“In kiln-heating limestone, occasion
Of fresco’s first step: calcination.
The process, applied:
CO2 thus ‘excised,’
Yielding quicklime for next needed station.”
The 15 April 2024 Twitter limerick was the first of three sequential verses summarizing the fresco cycle. The sequence is a fitting theme with which to begin the Spring 2025 posts: fresco is Italian for “fresh” (hence the post title), and this art form is a fascinating one to view from a chemistry perspective, among many others.
“In kiln-heating limestone, occasion /
Of fresco’s first step: calcination.”
The fresco cycle consists of three steps: calcination, slaking, and carbonation (also called carbonatation). While I’ve examined the overall process before, this sequence provided a chance to explore each step in greater detail.
The first step, calcination, is represented symbolically via the equation:
CaCO3 (s) + heat → CaO (s) + CO2 (g)
Alternatively, the reaction could be shown with calcium carbonate (CaCO3) as the single reactant with a triangle, representing heat, written over the arrow (noting here what would be a fascinating tangent on its own: some sources trace this shorthand to the alchemical symbol for fire). Calcium carbonate is also known as limestone; the more familiar name brings with it the bonus of simpler scansion.
“The process, applied: /
CO2 thus ‘excised,’ /
Yielding quicklime for next needed station.”
Heating calcium carbonate to high temperatures, as in Line 1’s kiln, drives the calcination step to the right, yielding calcium oxide (CaO) and carbon dioxide (CO2) as products.
Calcium oxide is referred to as quicklime in the fresco process, and it will be used in the “next needed station” of the fresco cycle: the slaking step, which is what provides the slaked lime (calcium hydroxide) that will become the actual fresco surface.
(As above, with the symbolic shorthand for heat, sitting with these terms during a break allowed me to explore an etymological question that often comes to mind in teaching this topic. The “lime” of quicklime and lime plaster comes from the Old English lim, reflecting its stickiness as a building material, whereas the “lime” of the citrus variety comes from the Arabic limah and Persian limu.)
Chemphasis 