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How are pigments permanently bound into wet plaster in buon fresco?

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Charles Sprague Pearce: Religion — Mural Detail of *Religion*, mural in lunette from the Family and Education series by Charles Sprague Pearce, 1897; in the Library of Congress, Thomas Jefferson Building, Washington, D.C.

In buon fresco (or "true" fresco), pigments are permanently bound into the wall through a chemical process involving the plaster's composition. The plaster is prepared with slaked lime, which becomes saturated with an aqueous solution of hydrate of lime as the artist applies water-diluted pigments to the moist intonaco (the final, smooth layer of plaster).12

As the plaster dries and sets, it absorbs carbonic acid from the air. This reaction produces carbonate of lime, which acts as a permanent binder, cementing the pigment particles into the plaster itself. Because the colors are imbibed into the surface rather than sitting on top of it, they become an integral part of the wall, resulting in a highly durable mural that is resistant to aging.12