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Make a watermark

By David Shaw, 4 October 2018

If you want to write a secret message, we’ve got a simple technique for you to try. Time to work on your watermark!

You will need

• Two sheets of paper
• Towel
• Ballpoint pen
• Bowl or tray of water

What to do

1. Put one sheet of paper in the water and leave it to soak for a few minutes.
2. Pull the wet paper from the water, holding it above the bowl or tray, allowing the excess water to drip back into the container.
3. Place the wet paper on a table or desk, making sure it’s nice and flat.
4. Gently pat the paper dry with a towel.
5. Put the dry sheet of paper on top of the wet sheet, and quickly start writing your secret message on it. If you wait too long, the top sheet will get wet and start to rip!
6. When you are finished, take the top sheet of paper away – you might want to destroy it so no one can read your secret message.
7. Hold the wet paper up to a light – can you see the message?
8. Leave the wet piece of paper to dry.
9. When it’s dry, look at the paper – can you read the message now?
10. To reveal the message, wet the paper and hold it up to the light like before. Your writing will shine through!

What’s happening?

You may already know that paper is made from trees, but do you know how? Trees are chopped into tiny pieces releasing the fibres within. The fibres are then made into a soupy, watery mixture. This mixture is rolled, squeezed and dried, and the fibres get tangled and stick together.

The entwined fibres are what makes paper strong. When paper gets wet, the fibres unstick from each other a bit, which is why wet paper tears easily.

In this activity, you’re squeezing the wet paper between a table and a pen. The wet paper fibres are free to move just enough to thin the paper in those areas. If you look closely, even at the dry sheet, you might be able to see where the paper has thinned.

The other piece of the puzzle is the stuff between the tiny fibres – gaps. When paper is dry, air is in the gaps between the fibres causing light to reflect off the paper, making it appear opaque (not see-through). When water fills the gaps, less of the light is reflected and more passes through the paper, so it looks see-through.

It’s the combination of thin paper and water that work together to leave a watermark – a secret message that can only be revealed with water.

Did you know?

Historically, watermarks like this have been used with paper products to detect and prevent counterfeiting and forgery. Over time they have become more complex, so typically they do not need to be wet to be seen when held up to the light. The idea has now carried over from paper to digital watermarks, used to identify digital files.

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