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
Sistine Chapel congregations. These included the Christmas Day and Easter masses, at which the Pope himself was the celebrant. The other 27 masses could be held in a smaller, less public space, for which the "Cappella Maggiore" was used before it was rebuilt on the same site as the Sistine Chapel. The "Cappella Maggiore" derived its name, the Greater Chapel, from the fact that there was another chapel also in use by the Pope and his retinue for daily worship. At the time of Pope Sixtus IV, this was the Chapel of Pope Nicholas V, which had been decorated by Fra Angelico.