Due to a bitterly cold winter, Leonardo had been forced to wait until the spring of 1492 before he could begin work on priming the refectory wall at Santa Maria delle Grazie. Aided by Francesco and Nicoli, a promising new apprentice, Leonardo applied his experimental gesso to the wall of the refectory. Off-white in colour, gesso was made from animal glue and traditionally mixed with chalk. Here, however, the maestro instructed his apprentices to stir in pitch mastic with the aim of creating a hard surface that would withstand deterioration to the painting from moisture in the air and would therefore allow him to paint at his usual pedestrian pace. Ten layers of normal gesso, applied thinly, was sufficient to prime a surface but no one had ever attempted such a large-scale painting using an
