Glowing Invisible Ink Secret Messages (Harry Potter Science)

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Want to learn how to make glowing invisible ink secret messages?
I’m so excited to show you how today!
This is a magical Harry Potter-inspired science activity that kids (and adults!) will love.

I’m joining forces with some of my favorite bloggers to bring you a whole week of Harry Potter STEM activities. You’re going to love these fun magical teaching ideas!

Along with this glowing invisible ink secret message project, I’m also sharing:

• Dry Ice Crystal Balls

• Arithmancy Math Activities

Our Version of the Marauder’s Map

This glowing secret message activity was our take on the magical Marauder’s Map from Harry Potter. While it’s not an actual map, you can easily turn this idea into a treasure hunt, a mystery puzzle, or a hidden adventure!

When we tried it, my daughter wrote a series of invisible clues to lead her brother on a scavenger hunt around our backyard.

Watch it in action here:

How to Make Glowing Invisible Ink

Did you know that laundry detergent glows? Cool, right?  We have tried a few different invisible ink experiments to decide which is our favorite. This glowing invisible ink is the best one we have tried.

Supplies Needed:

• Laundry detergent (with phosphors)

• Water (a few drops)

• Fine paintbrush or cotton swab (q-tip)

• Black light or UV flashlight

• White or light-colored paper

Directions:

Prepare the invisible ink:

Add just a tiny bit of water to your laundry soap to thin it slightly.

Write your secret message:

• Use a fine paintbrush (smaller tips work best!) or a cotton swab to write on paper.

• Try to make the letters thin so they dry faster and stay more hidden.

Let it dry completely:

Drying may take 15–30 minutes depending on how much liquid is used.

Reveal the magic:

Shine a UV flashlight or a black light over the paper to see the glowing words appear!

Our message starts with ‘I solemnly sear that I am up to no good,” because that is the phrase used on the Marauder’s map! Then my daughter said to meet her under the whomping willow tree. 🙂 From there she led her brother on another series of clues.

We have a little UV flashlight that we used to reveal the messages. This was perfect, but any black light or UV light will do!

The Science Behind Glowing Invisible Ink

Why does laundry soap glow?
Laundry detergents often contain phosphors, special substances that absorb invisible ultraviolet (UV) light and re-emit it as visible light.

• Under normal light, phosphors are hard to notice.

• Under black light, they fluoresce — meaning they glow brightly!

Phosphors are added to laundry soap to help clothes look whiter and brighter in sunlight.
In this project, the fluorescence makes your invisible message magically glow!

Download the Invisible Ink Marauder’s Map Now!

More Ways to Make Invisible Ink (Variations!)

If you want to experiment with different types of invisible ink, here are a few more methods to try:

1. Lemon Juice Invisible Ink (Heat Reveal)

• Supplies: Lemon juice, cotton swab, paper, iron or lamp

• How it works:

  • Write a message with lemon juice.

  • Let it dry.

  • Gently heat the paper with an iron (no steam) or hold it close to a warm lamp (carefully!) until the message browns and appears.

• Science: Lemon juice oxidizes (reacts with oxygen) and turns brown when heated.

2. Baking Soda Invisible Ink (pH Reveal)

• Supplies: Baking soda, water, cotton swab, grape juice

• How it works:

  • Write with baking soda solution.

  • Let it dry.

  • Brush or dab grape juice over the paper to reveal the secret writing.

• Science: Grape juice reacts with the alkaline baking soda and changes color, revealing the hidden message!

3. Cornstarch and Iodine Ink (Chemical Reveal)

• Supplies: Cornstarch paste, iodine solution

• How it works:

  • Write with cornstarch paste.

  • After drying, paint iodine solution over the paper.

• Science: Iodine reacts with starch to turn deep blue-black, exposing the writing.

Magical Extensions and Ideas

Want even more magical fun?

• Create a full Marauder’s Map:
  Map out your house or yard with hidden passages, clues, and secrets!

• Treasure Hunt:
  Hide invisible ink clues around your home or yard leading to a prize.

• Spy School Activity:
  Train your kids as secret agents using different invisible ink methods.

• Science Fair Experiment:
  Test which invisible ink methods are easiest to reveal, fastest to dry, or most invisible under normal light.

See more Harry Potter Themed Activities:

Harry Potter STEM: Arithmancy Math
Dry Ice Crystal Ball Experiment
Harry Potter Escape Room for Teaching

Creating glowing invisible ink messages is a magical blend of science, art, and storytelling.
Whether you’re hosting a Harry Potter party, a STEM day, or just want a creative afternoon, this project will spark excitement, curiosity, and a love of science!

Mischief managed.

Check out the rest of the awesome Harry Potter STEM ideas by hopping over to Babble Dabble Do.