Green Verdigris

The Chemistry of Green Verdigris

Verdigris is a synthetic pigment that ranges from a blue to a bluish-green color. There are two general forms of verdigris, basic and neutral, consisting of copper acetate with different combinations of H2O and CuOH. Neutral verdigris has the chemical formula Cu (CH3COO)2·H2O and was the preferred pigment in the Middle Ages. Verdigris is synthesized by hanging a copper plate over hot vinegar in a sealed pot and then scraping the green product off the plate.

In many respects verdigris is similar to Egyptian blue. Both obtain their distinctive color from copper ions surrounded by oxygen and take advantage of d-d transitions. They are distinct from one another because the metal resides in a different chemical environment. Where the copper in Egyptian blue is surrounded by the oxygen atoms of a silicate, the copper in verdigris

Statue of Liberty, New York.

Photo: Daniel Schwen

Fig.4 The chemical structure of verdigris. Image: Dalton Transactions 46 (2017): 14847–58

is acetate (CH3COO2-). This subtle yet important difference causes a precise ordering of the metal d-orbitals and thus produces a unique color. Verdigris has a peak absorption in the range of 474 nm and 672 nm, which corresponds to the blue and red range respectively. It should be noted that over time verdigris starts to turn brownishred when exposed to oxygen. This is because the copper reacts to form oxide compounds such as CuO and Cu2O.

Green Verdigris

in Art

Fortunately, the Mead’s fresco of Saint Sylvester was able to preserve most of its original colors. This life-size portrayal of the saint was originally part of a complex wall decoration in the Romanesque Church of Santa Coloma in Andorra, dating to the twelfth century. Sylvester was the Pope and head of the Roman Catholic Church, from 314 to 335. A circular halo, a cross, and an extended open palm symbolize his sanctity and the pope’s authority.

The Master of Santa Coloma, the celebrated muralist who is thought to be the maker of this work, used a restricted palette for his wall paintings. His method of applying pigments to fresh and damp plaster

Attributed to Master of Santa Coloma

(Andorran, 12th century), Saint Sylvester, fresco possibly from the Church of Santa Coloma, 1150–99. Fresco, transferred

to canvas. Museum purchase, 1941.9.

Photo: Petegorsky/Gipe Photography

required using inorganic pigment compounds rather than natural dyes; this allowed the color pigments to withstand the passage of time. The artist most likely employed smoke-derived black for the contours, ocher for the yellow halo and the coat’s goldenlike decorations, and cinnabar’s red for facial details. The blue-green of the saint’s ornate robe is likely verdigris, the man-made extract pigment that was often used during this period since antiquity. references

“Apostles from Sant Romà De Les Bons.” Museu Nacional d’Art de Catalunya. Museu Nacional d’Art de Catalunya, n.d. https://www.museunacional.cat/en/ colleccio/apostles-sant-roma-deles-bons/mestre-de-santa-colomadandorra/015783-000.

Anthony, Edgar Waterman. “II. Technique.” Essay. In Romanesque Frescoes, 61–65. Princeton University Press, 1951.

The Editors of Encyclopædia Britannica. “St. Sylvester I.” Encyclopædia Britannica. Encyclopædia Britannica, Inc., April 30, 2020. https://www.britannica.com/ biography/saint-sylvester-ı.

Palomares, Susanna Vela. Magister Sancta Columba: La Pintura romànica Del Mestre De Santa Coloma i La Seva època. Viena Art, 2003.