Underdrawing

Underdrawing is the preparatory sketch an artist lays down on the primed ground before applying paint, mapping out composition, contours, and sometimes tonal shading. Typically executed in charcoal, chalk, or diluted ink, it is normally invisible once paint covers it — but because most drawing materials absorb infrared light differently from the paint layers above, conservators can reveal it using infrared reflectography, and denser materials show under X-radiography. Underdrawings are invaluable evidence of process: they show whether a composition was fixed from the outset or reworked freely, help distinguish autograph works from workshop copies (which often trace an underdrawing mechanically), and have been used to study masters from Jan van Eyck to Botticelli in forensic detail.