Glossary · Technique
Fresco
Mural painting on freshly laid wet plaster — pigments become chemically bonded into the wall surface as the plaster carbonates and dries. The most durable mural technique, and the one Leonardo famously refused to use for the Last Supper.
Definition
True fresco (buon fresco) requires the painter to apply pigment suspended in water directly to fresh, wet plaster (arriccio scratch coat + intonaco finish coat). As the plaster dries, it absorbs calcium hydroxide from the air and becomes calcium carbonate — locking the pigment permanently into the wall surface. The result, when done well, is extraordinarily durable: Giotto's Arena Chapel frescoes survive after 700 years.
The constraint is severe: the painter must work quickly, completing each day's section (giornata) before the plaster dries, and certain pigments that react badly with the alkaline calcium environment cannot be used.
Why Leonardo Refused True Fresco
For the Last Supper (1495–98), Leonardo specifically chose not to use true fresco. He painted on a dry wall preparation of gesso, pitch, and mastic using what appears to have been a tempera and possibly oil-based technique. His reasons were almost certainly artistic:
- True fresco forced rapid, irreversible decisions — incompatible with Leonardo's slow, highly reworked style
- The limited palette of fresco-compatible pigments would have prevented his subtle color effects
- He could not apply glazes or sfumato over true fresco
The result was catastrophic for preservation: the paint began peeling within Leonardo's own lifetime. By 1556, Vasari described it as "a muddle of blots."