Fig. 1. Materials used for colouring the 'imitation' emerald: glass muller and slab for grinding, linseed oil, verdigris (copper green - made by the author by exposing copper to vinegar) and rock crystal.
Project (2012-2014)

Understanding Light through Art. Emeralds and the Artisan’s Contribution to Optical Knowledge

This Working Group project looked at artisans' contribution to optical knowledge. Since antiquity, the emerald has been one of the most precious stones money could buy. It was prized for its lush green color, its saturated translucency, its hardness and rarity. It is therefore not surprising that this popular stone was often worked by goldsmiths and set into costly metalwork pieces. Moreover, because of its rarity and preciousness, the emerald was also one of the most frequently imitated gems, and many recipes survive that describe how to create a substitute by the use of green glass, special pastes, or by coloring a cheaper transparent mineral with a copper green pigment called verdigris. This Working Group project included historical reconstructions of the latter practice, see fig.). In the fifteenth century, such recipes were frequently bundled together with painting instructions, of which the so-called Italian Bolognese Manuscript is a good example. Technical research into late medieval panel paintings shows that indeed painters also used the copper green pigment verdigris for the imitation of precious green stones.

Besides these economic and artisanal fascinations with the emerald, the gem also prompted the interest of natural philosophers. Numerous lapidaries and encyclopedias, both north and south of the Alps, show that translucent gems such as emeralds were studied to help understand the behaviour of light. As the emerald is naturally characterized by cracks and fissures, goldsmiths often nourished these jewels with oil to increase their saturation and translucency. Philosophers such as Marbode of Rennes and Konrad of Megdeberg write about this practice to explain how jewellers influenced the optics of a stone (its hue, lustre, and saturation). Furthermore, natural philosophers used artisanal practice to inform their theories about the genesis of gemstones in nature. For example, both Al-Biruni and Albertus Magnus write that emeralds grow out of copper because the "rust of copper"—being the pigment verdigris—is green.