Artist's books have been one of Kiefer's central means of expression since 1968. Unlike many artist's books, which are typically issued as multiples, Kiefer's books are singular, handmade pieces and, in this sense, more like paintings or sculptures. Iconoclastic Controversy (Bilderstreit, 1980) belongs to a series of paintings and books with the same title, produced between 1977 and 1980, that refer to the conflict that brewed in the Byzantine Empire in the eighth and ninth centuries between iconoclasts, who opposed the worship of religious images, and iconodulists, who advocated it. In the book in the Guggenheim Museum Bilbao collection, Kiefer names several emperors, popes, and patriarchs involved in this conflict. Each name is accompanied by a plus or minus sign, where the plus sign identifies figures who took an iconodulist stance and the minus sign those who positioned themselves as iconoclasts. He also names some figures whose posture remained ambiguous, such as Leo V, whom Kiefer depicts with both signs at once. Additionally, on the fifth page of the book, Kiefer mentions what might be considered the culminating events of the struggle: the Synod of Hiereia (the iconoclast council of 747) and the Council of Nicaea II (the iconodulist council of 787). Kiefer illustrates this ideological battle using images of tanks opening fire on the traditional objects of a painter's studio. Mark Rosenthal has suggested that Kiefer's resuscitation of the dispute over icons might be understood in relation to the contemporary debate over the use of photography as an art medium.