The Wikipedia (a poor source generally but in its current article on Leonardo's The Last Supper it appears to be on firm enough footing):
Medium
Leonardo painted The Last Supper on a dry wall rather than on wet plaster, so it is not a true fresco. Because a fresco cannot be modified as the artist works, Leonardo instead chose to seal the stone wall with a layer of pitch, gesso and mastic, then paint onto the sealing layer with tempera. Because of the method used, the piece has not withstood time very well – within several years of completion it already began showing signs of deterioration.
Damage and restorations
As early as 1517 the painting was starting to flake. By 1556—less than sixty years after it was finished — Leonardo's biographer Giorgio Vasari described the painting as already "ruined" and so deteriorated that the figures were unrecognisable. In 1652 a doorway was cut through the (then unrecognisable) painting, and later bricked up; this can still be seen as the irregular arch shaped structure near the centre base of the painting. It is believed, through early copies, that Jesus' feet were in a position symbolizing the forthcoming crucifixion. In 1768 a curtain was hung over the painting for the purpose of protection; it instead trapped moisture on the surface, and whenever the curtain was pulled back, it scratched the flaking paint.
A first restoration was attempted in 1726 by Michelangelo Bellotti, who filled in missing sections with oil paint then varnished the whole mural. This repair lasted very poorly and another restoration was attempted in 1770 by Giuseppe Mazza. Mazza stripped off Bellotti's work then largely repainted the painting; he had redone all but three faces when he was halted due to public outrage. In 1796 French troops used the refectory as an armory; they threw stones at the painting and climbed ladders to scratch out the Apostles' eyes. The refectory was then later used as a prison; it is not known if any of the prisoners may have damaged the painting. In 1821 Stefano Barezzi, an expert in removing whole frescoes from their walls intact, was called in to remove the painting to a safer location; he badly damaged the centre section before realising that Leonardo's work was not a fresco. Barezzi then attempted to reattach damaged sections with glue. From 1901 to 1908, Luigi Cavenaghi first completed a careful study of the structure of the painting, then began cleaning it. In 1924 Oreste Silvestri did further cleaning, and stabilised some parts with stucco.
During World War II, on August 15, 1943, the refectory was struck by a bomb; protective sandbagging prevented the painting being struck by bomb splinters, but it may have been damaged further by the vibration. From 1951 to 1954 another clean-and-stabilise restoration was undertaken by Mauro Pelliccioli.
Major restoration
The painting's appearance in the late 1970s was badly deteriorated. From 1978 to 1999 Pinin Brambilla Barcilon guided a major restoration project which undertook to permanently stabilize the painting, and reverse the damage caused by dirt, pollution, and the misguided 18th century and 19th century restoration attempts. Since it had proved impractical to move the painting to a more controlled environment, the refectory was instead converted to a sealed, climate controlled environment. Then, detailed study was undertaken to determine the painting's original form, using scientific tests (especially infrared reflectoscopy and microscopic core-samples), and original cartoons preserved in the Royal Library at Windsor Castle. Some areas were deemed unrestorable. These were re-painted with watercolour in subdued colours intended to indicate they were not original work, whilst not being too distracting.
This restoration took 21 years and on May 28, 1999 the painting was put back on display, although intending visitors are required to book ahead and can only stay for 15 minutes. When it was unveiled, considerable controversy was aroused by the dramatic changes in colours, tones, and even some facial shapes. James Beck, professor of art history at Columbia University and founder of ArtWatch International, has been a particularly strong critic.
That’s quite a history lesson ... thank you for sharing it.