The Art of Fresco: Lime and Sand
by Sr. Lucia Wiley, CHS
The best lime for fresco is pure white in color. This is important because of both the transparency and the bleaching ability of lime. Lime appears opaque when freshly painted, but over the first year of the fresco's life, the tonality of the painting will become even more unified because of the lime's bleaching ability which creates a snowy white reflecting surface under the colors. Carbonization continues for years and will slowly bring more depth and richness in color. Transparency will continue to grow in the depths and luster in the lights.
Burned and slaked lime is calcium hydroxide and for fresco this mixture must contain no gypsum. Lime which has burned over wood fire is the best. Next the lime must be slaked for as long as possible, at least 24 hours. Slaking is the process of adding water to the powdered lime to turn it into lime plaster. The chemical reaction that takes place here gives off great heat, so slaking must be done with great care to avoid getting burned.
Using the slaking box, sift the lime into the box slowly, adding the fresh lime at the top of the box. At the same time allow a slow stream of water to pour into the upper box from a water-hose. Do not pour large amounts of lime in at one time. Use a hoe to slowly mix water and lime; it must be thoroughly wet, with no undisolved remnants remaining. Watch for hot dry piles which will overheat and burn the lime. As the lime-plaster becomes thoroughly mixed, hoe the lime-plaster toward the gate which leads to the lower box.
The lime plaster must be stored for a minimum of a year, hopefully for two years. Storage in a pit is best, but steel drums which have been painted on the inside with asphalt varnish or baked lacquer also work.
Sand makes plaster porous and thus facilitates the transformation of caustic lime into carbonate of lime. The texture of sand is important. Coarser sand is best for the undercoat and fine sand for the upper coats of plaster (there will be specific grain sizes suggested later.) All the sand should be of a uniform grit and the grains should be angular in shape. They should feel sharp and not roll like a ball between the finger. The idea is to squeeze the mortar tight.
An artist must be particular about the type of sand mixed into the lime plaster. And once the proper sand is chosen, the sand must be washed and stored off the ground.
Sand must be free of loam, clay, mica, and gypsum. Humus and loam in sand are dangerous because they decompose when exposed to air and cause efflorescence. Clay mixed in with the sand causes cracks in mortar. Water soaked sand containing clay weeps, exudes moisture, and makes it difficult for colors to adhere. Sand containing mica flakes will eventually split off from the surface of the fresco. Never use sand containing cement or gypsum because of efflorescence and decay.
The safest sand comes from solid rock - gneiss, granite or porphyry. Ground pumice and unglazed triturated porcelain work well. Sand made from marble is also good and most desirable in certain instances. The decision to use sand or marble is personal. Sand gives the wall a warm tone and marble dust or meal makes the background wall whiter.
Sand washing is easily done with:
Sift the sand through the larger screen rinsing it well with water. Dry the sorted sand sizes separately. When dry store in separate bags. The final sifting is done directly into the mortar box. Marble meal and dust is washed just like the sand. The coarser sand is used in the first two coats of lime-plaster; the finer sand is used in the 3rd coat and the final coat or intonaco.
© the estate of Lucia Wiley; used by permission