One of Chagall's major contributions to art has been his work with stained glass. This medium allowed him further to express his desire to create intense and fresh colors and had the added benefit of natural light and refraction interacting and constantly changing: everything from the position where the viewer stood to the weather outside would alter the visual effect (though this is not the case with his Hadassah windows). It was not until 1956, when he was nearly 70 years of age, that he designed windows for the church at Assy, his first major project. Then, from 1958 to 1960, he created windows for Metz Cathedral.