This text describes the methodology for and the results achieved in the separation of an oil wall painting from an underlying al fresco painting. This intervention was carried out by technicians working for the Opificio delle Pietre Dure, Florence, on a painting by Andrea Lilli in the cloister of the ancient monastery of Sant'Angelo Magno at Ascoli Piceno. To insure the success of the operation, we had to work out some variations to the usual technique. However these variations did not require materials different from those commonly in use in wall painting restoration; they did not deviate radically but only marginally from the accepted methodology. In this intervention, the use of an absorbing cellulose-water blend, with the addition of ammonium carbonate, turned out to play a crucial role in softening the oil layer, and thus enabling the animal glue to adhere to the surface of the canvas as necessary for the detachment. The other phases of this intervention were carried out with extraordinary diligence and care, on account of the exceptional character and delicacy of this kind of work. -- AATA