I've been planning my next kids art club session and this is one of the activities they'll be doing.
First you thickly cover a sheet of paper with wax crayon. The more colours the better and if your arm's not aching you're not doing it right
Then paint a layer of black paint over the crayon. I used poster paint.
Once the paint has dried (which will hopefully be within the 2 hour session on Saturday) scratch away at the paint with the end of a paint brush.
This is my example to show the children on Saturday. Can't wait to see what they come up with!