gems, beryl, emerald, oiling, treatments

Through the case of the emerald, this chapter examines artisanal explorations of light and their influence on ideas concerning optics and vision within the broader history of perspective. The modern distinction between optics and perspective as projective geometry was not yet apparent during the medieval and early modern periods.¹ Influential treatises on perspectivist optics, such as the Latin translation of Ibn al-Haytham’s Book of Optics (De aspectibus, c. 1200), as well as works heavily indebted to it — including Roger Bacon’s Perspectiva (c. 1265), Witelo’s Perspectiva (c. 1275), and John Pecham’s Perspectiva communis (c. 1280) — typically focused on theories of light, colour, and vision. Over the course of the fifteenth century, however, mathematical perspective became an important tool through which artisans could create the illusion of spatial depth upon a two-dimensional surface. The history of this discovery and its subsequent development later became a central subject in the history of art and science. Because of this emphasis on perspective as projective space, considerably less attention has been given to the artisan’s creative manipulation of, and practical understanding of, the optical properties of materials.² Yet this applied knowledge of the interaction between light and artistic materials found important resonance among early writers concerned with the generation and medicinal virtues of natural substances. To investigate this history of practical optics, the present chapter therefore moves away from the traditional corpus of perspectivist optics — as constructed, for example, by David C. Lindberg in Theories of Vision (1976) and more recently by A. Mark Smith in From Sight to Light (2014) — and instead turns toward medieval and early modern recipe collections and works of natural history, including encyclopaedias and lapidaries. More specifically, I will show how the distinctive optical properties of the emerald stimulated not only admiration for the stone itself, but also encouraged the development of inventive techniques for working and imitating this precious mineral. These emerald imitations, in turn, gave rise to theories concerning the green gem’s natural formation, value, and uses. Indeed, the emerald was regarded as intriguing and beautiful not merely because of its unusual optical qualities; its deep translucent green colour had long been considered beneficial to human eyesight. In this way, the emerald — together with the various practices devised for its imitation — exerted a surprising and enduring influence on the furnishing both of the scholar’s study and the artisan’s workshop.