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La calce
The wall is the same stone you started with, run through fire and water. That is not poetry. It is the lime cycle.
Authentic Italian decorative plaster is not a polymer. It is calcium. Fire takes the carbon dioxide out. Water slakes it. Air puts the carbon dioxide back. You are spreading stone that will become stone again.
Start with limestone or dolomitic limestone — calcium carbonate, CaCO3 (dolomite also carries magnesium). Italian finishing limes are often from specific quarries; the chemistry of the stone shows up in the putty.
The stone is burned in a kiln, traditionally slow, around 900°C (about 1650°F), long enough to drive off carbon dioxide. What remains is quicklime — calcium oxide, CaO. The stone loses a large share of its weight. This is calcination.
CaCO3 + heat → CaO + CO2
Sawdust or wood is a traditional fuel in some Italian kilns, in part to avoid sulfates from coal. Over-burned or under-burned stone makes bad lime. This is a craft before it is a bucket.
Water is added to quicklime. The reaction is violent and hot (exothermic). Steam. The oxide becomes hydroxide: lime putty, calcium hydroxide, Ca(OH)2, with free water. Italians call the good, fatty putty grassello di calce.
CaO + H2O → Ca(OH)2 + heat
This is not the same as opening a bag of Type S hydrated lime from a US builder supply and stirring it into a bucket. Factory dry hydrate is useful for mortar. Traditional finishing putty is slaked in excess water and then left alone. The crystal form of the hydroxide changes as it sits. Plasticity improves. Unslaked bits, if they stay in the mix, can later blow out of a polished wall. The putty is filtered.
The fresh putty is stored wet, covered from air, in pits or vats. Air would start carbonation too soon. Romans wanted years. Modern Italian finishing lime is commonly aged months to a couple of years. Twenty-four months is a figure you will see on serious finishing grassello. Three months may be enough for coarser mortar. For a burnished interior coat, older is better.
Aged grassello can be stiff enough to cut. Solids around 40%+ are typical of a working putty. This is why a real lime plaster feels like it does under the trowel, and a latex paste does not.
The putty is mixed with marble flour or marble sand of a chosen grain. Fine flour + putty → grasello / stucco veneziano. Larger marble → marmorino. Still larger, often torn in the set → travertino. Sand instead of marble → intonachino. Pigment must be lime-safe or the color dies in the alkali.
On the wall, water evaporates and carbon dioxide from the air reacts with the hydroxide. You get calcium carbonate again, and water. The plaster has returned to the family of the original limestone.
Ca(OH)2 + CO2 → CaCO3 + H2O
That takes days to weeks to advance, and longer to complete in depth. It is not “drying” like paint. If you trap it under a plastic sealer too soon, you choke the reaction. Lime wants air. That is why we talk about breathable walls, and why historic fabric and lime belong together.
Aerial lime (this cycle) will not set underwater. Hydraulic lime and pozzolans (volcanic ash, brick dust / cocciopesto) can set with less air. Venetian exterior bases often used cocciopesto for that reason. Do not confuse the two in a spec.