On medieval objects of art, irreversible alterations are often found which are so serious that they cannot be restored. Only through technical examinations one can comprehend the original appearance. The alteration of the coloring on a panel from the 13th century and a cross from the 12th century which was caused by deterioration of the orpiment to a grey tone without color is discussed. Orpiment was often used in Norway in the Middle Ages as a yellow color. A polychrome wooden sculpture St. Olav from Fresvik (Norway) is missing parts of the figure, but much worse are the losses of large parts of the ground and paint layer. The ground and paint layer are on the cheek 7 mm and on the knee 15 mm thick, and thereby occurs an unclear definition of the forms.