Verdigris

Verdigris, a green pigment, has been used throughout history in various applications, ranging from painting to medicinal treatments. “Verdigris” refers to the copper salts of acetic acid, predominantly copper acetate, and has long been associated with the corrosion products formed on copper, bronze, and brass when exposed to air and acetic acid. While verdigris is often referred to simply as copper acetate in modern times, it was historically a more complex mixture, sometimes containing other copper salts, depending on how it was manufactured.[i]

Historical Usage

The use of verdigris as a pigment dates back to ancient Rome and Greece, where it was frequently mentioned in literature. [ii] Its popularity surged during the Middle Ages and Renaissance when it became the go-to green pigment for European painters.[iii] Verdigris was used to produce bright green hues, particularly in the drapery and landscapes of Renaissance paintings, and was often combined with other pigments like lead-tin yellow and lead white to achieve varying tones.

One notable aspect of verdigris is its dual role as both an opaque pigment and a glazing medium. Artists often glazed verdigris over a base color of lead white or lead-tin yellow, creating rich, saturated green shades. [iv] Despite its widespread use in the 13th through 17th centuries, by the 18th century, its use declined. This was due to the introduction of new, more stable green pigments like emerald green and viridian, which replaced verdigris due to their superior color retention and fewer degradation issues.[v]

Chemical Composition and Properties

Verdigris is a collective term for several types of copper acetates, which vary in color from green to blue, depending on their exact composition. These copper salts include both basic and neutral forms, with the basic verdigris slightly more prevalent in historical records. The most common chemical form of verdigris is basic copper acetate, but the pigment can also consist of mixtures involving copper chlorides or copper carbonates. [vi]

In terms of preparation, verdigris was historically produced by exposing copper plates to vinegar or other sources of acetic acid. The copper would react with the acetic acid vapors, forming verdigris on its surface, which was then scraped off. Notably, verdigris production became a specialized industry, with regions like Montpellier in France becoming hubs for its manufacture by the 18th century.[vii]

Optical and Material Properties

Under the polarizing microscope, verdigris displays clear pleochroism, shifting from light pale green to deep blue-green hues. This property made it a valuable pigment for artists aiming to achieve varied effects in their works. Verdigris was also known for its poor hiding power, mainly when used in oil-based paints, leading artists to mix it with lead white or lead-tin yellow for added opacity.[viii]

One of verdigris’s more interesting characteristics is its tendency to change color over time. When freshly applied, verdigris paints exhibit a vibrant blue-green hue. However, the color shifts toward green over time, especially in the first few months. This aging process was further influenced by the binding media used. For example, verdigris in oil and tempera tended to darken less than in aqueous media like egg tempera. [ix]

References

[i]  A. Buntrock, Genre/e’s Lehrbuch derFarbenfabrikation , vol. 2 (Braunschweig, 1909) 373-380.

[ii] E. R. Caleyand J.F.C. Richards, Theophrastus on Stones, OhioState University (Columbus, Ohio, 1956). Matthiolus, ed. and transof Dioscorides nE(.Jt VAt]~ iar(.JtX11~ (On Pharmaceutics) French ed., 5, no. 91 (Lyon, 1655), 501.

[iii] Robert L. Feller et al., eds., “6 Verdigris and Copper Resinate,” in Artists’ Pigments: A Handbook of Their History and Characteristics v.2 (Washington: National Gallery of Art, 1993), 131–47.

[iv] Feller et al.

[v] Lefranc (Paris), Beaux-Arts: Prix Courant (Paris, 1928), 7.

[vi] Feller et al., “6 Verdigris and Copper Resinate.”

[vii] K. C. Bailey, The Elder Pliny’s Chapters on Chemical Subjects (London, 1929). A. Buntrock, Genre/e’s Lehrbuch der Farbenfabrikation , vol. 2 (Braunschweig, 1909) 373-380.

[viii] Feller et al., “6 Verdigris and Copper Resinate.”

[ix] Feller et al., “6 Verdigris and Copper Resinate.”