“Hungarian Mine Green”, a Semi‐Natural Copper Pigment from Banská Bystrica Region (Slovakia) ‐ Analytical Evidence and Laboratory Replication

A comprehensive approach is taken to investigate the "Hungarian mine green" pigment, including a historical overview, analysis of the pigment on artworks, and its laboratory replication. It is known that in the past, the pigment is collected in wooden reservoirs in which copper (Cu) compounds precipitated from drainage water during copper mining at Špania Dolina-Piesky and Ľubietová deposits, Slovakia. Microsamples of four polychrome wooden sculptures from the 16th-17th centuries are examined. Posnjakite (Cu₄SO₄(OH)₆·H₂O) and malachite (Cu₂(CO₃)(OH)₂) are most frequently detected by X-ray powder microdiffraction. In rock samples from the mine site, brochantite (Cu₄SO₄(OH)₆) and malachite are dominant, and in recent precipitates, only langite (Cu₄SO₄(OH)₆·2H₂O) is detected, remained in contact with leaking water. In the laboratory, the pigment is prepared by gradually enriching the starting CuSO₄·5H₂O solution with NaHCO₃ in two series of experiments. The initial concentration of the reactants reflected the ratio of Cu²⁺: HCO₃ ^- = 1:1 found in the mine waters at the site, from which langite crystallizes. However, langite does not formed, while brochantite gradually transformed into posnjakite and subsequently malachite. The co-occurrence of basic copper sulfates and carbonates and the characteristic grain morphology proved to be the main indicators of the pigment in artworks.