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A month ago, I got a message from the OHM2013 speakerdesk:
❝Since you you'll be giving a presentation, lecture or workshop at OHM2013 (for grownups), we'd like to ask you; do you feel like giving a little talk about your subject for kids?❞
Thinking that my talk (about federation of online services, where you can have a choice of service providers or even run your own server rather than be beholden to big companies like Google, Facebook and Twitter (PDF of the slides, explanation in Dutch) might be a tad too abstract for kids age 6 - 10, I proposed something else instead: writing secret messages in invisible ink.
So I got to experimenting a bit. A well-known type of invisible ink is lemon juice or sugar water. After writing with it, you heat the paper, the ink turns brown and the message appears. Another option is to write with something that is acidic or basic, and then use red cabbage juice to bring out the message. This has the advantage of not requiring a heat source. So I got a cabbage, cut it into small pieces and cooked it with only a little water to obtain the dark purple juice. You can then easily create your own litmus paper by soaking up some juice in coffee filters.
About 25 kids age 6 - 10 participated. They really do seem to get younger every year... they're so small! I wanted to get them involved, so I brought several kinds of fruit and let them squeeze out juice to be used as ink. The onion wasn't a success, it's almost impossible to get juice out of it and it's not very acidic so it doesn't make for very good ink anyway. Apples are also hard to juice by hand, but are supposed to be quite acidic if you get the right kind. The juice we got was definitely weaker than the lemon juice, though. Grapes are fairly easy to juice, but it helps to have a coffee filter. Use white/green grapes or the ink is too visible... But lemons give you the most bang for your buck by far: just cut them up and squeeze, and you get plenty of juice without much pulp and it's nice and acidic.
The kids were completely unfazed by the fact that cabbage juice turns different colors when exposed to fruit juice and soap, by the way. They also had no problem drawing pictures using the "invisible" ink. (I guess drawing is more fun than writing.) In practice, the ink is not that invisible, because you can see where the paper is wet. I brought a bunch of small brushes to use for this, and that seemed to work well.
After finishing, we let the drawings dry in the sun. Then finally we got all the kids back inside and put all the drawings on a table, and applied the cabbage juice. I put the juice in a spray container, but this didn't work all that well: the juice doesn't get everywhere evenly, and a lot gets into the air, not smelling all that great. Using a big brush to paint it over the drawings worked better.
It takes a minute or two for the reaction to happen, turning the cabbage juice blue on unpainted parts of the paper, and red/purple where there's fruit juice on the paper. Putting cabbage juice on a drawing takes half a minute, so having all the kids come back inside before we started doing this was not the best approach with just one big brush. Having them come in as their drawing had dried would have been better.
If I'm going to do that again, I want to investigate some alkaline materials and see if we can get additional colors. Cabbage juice can be red, purple, blue, green or yellow, so there should be lots more possibilities than just the red/purple/blue that we got with fruit juices as ink. Also, it's easy enough to manufacture the cabbage juice on the spot, it doesn't have to cook long and you can just rip the leaves by hand if you don't want to use a knife. You can also buy prepared canned red cabbage and squeeze the juice out. I don't think the stains are too hard to get out, but you may not want to wear anything too new or too white.