Drawing Techniques: a Guide
Process: Underdrawing  
Artists began not only finished paintings, but also working drawings with preliminary sketches on the support. They often used impermanent or less visible materials, like stylus, charcoal, or black chalk, to execute these underdrawings, because they intended to erase them or cover them over with more permanent media like ink, wash, or white heightening. Occasionally slight traces of the underdrawings remain visible to the naked eye. Even if they are completely covered over, researchers can sometimes see the underdrawings by means of laboratory techniques like infrared reflectography.

Giuseppe Cades. Coronation of the Virgin. MMLV Fine Arts, New York