Stability of dioptase in egg tempera.

Here is an excerpt from David Scott’s 2002 publication, “Copper and Bronze in Art” (pg 255): “Dioptase, CuSiO3H2O, a common silicate, is usually found as a massive vitreous green cluster, associated with other copper mineral deposits, although individual crystals may also sometimes be found. Dioptase can be a translucent, emerald-green color. The mineral is seldom used as a gemstone, but small crystals have been emerald cut (Newman 1987), and some clusters have been set in jewelry. The misnomer “copper emerald” is the name for dioptase in the jewelry trade. Dioptase was one of the pigments – along with ochre, carbon black, and lime white – identified on a cache of lime plaster statuettes, approximately eight thousand years old, from the pre-pottery Neolithic site of ‘Ain Ghazal, Jordan (Tubb 1987). The dioptase was found in the eyeliner on the statuettes, and this appears to be the only recorded use of this mineral as a pigment in the ancient world, although there must be other examples that have not yet been reported in the literature.” As for lightfastness it has been reported to possess relatively good lightfastness although Kremer Pigmente states that it can change its color at higher temperatures or if exposed directly to the sun (most likely due to driving off the waters of hydration). As it is a copper silicate, it will likely be more stable than verdigris and even the copper-carbonates (e.g. malachite and azurite). The copper is less likely to readily disassociate from the silicate group as compared to the acetate group in verdigris and the carbonate group in other pigments. As with all copper-based pigments, dioptase should be handled with care but as it is only readily soluble in highly acidic solutions it is generally considered less toxic than verdigris when painting with water-based binders (e.g. tempera) as the latter is soluble in both water and alcohol. At present, not enough is known about the long-term properties of this pigment in oil-based mediums.