Tempera painting probed on the molecular scale

Detail of unfinished painting The Virgin and Child with Saint John and Angels or 'The Manchester Madonna' (Michelangelo, ca. 1497, tempera on wood, The National Gallery London). Note the use of a terra verde base layer for the rendering of flesh. In contrast to the oil painting technique that supplanted it at the end of the 15th century, tempera painting, practiced on wood panels, walls or canvas has received little attention on the physico-chemical scale. This painting technique, which has been practiced since Antiquity, is characterised by pigments applied in a water-based binding-medium, often egg-yolk. In order to understand the properties and appeal of this mixture on a molecular scale, a CNRS team, from Sorbonne Université and ESPCl 1 ,reproduced 15th century recipes that a Tuscan painter Cennino Cennini had consigned to paper, using egg-yolk and a clay-based pigment, green earth ( terra verde) . This combination was used on a large scale in works dating from the Middle Ages, as a base-layer for gilding and the rendering of flesh. By measuring its flow properties and characterising its molecular organisation, the team has shown the formation of a network between the proteins of the egg-yolk, the water molecules and the clay particles of the pigment renders the mixture more viscoelastic.