Luca was apprenticed in his youth to a goldsmith; but of what he wrought before the age of forty-five, we know but little.[97] At that time his faculty had attained full maturity, and he produced the groups of dancing children and choristers intended for the organ gallery of the Duomo.  Wholly free from affectation, and depending for effect upon no merely decorative detail, these bas-reliefs deserve the praise bestowed by Dante on the sculpture seen in Purgatory:[98]—­

    Dinanzi a noi pareva si verace,
    Quivi intagliato in un atto soave,
    Che non sembrava immagine che tace.

Movement has never been suggested in stone with less exaggeration, nor have marble lips been made to utter sweeter and more varied music.  Luca’s true perception of the limits to be observed in sculpture, appears most eminently in the glazed terra-cotta work by which he is best known.  An ordinary artist might have found the temptation to aim at showy and pictorial effects in this material overwhelming.  Luca restrained himself to pure white on pale blue, and preserved an exquisite simplicity of line in all his compositions.  There is an almost unearthly beauty in the profiles of his Madonnas, a tempered sweetness in the modulation of their drapery and attitude, that prove complete mastery in the art of rendering evanescent moments of expression, the most fragile subtleties of the emotions that can stir a tranquil spirit.  Andrea della Robbia, the nephew of Luca, with his four sons, Giovanni, Luca, Ambrogio, and Girolamo, continued to manufacture the glazed earthenware of Luca’s invention.  These men, though excellent artificers, lacked the fine taste of their teacher.  Coarser colours were introduced; the eye was dazzled with variety; but the power of speaking to the soul as Luca spoke was lost.[99]

After the Della Robbias, this is the place to mention Agostino di Gucci or di Duccio,[100] a sculptor who handled terra-cotta somewhat in the manner of Donatello’s flat-relief, introducing more richness of detail and aiming at more passion than Luca’s taste permitted.  For the oratory of S. Bernardino at Perugia he designed the facade partly in stone and partly in baked clay—­crowded with figures, flying, singing, playing upon instruments of music, with waving draperies and windy hair and the ecstasy of movement in their delicately modelled limbs.  If nothing else remained of Agostino’s workmanship, this facade alone would place him in the first rank of contemporary artists.  He owed something, perhaps, to his material; for terra-cotta has the charm of improvisation.  The hand, obedient to the brain, has made it in one moment what it is, and no slow hours of labour at the stone have dulled the first caprice of the creative fancy.  Work, therefore, which, if translated into marble, might have left our sympathy unstirred, affects us with keen pleasure in the mould of plastic clay.  What prodigality of thought and invention has been lavished on the