Della Robbia's earliest surviving freestanding sculpture is the white tin-glazed terracotta "Visitation" in the church of San Giovanni Fuoricivitas of Pistoia, dating to 1445. Although the date of della Robbia's first work in colored glazed terra-cotta is not known, his demonstrated control of this medium secured him two major commissions for the duomo of Florence: the large reliefs of the "Resurrection "(also from 1445) and the "Ascension of Christ "(1446). The pliant medium of baked clay covered with a "slip" of vitrified lead and refined minerals permitted a lustrous, polished surface capable of reflecting light and color that was beautifully appropriate for architectural sculpture. Whether animating the vast, somber space of the Cathedral or in the series "Twelve Apostles "gracing the pristine surfaces of the small Pazzi Chapel (1443–1450) in Florence, della Robbia's reliefs in this medium achieved a high level of mastery.