“A creative process, wall-stationed,
With paintings long-lasting, emblazoned;
The technique, pervasive
On surface abrasive,
Forms fresco through carbonatation.”
The second mechanism poem from NaPoWriMo2022 was the most general of the verses from that week, posted on 5 April 2022. It concerns the chemistry behind frescoes, an artistic medium seen widely throughout history and cultures, thus justifying to a reasonable extent (I hope) the pun in the post title.
The topic of fresco chemistry involves much fascinating science, art, and history. As with last week’s entry, the chemical reactions of interest here deserve some preliminary framing and additional words. Unlike the SN2 mechanism and others that will be described in future posts, this is a process reflecting general inorganic chemistry steps, rather than the specific electron pushing of organic molecule depictions. However, it still seemed to fit reasonably well in this week. Here, the abbreviations in parentheses denote the phases of the chemicals of interest: (s) for solid; (l) for liquid; (g) for gas; and (aq) for aqueous solution.
Step One: CaCO3 (s) → CaO (s) + CO2 (g)
Step Two: CaO (s) + H2O (l) → Ca(OH)2 (aq)
Step Three: Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s)
The fresco cycle shown here is a three-step process. The first two steps prepare an artist’s materials for this artistic medium. The first is called calcination: calcium carbonate (CaCO3) is heated to yield calcium oxide (CaO) and carbon dioxide (CO2). The second is called slaking: calcium oxide is mixed with water (H2O) to form calcium hydroxide [Ca(OH)2], known to fresco artists as lime plaster.
The third step is the chemistry behind the fresco formation itself and the focus of the poem.
“A creative process, wall-stationed, /
With paintings long-lasting, emblazoned…”
Frescoes consist of two layers of lime plaster used to coat a wall and create a painting surface (“wall-stationed”). In the rougher layer, the arriccio, plaster is mixed with coarse sand and applied directly to the wall. The intonaco layer (lime plaster and fine sand) is then applied over the arriccio layer to become the actual painting surface.
In fresco chemistry, pigments are painted directly on a surface consisting of calcium hydroxide [Ca(OH)2]. As the fresco dries, the calcium hydroxide reacts with the carbon dioxide (CO2) in the air and forms the stable and long-lived compound calcium carbonate (CaCO3): trapping the pigments, leading to an image that will be “long-lasting [and] emblazoned” on the wall.
A fresco artist ideally applies an intonaco layer only to an amount of wall feasible to finish before the plaster dries. The corresponding term is giornata, for “a day’s work.”
“The technique, pervasive /
On surface abrasive…”
The technique is “pervasive”: seen in many eras and locations throughout history. Calcium hydroxide is a basic compound, bases are caustic; lime plaster is a “surface abrasive.”
“…Forms fresco through carbonatation.”
The last reaction in the sequence shown is called carbonatation, as it forms calcium carbonate. It is sometimes called carbonation, but the first term is less ambiguous (multiple processes are denoted as carbonation)… and scans more readily in this limerick!
This reaction is key to the buon fresco (“good fresco”) technique, in which the artist is painting on fresh lime plaster, aiming to cover their giornata. A related technique is called fresco a secco (“dry fresco”), in which the artist uses paints on an already-dried surface.
Chemphasis 