The time between 1640, when Álvaro Alonso Barba published his Arte de los metales (Art of Metals), and 1761, when Francisco Xavier de Gamboa published his Comentarios a las ordenanzas de minas (Commentaries on Mining Ordinances), was a transitional period in which scientific mining and metallurgical knowledge in Spanish America became operative and practical and replaced the long-standing Spanish (and European) medieval tradition of the theory of minerals and metals. Barba was the last representative of this tradition, inheriting the complex world of the classical writers, the medieval alchemists, the first news from the new American lodes, and the first steps of the Scientific Revolution. On the contrary, Gamboa did not worry about classical and medieval theories, nor did he worry about the problem of metal generation. He was concerned only about the most effective system with which to profit from mining using all the paraphernalia that the science and experience of two worlds could furnish. Both handbooks, very successful in their respective times, show clearly two different approaches to the principal question: how to best manage, from two such different perspectives, the American treasures.