La Charité by Andrea del Sarto (in The Louvre Museum), restored in 1980-81, was the focus of collaboration between the Laboratory and Restoration Service. Two aspects are reported: the complex problem of cleaning the blues in the sky and an attempt to explain Robert Picault's method of transfer in 1750. The history of the restorations undergone by this painting begins with the spectacular transfer of 1750 when Picault replaced the original support without damage to the painting. A study of the painting with the help of various scientific methods is done, followed by an explanation of the process used to clean the sky. Throughout this report, it is shown that collaboration with the Laboratory of Research of the Museums of France was extremely important for the success of the project. Finally, comparing several old texts with the chemical technology of 1750, by a chemist from CNRS familiar with the technical knowledge of paintings, enabled us to put forth a hypothesis concerning Picault's secret .