Tech: Using invisible ink

 

Schrödinger's Cheshire elephant, drawn with invisible ink.

My playmate and guardian angel, Barbara Braxton, a teacher-librarian, has always been in the habit of posting challenges at me, usually because she wanted something different for some students who were using her library. I, in turn, would always push the something different envelope, and I started out like this:

Spies often need to share the proceeds of their crimes, so let’s begin there. If a cake is to be shared between two people, one person cuts the cake in two, the other person chooses who gets which half. How would you do it with more than two people? There is a way!

When three or more people share a cake, the first person cuts a slice. If somebody thinks it is too large, they can take a piece off the slice, but then it becomes their slice.

Then the next person takes a share, and so on.

After that, I went to invisible ink.

Alphonse de Neuville’s 1880 painting The Spy shows a French soldier, in civilian clothes. The time is the Franco-Prussian War, and he has been captured by Prussians as he tried to get into Metz, which the Prussians were besieging. He was probably a messenger, but as he is not in uniform, he is likely to be shot as a spy. Will they find secret messages in invisible ink? Will he be shot? Over to you!

You will need one of these: a pipe cleaner, a toothpick, or a small paint brush to use as your ‘pen’, or you can try making a quill pen. Steel pens were never used for invisible ink writing, because they left fine scratches on the paper that could be seen in a slanting light. Use your ‘pen’ to write a message on a piece of paper. Do not use too much ‘ink’, or the paper will wrinkle, drawing attention to the secret writing.

Invisible ink from milk

You need a small bowl and a small amount of milk. Use your chosen ‘pen’ to write a message and let it dry completely. The person who gets your message must heat the paper so the message reappears. This can be done by ironing or holding the paper above an old-fashioned (incandescent) 100-watt light bulb or a radiator, but watch out for burns! When milk is heated, it turns brown before the paper does, and the invisible message appears.

Invisible ink from a lemon

You need a small bowl, some lemon juice (or a lemon, a knife, a board and a squeezer), and one of the ‘pens’ mentioned above. You can write with lemon juice in the same way as with milk and read it after it is dry by heating it. You can also read it by spraying the paper lightly with red cabbage water. Use a plant misting bottle (see the note at the end) to spray on the red cabbage water.

With heating, the writing appears gradually because heat causes a chemical change in the lemon juice. The juice chars at a lower temperature than paper, so the writing appears faint and brown. Red cabbage water is an indicator for acids and bases. Since lemon juice is an acid, the red cabbage water interacts with the dried lemon juice and turns a different colour, so the secret message reappears.

Making red cabbage water solution:

a. Carefully chop part of a large red cabbage into small pieces on a board with a kitchen knife.

b. Simmer the cabbage pieces in hot water until it turns into a deep shade of purple.

c. Allow the water to cool, and refrigerate it when not in use.

Be careful with the red cabbage solution, because it can stain your clothes. Be sensible around sharp knives and boiling water.

Invisible ink from vinegar

You will need a small bowl, some vinegar, and a suitable writing instrument.

Write your message in vinegar and use red cabbage juice to read the message, as in the previous method. Vinegar is also an acid.

Invisible ink from starch

You need 5 mL (1 teaspoon) of corn starch in 60 mL (¼ cup) of water, and some way of heating the starch solution gently: half a minute in a microwave is about right.

Stir the starch and let it cool. Write your message on paper and let it dry.

To read it again, wipe the surface of the paper with a sponge that has been wetted in iodine and water. The iodine mixture should be 10 drops of standard iodine solution in 60 mL (¼ cup) of water. The message will show up dark purple on a light purple background.

Safety tip: Iodine can be used on cuts to kill germs because it is poisonous to living things. Drinking or eating iodine will make you sick. Wear gloves and wash up carefully afterwards. The dry corn starch message has a lot of starch. The iodine reacts with starch, and turns dark purple.

Note: Some types of paper may contain starch as a filler, which may stop you reading the message. This is a good way to discover the value of thinking ahead—and there is also a project idea here. Newspaper should be starch-free and so it should work well.

Invisible ink from baking soda

You need baking soda, sodium bicarbonate or sodium hydrogen carbonate (same thing, three names), water and grape juice concentrate.

Mix about 60 mL (¼ cup) of baking soda and 60 mL (¼ cup) of water. Write with this mixture on paper, using a brush. Let it dry completely. To read the secret message, paint grape juice concentrate across the paper from top to bottom with a paint brush or a sponge.

Does it work with regular grape juice?

Tip: Grape juice stains things. Be careful not to spill it.

The acid grape juice interacts with the alkaline baking soda to produce a different colour making the secret message appear. Perhaps you can explore a few other fruit juices, to see if any of those work.

I haven’t tried this one, but I think you will need to avoid flooding the page with grape juice, as that may wash the message away, so use a damp brush, or spray it with a misting bottle.

Invisible ink from lemon juice, honey and glycerine

I appear to have found this on the web: it was in my notes, but I don’t think I wrote it, so I make no claim to it. Also, I have not tried it yet. You are on your own, Secret Agent!

You need lemon juice, runny honey and glycerine. My notes say the mix should be about 4:4:1 lemon : glycerine : honey. Glycerine can be bought at any pharmacy, and it is safe enough—but as a rule, never drink anything in the lab. Mix well and store in the refrigerator.

1. Dip a cotton swab or a brush into the ink and use it to write the secret message on a piece of paper.

2. Allow the message to dry completely.

3. To read the invisible ink, spray the message with the red cabbage water.

You can buy a misting bottle for anything from $2 upwards, but I use the ones that come with window cleaners, bathroom cleaners and stain removers. You just need to rinse and wash them very well.

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