Lime Putty is the main ingredient of the buon fresco painting.Preparation of painting surfaces for fresco involves the application of plaster of increasingly finer texture.
The first step (which in nowadays mainly done by the factories) is the heating (calcination) of marble or limestone (calcium carbonate, CaCO3) at 800-900ßC to make porous lime (calcium oxide, CaO).
HEAT CaCO3(s) ----> CaO(s) CO2(g)
To form the plaster for fresco work, the lime is "slaked." The slaking process, which requires the addition of 2 or 3 molecules of water for each molecule of lime, yields calcium paste or lime putty, an aqueous gel of thin crystals of calcium hydroxide.
CaO(s) H2O(l) ----> Ca(OH)2(s) HEAT
Excess water acts as a lubricant so that the crystals slide easily over one another. Historically, lime was slaked in pits or troughs over a period of at least six months to obtain lime putty of the desired consistency. Artisans in Michelangelo's time use plaster aged for as long as ten years. Fresco plaster itself is made from the slaked lime and varying portions of sand or marble dust. Generally, walls are plastered with several layers of such fresco plaster in order of decreasing proportions and particle size of sand. Hardening of the fresco plaster on the wall includes several simultaneous physical and chemical process: the absorption of water into the wall, evaporation of water from the surface, and the carbonation of the slaked lime by carbon dioxide, CO
Ca(OH)2(s) CO2(g) ----> CaCO3(s) H2O(l)
Only high calcium lime putty should be used for fresco painting, the best would be lime putty produced from white marble. I have tried a wide variety of limes during the last 11 years and concluded that even the high calcium limestone is quite inferior to the white marble lime putty in it's plasticity and absorption of color.
The actual heating process also affects the quality of lime putty. Ideally Marble, Calcite (crystalline calcium carbonate) or Limestone for fresco should be burned in wood or electric kilns. Coal and gas burning kilns, the least expensive methods widely used for industrial production of hydrated limes, result in lime putty in which calcium hydroxide is partially changed to calcium sulfate (gypsum) which interferes with setting of the plaster consequently affecting the painting and color absorption.
"Marble, limestone and calcite are all forms of crystalline calcium carbonate, which is quarried and stacked in lime kilns, where it's carbon dioxide is driven off by heating. The result is quicklime or caustic lime (calcium oxide).
The stone fragments retain heir original shape throughout the burning process and only become white and a little lighter...
.. Quicklime is slaked with water, a process producing much heat during which the stones increase in volume and eventually disintegrate. The resulting slaked lime or calcium hydroxide is a voluminous white powder that can be packed in sacks and conveniently shipped to building sites. If during the slaking, however, more water is added than is necessary for the formation of calcium hydroxide, a pulp results that is stored in pits for as long as possible, and therefore is sometimes called pit lime.
Not all kinds of lime can be used for fresco painting. Lump hydraulic lime and dolomitic lime are quite unsuitable. Only white lime meets the necessary requirements. As quicklime, it should only contain 95% CaO and less then 5% magnesia."
Kurt Welhte "The Materials and Techniques of Painting" © 1975 Prentice Hall Press.
The High Calcium Slaked Lime Putties are difficult to find locally (in USA), at the FrescoSchool.org we use imported from Italy "Calce Florentine" - High Calcium Lime putty sold by the www.FrescoShop.com at the www.TrueFresco.com/frescoshop
iLia Anossov (fresco)
www.FrescoSchool.org