Multicolour pictures with curcumin ink

A while ago we had a play with curcumin, which gives turmeric its yellow colour.  Curcumin turns a deep purple/red colour when it's added to an alkali (and changes back to yellow when acid is added again).  When we first played with it, I extracted curcumin from turmeric, but subsequently found that the natural yellow food colouring I have (from Tesco in the UK) is curcumin-based. It's less faff to use the food colouring if you can buy it or have it in the cupboard (and doesn't require a spirit which you may want to save for drinking at the moment...).

We used yellow food colouring for some colour changing painting on cotton, but if you can't get curcumin-based yellow food colouring, it would also work with turmeric (follow the link above for instructions).  Anyway, it's been a while since we used curcumin's lovely colour change and I thought it would be nice to do a Spring-themed activity.

Flowers ready to paint

I put some cotton wool in a solution of bicarbonate of soda (a teaspoon in about 50ml water) and then dried it outside - this solution is alkaline.  I used a side of a cereal box as the background for a flower picture (intended to be daffodils, but I realise they don't naturally occur in a purple colour…).  I made some stems and leaves from green paper, and stuck them on.  A more arty or older child might enjoy doing this, or drawing them on afterwards, but mine isn't really into craft and I wanted something easy for him to do.  I stuck with PVA glue some of the dried bicarbonate of soda soaked cotton wool and some cotton wool straight from the pack to make the shape of flowers, and let it dry.

Finding the petals that turned purple

My son then 'painted' the flowers with a brush and a diluted (1 part in 4) solution of the curcumin food colouring.  The untreated cotton wool stayed yellow, but the bicarbonate of soda soaked cotton wool turned deep purple/red!  He enjoyed guessing whether each piece of the cotton wool would stay yellow or turn purple and then testing it with his brush.  It's worth saying that it works best if not too much colour is put on as otherwise it soaks out from the cotton wool, and the bicarbonate of soda which dissolves in the purple/red bits will spread and adjacent yellow colouring will go purple too; this wasn't a huge problem for us as my son was keener to find out what colour it would go than to completely paint the white cotton wool.

Finished picture

We briefly talked about the science behind it, and he remembered that curcumin works as a pH indicator, changing colour depending on whether it's in acid or alkali.  I also invited him to make his own picture as I had more cotton wool, cardboard and green paper, but he wasn't keen - other children may find this fun.  He did, however, want to show his grandparents his picture on a video call...