During the process of recycling, paper is broken down by water. Non-waxy and non-oily pigments - like coloured pencils, charcoal and chalk - do not interfere with this process. On the other hand, wax crayons and oil pastels create a layer that not only cannot be removed but stop the underlying paper from being properly recycled.

This is why art made with water soluble colours (pencil, water based paints, chalk) are accepted in the recycling system but not those which are not water soluble (wax and oil-based colours).