Fresco Lime Putty - mystery revealed! Slaking, Chemical composition and aging of the putty lime with special chapter on problems with magnesium in dolomitic lime (common construction lime) for buon fresco.
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.
Preparation of painting surfaces for fresco involves the application of plaster of increasingly finer texture. The first step (which in nowdays mainly done by the factories) is the heating (calcination) of the 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, CO2.
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.
Colonial Aged Slaked Lime Putty
is a pure aged putty lime of the best quality. The only intonaco putty lime we use in our studio. This is the finest putty for intonaco and other plaster coats as well as variety of plaster finishes such as marmorino, Venetian stucco, scraffito, and scagliola. Aged Colonial Fresco Lime Putty on most of our large scale fresco installations.
In addition to "Colonial Fresco Lime Putty" for underlaying plaster coats (all coats of fresco except intonaco) in building movable fresco panels
"Arriccio Lime Putty"
can be used. Although it has high magnesium content, after over six years of testing by our studio we concluded it is safe to use and has additional benefits such as faster setting time (will set faster and achieve its hardened stage much sooner than "Colonial Fresco Lime Putty" - days as opposed to weeks) and lower cost. Arriccio Lime Putty is also ideal for ornamental plasterwork and it's restoration.
Practice Lime Putty
To gain experience in plastering at a minimal cost (pure calcium lime putty – is expensive) it is possible to prepare a “Practice Lime Putty” from common Type-S Hydrated Construction Lime. Type-S Hydrated Lime is manufactured from Dolomitic Limestone (Dolomitic limestone contains 35 to 46 percent magnesium carbonate). Being a great lime to practice fresco plaster application techniques, this lime is NOT suitable for the actual painting in fresco due to fast setting and poor adhesion/binding of colors and efforescence on the color layer. However it is widely used in construction and is available at very low cost almost at any building supplies retailer.
For your convenience, a short video from the FrescoSchool.org demonstrating the process making “Practice Lime Putty” from Type-S Hydrated Lime is available free of charge the at
Fresco School’s Video Channel