Fresco Lime Putty - putty lime, slaked lime, calcium lime, lime putty.

Fresco Lime Putty & Common Lime

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. 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 + CaCO_3(s) ----> CaO_(s) + CO_2(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 pasteor 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) + CO_2(g) ----> CaCO_3(s) + H₂O_(l)

"Calce Florentine" 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. "Calce Florentine" comes from the same source (pit) that is being used on major fresco restorations in Italy.

In addition to "Calce Florentine" 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 "Calce Florentine" - days as opposed to weeks) and lower cost. Arriccio Lime Putty is also ideal for ornamental plasterwork and it's restoration.

to be continued...