In what technique did Michelangelo paint the Sistine Chapel ceiling?

Answer

He painted onto wet plaster between 1508 and 1512.

Context

Buon fresco means painting with water-based pigments onto fresh lime plaster. As the plaster dries, the colour becomes part of the wall. Britannica explains that Michelangelo applied colour quickly to the wet surface, and that some scholars think he used fresco secco, painting on dry plaster, for finer details such as faces. Because plaster is laid in sections that must be painted before they set, the work shows visible joins in the finished ceiling.

Michelangelo was trained mainly as a sculptor and was new to fresco. He and his assistants relied on hundreds of his drawings to transfer outlines to the plaster, and he used them less as he grew confident. One popular belief is that he painted lying on his back, but Britannica says he stood on a large scaffold with his head tilted back.

The Vatican Museums record that Pope Julius II entrusted him with the work in 1508 and that it was finished in October 1512. The pope inaugurated the chapel with a Mass on 1 November 1512, the feast of All Saints.

The scheme includes nine central panels of Genesis, from the Creation to the Fall, the Flood and Noah's family, with five Sibyls and seven Prophets seated around them, and the Ancestors of Christ in the spandrels and lunettes. The pope's original plan was reportedly much simpler, centred on figures of the apostles.