Turmeric and invisible ink

As the small boy enjoyed watching parts of pictures disappear when seen in infrared light, I thought about trying invisible ink drawings.  There are several ways of doing them, some more toddler-compatible and colourful than others.  I thought I'd give turmeric a go as I knew we had a jar on our spice rack which is so far past it's use by date that I'm a little dubious about cooking with it.

Turmeric contains curcumin, which is bright yellow.  However, curcumin is sensitive to pH and changes to a red/purple in the presence of an alkali.  It wouldn't be great in our red cabbage pH indicator experiment though, as it stays yellow when you add acid rather than giving a further colour change.  Turmeric solution should give a really obvious colour change if brushed across paper which has something drawn on it with an alkaline invisible ink.

I first tried to dissolve it in hot water, and whilst it smelt delicious and reminded me of Moroccan souks, it didn't seem to dissolve very well and I got a very feeble colour change when I tried it.  A little Googling suggested curcumin is pretty insoluble in water, but very soluble in ethanol (in alcoholic drinks) and and also in isopropanol (sometimes sold as 'rubbing alcohol').  I decided to try improvising with something we had in the drinks cupboard instead of buying anything else.

We've a selection of various unpleasant-tasting spirits from our travels, and top of the list of things we'll never actually drink is BrennivĂ­n from Iceland.  It's 37.5% ethanol (and flavoured with caraway which I really dislike), so I thought it was worth a go as a solvent to try and dissolve curcumin in.  Indeed it works nicely and gives a vibrantly yellow liquid with a very visible colour change if you add about a level teaspoon of turmeric to a shot glass of the spirit (with a lot of undissolved sediment which settles to the bottom).  The smell of the combined BrennivĂ­n and turmeric mixture is quite bizarre, but probably adds to the mystique of the invisible ink... I'm pretty sure you could use any uncoloured alcoholic beverage with a high percentage of ethanol, but I haven't tested it.

I drew some shapes and words on some sheets of white A4 paper with a concentrated solution of bicarbonate of soda in water and left to dry overnight.  If you held them up to the light, you could see a little wrinkling where the drawings were, so it wasn't truly invisible, although you could probably improve on this with higher quality paper.

The small boy was pretty lethargic this morning as he's got a cold and had a busy day yesterday, so I thought a nice sitting down activity might be good and suggested the invisible ink.  Initially he wasn't terribly keen, but - like many other small children - he has an invisible friend, although in his case, it's an invisible gecko rather than a person(!) and he goes to lots of places with us.  I'd attempted to draw a gecko on one of the sheets of paper last night (although my art skills are pretty poor) so I asked if he'd like to try and find the invisible gecko.  He suddenly gained some enthusiasm for the idea...

I took some of the clear yellow liquid from the turmeric and alcohol mix and put it in a little pot for him to use.  It stains everything yellow and is pretty hard to remove, so a used yoghurt pot or similar is ideal.  I also gave him a big children's paintbrush and one of the sheets of paper I'd drawn on, and put his painting apron on (as I didn't want his clothes to be permanently yellow either).  He dipped the brush in the pot and dabbed it on the paper, and remarkably hit a spot where it was partly on the bicarbonate of soda drawing and partly not, so some of it turned a bright red/purple.  He was pretty keen to find the rest of the picture, and did a bit more dabbing with his brush.  This one wasn't the gecko, but his name and some stars, and he could see the start of some letters.  I suggested some brush strokes instead to spread the turmeric solution around; he's usually averse to the suggestion of painting like that with paint, but in his enthusiasm to find the picture, he happily painted away with the yellow.

We tried the gecko next, and he was quite excited to find it had stripes.  He wanted to find its head, eyes and legs and did some speedy brush strokes to uncover most of the invisible ink drawing.  I'd also written the word 'gecko' (as he's quite into trying to read words phonetically at the moment) and he enjoyed finding that too and sounding it out.  A fair bit of yellow was spread around his table, and I haven't managed to get it all off, so I'd suggest using something waterproof underneath if you're worried about yellow stains.

Finding the stripes of the gecko

The colour change is very easy to see

I mentioned to my son that the yellow was from something called turmeric, but to be honest it didn't mean anything to him as he's never cooked with it.  I tried to explain that the invisible ink was an alkali and the turmeric contained an indicator like red cabbage, and he instantly asked to 'do science' next.  I thought I'd try branching out a little from our usual red cabbage indicator experiment and get him to test the remaining turmeric solution with his acid and alkali (vinegar and bicarbonate solutions respectively).  He was keen on the idea, so I put the yellow solution into an empty soft cheese pot.  The pot is white inside so makes any colour change nice and obvious.  It also avoided our nice science set getting permanently yellow stained.  He first added some acid, and there was no colour change.  He then added some alkali, and nothing happened.  After a lot more alkali, there were flecks of purple, and the solution was a little more orange, but it didn't really work well as a liquid indicator and he got bored of trying after 5 minutes of pipetting.  I tried myself a bit later, and had to put a lot of bicarbonate of soda powder in the liquid to get a noticeable colour change.  We moved onto doing some colour mixing, which has recently been extended after some experimentation by the small boy to making brown by mixing red, blue and yellow.

Attempting to change the colour by adding a lot of bicarbonate of soda solution

Overall, the small boy and I both liked the effect of the invisible ink and the bright colour change and it was achievable with things we had in the cupboard, but it's pretty messy!