Luca della Robbia created the half-length glazed terracotta Madonna and Child (Fig. 54), today at the Museo degli Innocenti in Florence, sometime around 1450. This high relief sculpture features the infant Christ, seated in his mother’s arms and displaying a banner that proclaims: “I am the light of the world” (“EGO SVM LVX MVNDI”; John 8:12). As others have noted, ambient light plays bewitchingly over the figures as if to affirm the truth of these words.1 The shining white surfaces that render mother and child so attractive were a key feature of the new glazed terracotta medium famously invented by Luca himself. In recent decades, scholars have understood the white color and reflectivity of the glaze to communicate, in an ideal fashion, the purity and holy resplendence of Christ, the Virgin, and (elsewhere) the saints. Yet, the sensitivity of such glazes to real lighting conditions has not received a detailed analysis in the literature on Luca or his artistic heirs in the Della Robbia and Buglioni family workshops. Drawing on ideas in texts by Cennino Cennini, Leon Battista Alberti, and Lorenzo Ghiberti, this essay will show fifteenth-century audiences to have been keenly aware of a distinction – and tension – between effects of light that animate the surfaces of sculptures and those that describe physical form. Moreover, it will suggest that this visible distinction could usefully remind Luca’s viewers of the duality of Christ and the Virgin – as holy yet human – and picture the theological differentiation between lux and lumen alluded to by Christ’s proclamation.