THE CASE OF THE BLANK PIECE OF PAPER
You have found a piece of paper carefully hidden in a drawer, but there is nothing on it. So why was it preserved? Maybe there is something important written on it in invisible ink! How will you make the message readable so you can follow the clue it holds?
People have been sending messages using invisible ink for thousands of years. Really. The ink disappears after it dries and cannot be seen on the paper where it is written. And then the writing is made visible again using heat, ultraviolet light, powders, liquid reagents, or contrast colors.
How would you like to send a secret, invisible message to someone? And then reveal the writing in the answer you receive?
Here are two easy invisible writing methods to try.
1. Waxy writing
You will need:
Piece of white paper (we used printer paper)
White crayon
Watercolors
Paint brush
Small bowl of water
Write your message using the white crayon, which should be invisible on the white paper.
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To make the writing visible, paint over it with watercolors and let the paper dry. The crayon wax repels the paint, so the writing appears white against a now-colored background.
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2. Heat-activated ink
You will need:
Piece of white paper (we used printer paper)
Clear soda, like Seven-Up, containing sugar
Cotton swabs or paint brush
Iron
Ironing board or towel to fold under the iron
Write your message using the soda and cotton swabs or paint brush, and let it dry so it becomes invisible.
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To reveal the message, you will need an adult’s help. Put the paper on the ironing board or the multi-layer towel. Have the adult set the iron on the “cotton” setting and iron over the paper until the message is revealed.
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When heated, the sugar in the soda reacts with the oxygen in the air (oxidation), which turns the sugar brown – essentially caramelizing it!
There are variations on this type of invisible ink, using acidic liquids like lemon juice or vinegar. Dilute one with a little water and then write on the paper – this time a paper that absorbs more of the liquid than printer paper does. After it is dried, the writing should be invisible. Then iron the paper and again the writing will be revealed.
The lemon juice and vinegar also contain sugars, but their acids will react with fibers in the paper to create even more sugar for oxidation.
[Note to adult: To up the game, you could write a secret message, perhaps with a clue to where the child could find a treat or a small gift, and leave the paper somewhere the child could find it. Let the case begin!]
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For some other invisible ink methods, see:
https://www.youtube.com/watch?v=AOVD7WgFP2s (2:28)
http://seven-oaks.net/5-foolproof-invisible-ink-recipes/
Wikipedia provides interesting information for older children:
