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 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.
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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