The microclimate of Leonardo's "Last supper"

A microclimatic study was carried out in the refectory of the Church of Santa Maria delle Grazie in Milan and in the next room, in order to know the microclimate with reference to the conservation of the Leonardo's Last Supper. Several direct and indirect effects, due to the present-day exposition to crowds of people, have caused large and continuous variations of the main thermo-hygrometric factors and an increase of the deposition rate of airborne pollutants. This situation leads to a faster and faster decay of the mural, which is naturally in an unhappy situation due to the non durable painting technique used by Leonardo. This study analyzes the diurnal and seasonal cycles of the thermo-hygrometric gradients which exist between the mural and the air, the deposition processes that are each time induced, and the internal stresses forced on the painting by the lighting system, the central heating, and the presence of visitors.