The surface of ceramics is known to be very sensitive to particle beam irradiation, and visible brownish stains are created under bombardment. The reason for the visible damage is probably to be attributed to atomic displacements in the material. Whichever the reason, it is important to operationally define under which conditions of irradiation such materials can be safely analysed by ion beams for compositional characterisation. An extensive investigation of this problem has been performed at the external beam PIXE facility of the KN3000 Van de Graaff accelerator in Florence. Both modern materials, prepared according to the ancient tradition, and fragments of original works dating from Renaissance have been employed for the study. The induced damage effects have been measured as a function of beam current, integrated dose, sample temperature during irradiation. The damage recovery after irradiation has also been studied. It comes out that even at room temperature the induced stain progressively fades out and that heating the samples at higher temperatures can eliminate the problem even in a very short time. To be made quantitatively, all these studies have implied a preliminary definition of "damage". This has been assumed as the difference in the measured colour coordinates of the induced stain with respect to the surrounding unirradiated surface.