A mural is defined as any piece of artwork that is painted directly on a wall, ceiling or any other large permanent surface. Fresco, secco and mezzo-fresco techniques are still used by artists today, and these techniques are widely seen in famous murals painted by historically important artists, such as Michelangelo, in several famous churches and palaces in Europe and on walls painted by Mexican muralists such as Diego Rivera.
Fresco Technique
Fresco technique in mural painting was popular from the late 13th century to the mid-16th century. Frescoes are painted on a fresh and wet plaster so that the color pigments can dissolve into the lime water in the plaster. When the plaster dries, the lime water and the color pigments combine creating the shapes painted by the artists. Plaster stays wet approximately 12 hours, so you have to start painting in an hour after the plaster is prepared and laid on the wall or ceiling and stop painting hour or two before the plaster is entirely dry. The plaster does not actually dry. The calcium hydrate in the wet plaster reacts with the carbon dioxide in the air forming calcium carbonate that hardens the plaster. This chemical reaction makes binder unnecessary. Because of the limited time you have to work with the wet plaster, you should only prepare an area you know you will be able to finish with the given time.
Secco Technique
Secco technique is used on dry plaster. The chemical reaction between the color pigments and the plaster does not happen when dry plaster is used, making a usage of binder necessary in secco technique. The three binders you can use in secco murals include egg, glue and oil. You can paint the entire mural using secco technique or use secco technique on top of a fresco technique to correct mistakes, make changes, add small details or add colors, such as blues, that cannot be used in all fresco murals. Use secco technique only for murals meant for interior decoration because the technique cannot stand exposure to environmental factors such as rain or sunshine. The color pigments of secco murals, however, are more true to the shades you apply than the color pigments of fresco murals because the wet plaster dilutes and changes the color shades.
Mezzo-Fresco Technique
Mezzo-fresco technique is used to paint murals on nearly-dry plaster. The best way to test if your plaster is ready for mezzo-fresco technique is to press your thumb in the plaster. If you do not leave a thumb print but the plaster gives in, you can start painting your mural. Binder made of eggs, glue or oil is needed to secure your mural to the plaster because the color pigments only slightly penetrate into the plaster. The color pigments, however, do not change once the plaster dries like they do when fresco technique is used. Mezzo-fresco technique has widely replaced fresco technique since the end of the 16th century.
