Fresco (Italian: 'fresh') is the technique of applying water-based pigment onto freshly laid wet plaster. As the plaster dries, it absorbs the pigment and becomes part of the wall itself — which is why frescoes from Pompeii and Minoan Crete have survived two millennia. The true fresco (buon fresco) differs from the dry variant (secco), in which paint is applied to dry plaster: buon fresco is essentially indestructible if the wall survives; secco flakes. Michelangelo's Sistine Chapel ceiling (1508–1512) is the most ambitious fresco programme ever attempted by a single artist. He painted in sections ('giornate') the size that could be plastered and painted before drying — an enormous logistical puzzle. The Mexican muralists of the 20th century — Rivera, Orozco, Siqueiros — revived the tradition as political art.