I have recently discovered that you can use children's wax crayons in collagraphs to subtly lighten areas.
The wax makes the surface it is applied to smoother, so cancels out some of its ink-retaining qualities. Because you can vary the amount of wax you apply, you can create very subtle effects and tonal graduations - something which is not easy to achieve in this medium!
Wax crayons work well when applied to any surface that retains at least some ink, such as the mountboard itself, mountboard where the top layer has been peeled off, or areas with carborundum. When used on textured areas, the wax will emphasize the texture.
The photos below show a plate where I've applied neon yellow wax crayon to lighten quite a few areas, and the resulting print. (The colour of the wax crayon is of course irrelevant!)
