Invisible ink

One of Kid M and my partner’s favourite things to do together is play pirates. They have pirate names (Captain Flapjack and Pirate Porkpie) and make treasure maps. Not sure there is much more to it that sitting on M’s bed pretending it’s a boat and emptying an entire box of cuddly toys (the crew!) all over the bed, but they both love it. I decided to add a new level to the map making by introducing them to invisible ink.

What you need:

  • Paper and pens to make your map (essential – feel free to jazz it up with tea stains to make it look more like a genuine pirate map).
  • Lemon juice OR milk OR bicarbonate of soda (only one needed but it is essential. If using bicarb then mix one teaspoon of bicarb with one tablespoon of water).
  • Paint brush or cotton buds (not essential as you could use a finger I guess, but helps to paint the invisible ink on the paper).
  • Oven (or some other form of heating device, see tip/extra bits for more, essential – heat oven to 150 degrees C).

img_5346What to do:

  • Draw a treasure map on the paper.
  • Mark an X on it with your choice of invisible ink (lemon, milk or bicarb + water) and wait for this to dry.
  • Pop in the oven for 10 minutes to see the X reappear.


Tips/extra bits:

  • We did a couple of maps and turned it into a game. M and Tom each made one, did some crosses and then they each had to guess where they thought the other person’s X were. For this to work really well make sure you don’t have too much liquid for your X so that it dries easily and you can’t spot it.
  • We tested all 3 ‘invisible inks’. I would say that lemon juice was the best for not being able to see it before going in the oven. The bicarb and water was a bit grainy but if you mixed it better than we did (or add a little more water) then it might be less so… In terms of the ones that came out the best the bicarb and water was definitely the best, followed by the lemon and then the milk.
  • You could also use juice from an onion or apple – we didn’t test these but they are meant to work in the same way.
  • Instead of putting it in the oven you can use an iron, hairdryer, hot radiator or hold over a lamp. The oven is probably the quickest and easiest.

The science bit

Oxidation is the secret here. The liquids used all contain carbon compounds, which are colourless at room temperature. The liquids are absorbed by the paper. Heating up the paper causes some of the chemical bonds to break down and the carbon to be released. The carbon oxidises (reacts with the oxygen in the air), which causes the paper to turn brown.

Explaining it to children

The liquid that you add turns brown when heated up due to something called oxidation. You don’t always need heat for oxidation to occur. Think about what happens when you cut up an apple and leave it on a plate? The apple starts to turn brown, and this is due to the same process; oxidation.


Experiment based on an activity from the fantastic Usborne Activities: 365 Science Activities book.

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