The technique of this school of painters can only be known by studying the works they made. No treatises have been left on their technique, like that of the Italian Cennino Cennini. The first written sources are from the 16th century. The quality of materials and painting technique has long been attributed to van Eyck's discovery of oil as a medium. Scientific examination has since shown that drying oil had been used before this period. The technique of the Flemish Primitives has been studied in collaboration with scientists, art historians, and restorers, and published in the <i>Corpus des Primitifs Flamands</i>, the first study being L'Agneau Mystique au Laboratoire. According to this source, van Eyck's medium was oil + x. Since then, others found oil + proteins with staining tests. The quality of the technique in the case of van Eyck is much more a question of pictorial vision than of materials used. The description of the technique of these painters starts with the oak support and frame, the ground, paint layer, pigments, superposition of different layers, and the conservation problems involved.