Artists are sometimes surprised to see one application of paint barely hide the drawing or painting underneath, and another color completely mask all that was below. Some wonder why sometime after they completed a painting, they begin to see lines of the sketch that before were completely unnoticed. (This effect is called pentimento.) Other artists wishing to apply a beautiful glaze are frustrated when the glaze kills the color below. These are common problems experienced by all painters at one time or another, but the reasons are little understood by them