The Frequent Occurrence of Atacamite in Medieval Swedish Murals

Copper trihydroxychloride [Cu2Cl(OH)(3)] exists with four polymorphs: atacamite, paratacamite, clinoatacamite, and botallackite. They have all been used as green pigments, usually under the name atacamite. For many years, atacamite was regarded as a pigment mainly used in ancient South America, China, and Egypt. However, the last decades have shown that atacamite has been found in European medieval paintings, and quite often in Sweden. This paper gives a brief overview of the history of atacamite and its polymorphs. Green pigments from medieval murals in 56 Swedish churches were analyzed. The results show that atacamite and malachite are common, while green earth is less frequent and green vivianite rare. In particular, atacamite often occurs in medieval wall paintings on the island of Gotland in the Baltic Sea. The atacamite minerals are rare in Europe and are not found in the Swedish bedrock. Their occurrence in Swedish murals and their origin is discussed. Atacamite may be synthesized by various methods and is in fact often observed on corroded outdoor bronze statues in marine surroundings.