Less messy colour-changing turmeric painting

When we were making our coloured oil and water mixes, there was an unfortunate incident involving yellow food colouring and the small boy's brand new white t-shirt.  In hindsight, I don't know why I ever thought it was a good idea to let him loose with brightly coloured liquids without something covering his clothes, but I won't be making that mistake again soon!

The yellow food colouring contains curcumin as the yellow pigment.  Curcumin is the molecule that gives turmeric it's yellow colour; the food dye we were using was Tesco's own brand (UK), but most 'natural' yellow food colouring seems to be curcumin.

I quickly got the curcumin-stained t-shirt off the small boy, made it damp and put some stain remover (Vanish) on it.  It turned red/purple as the stain remover is alkaline (and the curcumin is pH-sensitive).  All very pretty, but after a wash, the t-shirt still had a yellow stain which turned back to purple with more Vanish.  Rather annoying about the t-shirt not being white anymore, but it actually gave me an idea about a less messy way to play with curcumin and its colour changes.  We've previously to reveal invisible ink but it led to quite a lot of turmeric being painted on the table and on his apron as he's a messy painter.

Given that the yellow curcumin seems to bind well to cotton fabric, and stay on even in the washing machine, I thought we could make a yellow curcumin 'canvas' on which the small boy could paint with colourless bicarbonate of soda solution.  My rationale for this was to minimise more yellow stains which are near-impossible to remove from clothing and furniture (you'd understand why I was trying to think of lower mess ideas if you'd seen his painting style).  To dye the cotton I tried both curcumin, extracted from turmeric into a 37.5 percent ethanol (i.e. 'alcohol') spirit, and the yellow food colouring diluted in water.  Both worked fine, and the food colouring is less faff and cheaper than dissolving turmeric in alcohol.

We had a play with the unwashed, but dried, fabric first.  We got the same vibrant colour change from yellow to red/purple that we had with invisible ink.  However we found that the colour change happens a little slower than when revealing invisible bicarbonate of soda ink on paper with the turmeric solution (maybe 5 seconds to get the colour change rather than it being instant).  It makes it a little tricky to see where your brush strokes are if you're painting a picture, but for the level of artistic skill currently present in our household, it was perfectly ok.

The other fun thing we discovered is that you can make your picture disappear again by painting over it with vinegar; the curcumin reverts to yellow when the pH is neutral or acidic.  This gives a further layer of fun for the activity as you can erase parts of the purple picture to make a different picture.  After a quick wash, the canvas is good to go again with the acid and alkali removed so it's all yellow, although it fades a little from the original colour.

Erasing a bicarbonate of soda painting on curcumin-dyed cotton with vinegar

A note of caution on washing curcumin-dyed fabric though, even on the second wash, some curcumin does come out of the fabric, and thus we have a yellow colour changing cloth wipe which was accidentally washed with it...  I'm not sure Cheeky Wipes will want to market these, but our uniquely coloured cotton terry wipe lends itself to some fun too!

Colour changing cloth wipe looking a bit like a Halloween pumpkin!

All in all, a very small amount of food colouring and some old cotton fabric gave a low mess fun activity, and we can wash the fabric and do it again another day.