Colored pencils are usually made with a wax-based binder holding the pigment together. After some color layers have been applied to paper, the wax can be dissolved with a light wash of solvent to produce a smoother-looking blend and eliminate “speckles” from paper peeking through. It doesn’t make the color bleed, it just drops it in place.
Two popular solvents used for this purpose are Turpenoid Natural (citrus-based) and Gamsol (odorless mineral spirits). Turpenoid Natural is popular with artists who are concerned about toxicity, but when I tried it it added a yellowish tinge to my colors, and never seemed to evaporate completely, leaving a slight oily residue. Gamsol, on the other hand, doesn’t affect the pigment color at all and evaporates quickly and completely, with no noticeable residue.
See below my test on a small piece of Stonehenge paper: P = Pencil only; T = Turpenoid Natural; OMS = Odorless Mineral Spirits (Gamsol). Left column: medium pencil application; center column: heavy pencil application; right column: light pencil application. As you can see, the Gamsol didn’t change the color at all, it simply smoothed it–the heavier the pencil application, the more smoothing possible (since there’s more pigment to drop into place).
I’m liking Gamsol a lot–now what shall I do with this whole bottle of Turpenoid Natural?