Why do some of Andrea Mantegna's painted figures look sculptural or metallic?

Andrea Mantegna’s painted figures often appear sculptural or metallic because of his profound artistic engagement with the work of the Florentine sculptor Donatello.1

During his formative years in Padua, Mantegna developed a style that was heavily influenced by the wealth of sculpture Donatello produced for the high altar of San Antonio. Mantegna’s reaction to these works was a creative struggle or dialectic, which resulted in his own art retaining distinct echoes of Donatello’s sculptural approach. This influence manifested in his paintings through hard, even metallic, surfaces and a focus on monumental volume.1

This sculptural quality was a hallmark of his style, reflecting his broader interest in the Classical antiquity of ancient Rome, which he studied through both architecture and sculpture. While this essentially sculptural approach remained a key feature of his work for much of his career, it was only somewhat softened in the 1490s.1