Sistine Chapel ceiling of the Sistine Chapel. At the outset, the plaster, "intonaco", began to grow mold because it was too wet. Michelangelo had to remove it and start again. He then tried a new formula created by one of his assistants, Jacopo l'Indaco, which resisted mold and entered the Italian building tradition. Because he was painting "fresco", the plaster was laid in a new section every day, called a "giornata". At the beginning of each session, the edges would be scraped away and a new area laid down. The edges between "giornate" remain slightly visible; thus, they give a good idea of
Illusionistic ceiling painting sotto in sù". This was the prototype of illusionistic ceiling painting that was to become an important element of Italian baroque. Correggio at Parma took the illusionistic ceiling a step farther in his frescoes of Christ and the Apostles for the cupola at the San Giovanni Evangelista and in the "Assumption of the Virgin" in the dome of the Cathedral of Parma, which is Correggio's most famous work (1520–24); in these frescos Correggio treats the entire surface as the vast and frameless vault of heaven in which the figures float. In a visual continuity between the architectural interior and its