The term fresco refers to a number of techniques for painting images on a room surface. In buon fresco, pigments (mineral colors) are mixed with water and then applied to a layer of wet, lime-based (alkaline) plaster. Because the pigments become embedded in the plaster as it dries, no glue or other binder is needed to hold them in place and buon frescoes are very durable. However, some pigments, especially shades of blue, cannot be used in combination with wet plaster, and since pigments can be applied only as long as the plaster remains damp, mistakes can be corrected only by replastering and then repainting the affected area. In contrast, a Secco fresco is done on a dry surface, thus requiring a binder. Sometimes artists combined the two techniques, beginning a work in buon and then, once the plaster dried, using a secco to correct mistakes or add colors that could not be applied using buon.

The biblical scenes created by Michelangelo for the ceiling of the Sistine Chapel in the Vatican City are among the world's most famous frescoes. Commissioned in 1 508, Michelangelo began his work reluctantly, considering himself more of a sculptor than a painter, and spent four years perched on scaffolding. painting some 300 figures across about 5,000 square feet of ceiling. Despite his initial hesitation, Michelangelo followed the purest fresco tradition of the time, applying his pigments mixed in water directly to fresh lime plaster and choosing only those that are chemically compatible, avoiding azurite, vermilion, and lead-based colors, all of which deteriorate under the alkaline conditions of the lime. The incredible scale of the task required him to execute each scene in patchwork, with about five square meters worth of wet plaster applied per giornata, or "day's work." The use of plaster relatively low in lime, which dries more slowly, bought him some extra time, but it is clear that Michelangelo painted each giornata extremely quickly- -rarely, if ever, resorting to finishing a section with touch-up paint containing binder.

Over the years following their completion, the V atican frescoes accumulated a coating of soot and dirt that dulled and darkened the images. In 1787 the German writer Goethe made the following observation:“On 2nd February we went to the Sistine Chapel to witness the ceremony of the blessing of the candles. I thought it precisely these candles that over three centuries have blackened these splendid frescoes; this is the incense that has not only with its smoke covered over the sun itself of art, but with every year continues to dirty it and will finally engulf it in darkness. "

While Goethe's dire prediction did not come completely to pass, the slow darkening of the frescoes led to a characterization of Michelangelo as a relatively somber artist with lttle appreciation for color. Deposits of salt from rainwater let in by the leaking roof and from within the building materials themselves compounded the decline, leading to blistering of the paint. Eventually, a full-time restorer was appointed to maintain the frescoes. Salt crystals and dirt were removed by scrubbing with sponges dipped in wine, paints containing binder were used to touch up sections, and varnish was applied to "renew" the colors.

The met effect of several hundred years of this cosmetic maintenance was a dark film consisting of layers of dust, soot, and varnish entirely covering the frescoes. In 1980 a massive effort began to restore Michelangelo's work to its original glory. Restorers painstakingly removed the layers of grime and varnish, the touch-up paint, and the salt deposits with solutions appropriate for each layer. A dilute solution of acrylic paint was applied only when absolutely necessary to reinforce the existing paint.

Over the course of the next fourteen years, the bright colors that emerged were almost shocking to those accustomed to the dull earth tones that had been the status quo for centuries. Indeed, the work at the Sistine Chapel has led to vigorous debate about the role of restoration. Restorers often cast themselves in the role of interpreting the original artist's intent, using modem materials to override the effects of time. However, some critics believe that aging is part of the natural evolution of art and, therefore, new is not necessarily better.