 Complete the following phrase:  Unquestionably though, his most challenging project involved the Black Paintings of Francisco de Goya, which were painted in oil on the walls of Goyas home, the Quinta del Sordo. The images were transferred from the wall to canvases, using pictures taken by the French photographer Jean Laurent as a guide. Many of the paintings were smudgy, badly cracked, and smeared with plaster, so he took the initiative of restoring them to their original state, as much as possible. The homes owner at that time, Baron Émile dErlanger, had planned to sell them at the upcoming 