The deterioration of da Vinci's The Last Supper was noted as early as 1517, only ~10 years after its completion.  The abrupt corrosion is thought to have been caused by the tempera paint used on the drywall surface of the church and the altitude of the Santa Maria delle Grazie also makes the area prone to humidity and flooding (King).  Since the damage began, there have been multiple restoration efforts, with the most recent being that of Pinin Brambilla Barcilon (Barcilon).  After an intense quality examination of The Last Supper, Pinin Brambilla Barcilon was commissioned in 1978 to clean the surface of the painting and then remove the lasting effects of previous conservation efforts (Barcilon).  Further examination of the extremely damaged areas forced Panin to fill in the spaces with watercolor paint (Barcilon).  When the renovation effort ended in 1999, the church refectory room went through an extreme renovation, including climate-control and anti-humidity chambers, in order to minimize the effects of potential environmental pollution on the painting (King).

Sources:

Barcilon, Pinin Brambilla, et al. Leonardo: The Last Supper. University of Chicago Press, 2001.

King, Ross. Leonardo and The Last Supper. Bloomsbury Publishing, 2014.

Image Source: Steigerwalt, Guilia Louise. “Pinin Brambilla Barcilon: Leonardo Da Vinci's Restorer.” Italian American Bilingual News Source, 24 Mar. 2017, italoamericano.org/story/2015-4-8/Pinin-Brambilla-Barcilon.