a preparatory drawing for a painting sketched directly on a ground

a preparatory drawing directly on a ground, which is subsequently covered with paint. Such drawings are often executed with charcoal, chalk, pencil, or paint and brush. Varnish – a coating applied to the surface of painting. X-radiography – an imaging technique that is very helpful in revealing changes that may have occurred during the different stages of development of the painting. Pigments containing heavier metals absorb X-rays more than other pigments and X-radiographs register these differences, revealing changes of the composition by the artist as well as losses of original paint.

Underdrawing is the drawing done on a painting ground before paint is applied, for example, an imprimatura or an underpainting. Underdrawing was used extensively by 15th century painters like Jan van Eyck and Rogier van der Weyden. These artists "underdrew" with a brush, using hatching strokes for shading, using waterbased black paint, before underpainting and overpainting with oils.