Invisible Ink – That Works

posted by Gina

Age Range: 5 and up

How many of us have been sorely disappointed with the lemon-juice-and-candle invisible ink of myth and legend?  If your attempts were like mine, the paper would catch fire before a message appeared.  Besides, any secret messages written under careful adult supervision kind of defied the purpose.  In theory, a hot light bulb will achieve the same effect.  In reality, not always – especially with today’s energy-saving bulbs.

Just to be sure we remembered our previous failures correctly, my brave assistants and I set to work with lemon juice and a light bulb. As we suspected, little happened, other than my assistants growing bored with the entire process and wandering away.

But fear not, my secret-message-writing friends!  A tipster pointed me towards an invisible ink that does, in fact work each time.  Better still – no flames or hot bulbs are required, making this experiment one that does not need extinguishers at the ready.  

You will need:

• Baking soda
• Water
• Grape Juice
• Paper
• Q-Tip or similar
• Paintbrush

Procedure:
1.  Mix equal parts baking soda and water in a bowl or cup.  (You don’t need much at all – a spoonful will suffice.)

2.  Dipping the Q-Tip into the baking soda mixture, paint your secret message onto the paper.

3.  Let the paper dry thoroughly.

Here, as with lemon juice, we discovered that a secret message is not so secret when the paper shows your words via wet marks.  Even though the baking soda and water dries colorless, you can still see the writing.  To solve this problem:

4.  Write a decoy message across the paper.  This writing will obscure your tell-tale wet marks.

5.  Once the paper is dry, paint over the paper with the grape juice.  Et Voila!  Your secret message appears!

Why this works:
The baking soda and water solution is a base, while the grape juice is acidic.  The two react, turning a gray color.

Here and here are mostly kid-friendly explanations of PH Levels, acids, and bases.

I wonder what else might work, using the acid/base combo – perhaps a little home experimentation might offer some solutions?

Curriculum connection:  chemistry

 Related Detective Literature:

• The Nate the Great books (younger readers, 1st and 2nd grade)
• The Sammy Keyes books (2nd through 4th)
• The Encyclopedia Brown books (1st through 3rd)
• And those of us who loved them as kids will be delighted to know that the Three Investigator series is back in print (2nd through 6th)

Secret Code Links:

• http://youthonline.ca/spykids/
• https://www.cia.gov/kids-page/games/break-the-code/index.html
• http://www.nsa.gov/kids/home.shtml