solution changes colour when it meets the acidic message. Bicarbonate of soda solution is alkaline (also called ‘basic’) – grape juice is an indicator and it changes colour when it meets the alkaline message.
Invisible Messages and Black Light
Write a message on a piece of paper with a fluorescent security marker, and then shine a ‘black light’ (used in fish tanks) on the paper to read the message. Try it with a laundry liquid or powder that claims to make ‘whites whiter’.
A black light gives off ultraviolet (UV) light – light that has a wavelength that human eyes cannot properly see. UV light makes some chemicals glow (fluoresce). The police use UV light to identify stolen property marked with invisible fluorescent ink, and tropical fish keepers use UV light in fish tanks to make their fish glow brightly. Some laundry liquids or powders contain optical brighteners. These glow in bit of UV light that is in daylight, to make clothes look whiter.