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Revealed: Invisible ink invention that will surprise you

Donna Coutts, November 11, 2019 6:45PM Kids News

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There are still situations in which it’s important to send a secret message on paper, rather than by digital communications. media_cameraThere are still situations in which it’s important to send a secret message on paper, rather than by digital communications.


Reading level: green

Scientists have discovered a new kind of invisible ink.

It’s called water.

And all you need for this secret-messaging substance to work is to write with it on a super-hi-tech piece of paper coated with a chemical that includes a mineral called manganese.

Shine a UV* light on the coated paper and the water message becomes visible.

Supplied Money Bottled water, generic, shopping media_cameraWith the hi-tech, chemically coated paper, pure water becomes invisible ink that is able to be read under UV light.

When the message has been read and needs to be hidden again, get out your hairdryer, blow some warm air on the paper, the message is erased and the paper can be reused, which keeps costs low.

The chemicals on the paper absorb and emit* light and the scientists found that putting water on the paper disrupted the structure of the chemicals it was coated with. The water writing showed up as darker than the surrounding paper under ultraviolet light.

Using hot air from a hairdryer for 15-30 seconds, they were able to reuse the special paper up to 30 times because the heat reorganised the chemical structure of the paper coating.

Without the heat from a hairdryer, the message stayed visible for up to three months.

Supplied Editorial Shop Smart, Thinkstock pic for Beauty Spot media_cameraHot air from a hairdryer for 15-30 seconds completely wiped the message from the special paper and it could be reused.

The invention was by a group of scientists in China, led by Qiang Zhao of the Institute of Advanced Materials, Nanjing University of Posts and Telecommunications. Details have been published in the journal Matter.

Although much of modern communication is digital, there are still sometimes situations in which it is important to send messages on a piece of paper, such as when the recipient* doesn’t have internet access or a secure electronic device.

Invisible inks have long been useful for sending secret messages but the problem has always been how to dispose of a message once it has been revealed so no one else can read it.

Previously, the most readily available invisible ink was lemon juice.

Write with the lemon juice on regular paper, iron the paper with a hot iron and, voila*! the hidden message appeared.

The downside was that the paper had to be destroyed to hide the message so no one else could read it.


  • UV: short for ultraviolet
  • emit: give off or give out
  • recipient: person receiving
  • voila: pronounced vwah-la,a French word that is commonly used in English and means “there you are”


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  1. What do you need to read the message written with water?
  2. How many times were they able to reuse the special paper?
  3. What happened to the message without a hairdryer?
  4. Give two reasons why sending a secret message digitally may not be possible.
  5. What is one of the downsides of using lemon juice as invisible ink?


1. Secret Messages
Work with a partner and come up with a list of times or circumstances where you or an organisation that you know of could use this special paper to send secret messages.

Is there any need to use it at your school? Explain why or why not.

Time: allow 15 minutes to complete this activity
Curriculum Links: English, Critical and creative thinking

2. Extension
Can you think of some other ways to send messages secretly, either via paper or digitally? How important is it in the digital age to have things secure? Is going back to old-fashioned ways the most secure way to deliver confidential and secretive information?

Time: allow 10 minutes to complete this activity
Curriculum Links: English, Critical and creative thinking

After reading the article, with a partner, highlight all the openers you can find in blue. Discuss if they are powerful and varied openers or not. Why do you think the journalist has used a mix of simple and power openers? Would you change any, and why?

HAVE YOUR SAY: Have you ever used any kind of invisible ink or other spy trick to send a secret message? What works well?
No one-word answers. Use full sentences to explain your thinking. No comments will be published until approved by editors.

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