“Fresco” is Italian for “fresh.” There are two types of fresco paintings
Buon fresco is the true fresh. The real deal. Pigments are applied while the plaster is still wet and as the wet plaster dries the paint becomes embedded into the plaster itself. Technically speaking, the plaster does not "dry" but rather a chemical reaction occurs in which calcium carbonate is formed as a result of carbon dioxide from the air combining with the calcium hydrate in the wet plaster.
It’s called “fresh” because an artist is required to get the painting finished before the plaster hardens—like working in lacquer—speed is of the essence and this technique makes re-working the picture difficult without starting all over with fresh plaster.
In contrast to buon fresco, fresco secco is pigment applied on top of dried plaster. This is not long-lasting and in most climates the paint flakes off. Think of ever-impatient Leonardo’s Last Supper. Unless you are working in the ultra-dry climates of a place like the Buddhist cave murals of western China and central Asia, fresco secco will not last. So, we are lucky that Angelico and Signorelli were working in true fresco. This is especially important since Signorelli came on board almost fifty years after Fra Angelico had to drop out. Fifty years for which the chapel had been left untouched—Angelico’s scaffolding still in the same place in had left them and Angelico’s colors going strong.
It was in the fateful year 1500 when a contract was signed that stipulated that Signorelli would be paid 205 ducats to finish the faulted ceilings and 575 ducats to paint the walls. In addition, the painter was to be furnished with ultramarine, a certain quantity of food and wine, and a free lodging, with two beds.
Although he was supplied with the expensive ultramarine—as costly as gold—he had to supply the rest of the pigments in his painter’s palate. These fresco pigments included other blues, like azurite. It also included verdigris malachite; yellow ocher, burnt Siena, and bone black.
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