Why Not True Fresco?
Leonardo's technique was definitively not true fresco (buon fresco). In true fresco, pigments are applied to wet plaster; as the plaster dries, calcium hydroxide reacts with atmospheric CO₂ to form calcium carbonate, permanently binding pigments into the wall matrix. This chemical marriage makes fresco extraordinarily durable — but it also imposes severe constraints: the artist must work fast (before the plaster dries), cannot easily make corrections, and is limited in the subtlety of color blending.
Leonardo wanted none of these limitations. He chose to paint a secco (on dry plaster), using what the official Cenacolo Vinciano description identifies as "greasy tempera made by mixing pigments with egg yolk." Other analyses describe a combination of tempera and oil — a mixed technique that gave him the freedom to work slowly, revise endlessly, and build up translucent glazes for the atmospheric depth he demanded. The cost of this freedom was the painting's survival.
The Ground Layer
The complete wall preparation sequence, from innermost to outermost:
- Stone/brick wall — the refectory's north wall, a thin exterior wall of rubble-filled masonry
- Rough plaster coat (arriccio)
- Thin leveling layer — calcium carbonate mixed with magnesium and animal glue
- Lead-white oil imprimatura — the critical sealed ground layer
- Underdrawing — red chalk and black paint
- Multiple thin paint layers with translucent glazes
The imprimatura was an oil-based formula pigmented with lead white — a mixture of hydrocerussite (2PbCO₃·Pb(OH)₂) and cerussite (PbCO₃). Leonardo deliberately added lead(II) oxide (PbO) as a drying agent and thickener. This sealed the wall surface smooth and bright — but it also meant his paint layers had no chemical bond to the substrate.
Leonardo's Palette
Scientific analysis using X-ray fluorescence spectroscopy, micro-Raman spectroscopy, synchrotron X-ray powder diffraction (SR-XRPD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and micro-Fourier transform infrared spectroscopy (μ-FTIR) has identified a sophisticated and expensive palette:
Blues
Natural ultramarine (lapis lazuli) — Na₈₋₁₀Al₆Si₆O₂₄S₂₋₄ — served as the primary blue pigment, applied in the thickest layer for Christ's robe. Ultramarine cost up to eight ducats per ounce, more than annual rent, underscoring the commission's lavishness. Cross-sections from St. Bartholomew's blue cloak confirmed ultramarine alongside azurite (Cu₃(CO₃)₂(OH)₂). The tablecloth's blue stripes employ azurite-ultramarine mixtures over grey or black underlayers. The post-restoration analysis revealed three distinct original blues: lapis, periwinkle, and turquoise.
Reds
Christ's red garment required five separate coats of vermilion over a carbon-black primer — a layering process "completely unknown to fresco," as Ross King observed. A landmark 2019 study by Osticioli et al. in Spectrochimica Acta Part A identified the red lake pigment as kermes (an anthraquinone dye from kermes insects), precipitated with aluminum potash alum mordant — correcting earlier fluorescence-based identification as madder lake. Deep red kermes particles dispersed in the ultramarine layer of St. Bartholomew's cloak appear to modify the hue toward purple.
Other Pigments
The post-restoration analysis also revealed traces of gold and silver foil on apostles' robes — luxury materials that speak to Ludovico Sforza's investment in the commission. Lead white served double duty as both the ground layer pigment and a mixing white throughout.
The 2023 Chemical Discovery
A groundbreaking 2023 study by Gonzalez et al. in the Journal of the American Chemical Society (145(42), 23205–23213) analyzed Last Supper fragments using synchrotron techniques. The team detected plumbonacrite (Pb₅(CO₃)₃O(OH)₂) and — for the first time ever in a historical painting — shannonite (Pb₂OCO₃), the most alkaline lead carbonate form.
These compounds form when PbO reacts with drying oil in an alkaline environment, confirming Leonardo's deliberate manipulation of paint chemistry. He wasn't simply mixing pigments — he was engineering chemical reactions within his paint layers, a level of technical sophistication that aligns with his extensive scientific notebooks on optics, color theory, and material experimentation.
The Fatal Flaw
The fundamental catastrophe of Leonardo's technique lies in a single chemical fact: his paint merely sat atop a sealed dry surface, held only by the mechanical adhesion of organic binding medium to the smooth primer. Where fresco painters create a molecular bond, Leonardo created a physical one — and physics lost to chemistry, humidity, and time.
"He wanted to paint on the wall as if on panel — slowly, reflectively, with the freedom to revise. The price was eternity."