It was during this time that he painted The Last Supper in the dining hall of the monastery of Santa Maria delle Grazie, at that time intended to be the Sforza family mausoleum. This huge painting is reason enough to visit Milan. Leonardo completed only fifteen paintings because his active mind tended to jump from project to project. He also had a tendency for experimentation that meant many of his paintings did not last well.
In The Last Supper he did not use traditional wall fresco techniques and experimented with mixing his paints, hence the deterioration we see in the painting today had begun only a few years after Leonardo finished it. To protect the painting there are now strict controls on the number of people allowed in the hall at one time and it’s necessary to pass through glass doors and dehumidifying chambers on the way in: the engineering mind of Leonardo would have loved it. There are also two permanent exhibitions of Leonardo’s more technical work in Milan at the Museo d’Arte e Scienza, "Treatise on Painting" and "Leonardo: Citizen of Milan."