This section is from the book "The National Capitol. Its Architecture Art And History", by George C. Hazelton, Jr. Also available from Amazon: The National Capitol Its Architecture Art and History.
The art of fresco, which has long flourished in Italy, was known and practiced by the Egyptians, Greeks and Romans. The Germans, French and northern nations had no knowledge of the art. The French, in their mural decorations, paint in oils directly upon the wall, or in their studios at leisure upon canvas, which they afterwards press upon the wall, coated with white lead, until the picture sets and becomes firm. This was the process used in most instances in the decoration of the new building for the Congressional Library. Fresco is the Italian word for fresh ; the process is so called because the plaster is wet or fresh when the colors are applied. The wall or ceiling, before the decoration, resembles the first coat of plaster ordinarily given to a room. The colors are liquefied with water, and so naturally sink into the prepared background and become a part of it. As the wet plaster is much darker than the dry, it is very difficult so to apply the colors, which change materially in drying, that the tone is preserved harmonious throughout. It requires long experience and artistic judgment. Then, too, the pallet is meager. Mineral or earth colors only can be used, as the lime in the plaster, which is used for white, eats and destroys all other paints. No lakes, no vermilions, no carmines, as in oils, are at the artist's hand; he must produce his flesh tints by clever combinations with light and Indian reds. Brumidi's flesh tints are therefore worthy of study.