The eighth and ninth centuries were a formative period for medieval art. Court patronage was scarcely new to this period, but its focus and character had shifted with the decline of the cities. The expanded role given to images during the seventh and early eighth centuries led in the Byzantine world to the sharp reaction known as Iconoclasm, and ironically gave new force and definition to religious images. An important example of new forms and interpretations of traditional iconography is the image of the Crucifixion. The study of western European church architecture of the eighth and ninth centuries has been dominated for the past half-century by Richard Krautheimer's great article treating 'the Carolingian revival of early Christian architecture'. Sculpture in ivory and rock crystal was associated with Carolingian Francia during this period, but the other major medium for what may be thought of in the context of sculptural art, various kinds of metalworking, was widely employed across western Europe.