Glazed terracotta of the della Robbia; the round versus relief
Painted glaze rivals sculpture in permanence and beauty; only relief shares perspective
Leonardo answers the durability boast by pointing to painting fired as glass colours over metal and terracotta, above all the glazed work of the della Robbia family in Florence, which matches sculpture's permanence and surpasses its beauty because it joins the two perspectives. He then argues that a figure in the round shows only two real views, front and back, doing no more "demonstration" than a painting of the same view. Bas-relief, by contrast, is bound to perspective and rises far nearer to painting than the full round, which uses only the simple measurements taken from life. The leaf carries text only, with no illustration.
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Glazed terracotta painting of the della Robbia in Florence
Painting with glass colours over metals and fired clay, fused in the furnace and polished to a flat lustrous surface, is practised in France and Italy and above all by the della Robbia in Florence, who execute great works in painting on glazed terracotta. Though breakable like marble, it lacks the destroyers of bronze, equals sculpture in permanence, and surpasses it in beauty by joining the two perspectives.
A figure in the round shows only two views, front and back
The sculptor making a figure in the round makes only two figures, not the infinite ones for the infinite aspects it can be seen from: one seen from the front, the other from behind. A half-relief seen from the front demonstrates no more than a painting made in the same view, and the like holds for a figure turned to the back.
Bas-relief bound to perspective, nearer to painting
Bas-relief is of far more speculation than the full round and approaches painting in the greatness of its speculation, because it is bound to perspective. The full round concerns itself not at all with such knowledge, using the simple measurements as it finds them from life.