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Why are some colors difficult to use in lime plaster frescoes?

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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 the context of mural painting, specifically the buon fresco technique, certain colors are difficult to use because the medium relies on a chemical reaction involving slaked lime (calcium hydroxide). As the plaster dries, it absorbs carbonic acid from the air to form a permanent binder of carbonate of lime.1

Because this environment is highly alkaline, artists are restricted to using pigments that are lime-resistant, which are primarily earth pigments. Many mineral colors, such as certain blues, are chemically unstable when exposed to lime and can be altered or destroyed by the reaction. Consequently, if an artist wishes to use these sensitive colors, they must apply them over a dry surface using a different medium, such as egg tempera or casein, rather than incorporating them directly into the wet plaster.1