Crocoite PbCrO_4 and mimetite Pb_5(AsO_4)_3Cl: rare minerals in highly degraded mediaeval murals in Northern Bohemia

摘要

Mural paintings of exceptional quality, which can be discerned in spite of their extensive mechanical damage and colour fading, have been uncovered in the church of St. Gallus in Ku?ívody, Northern Bohemia, dated to the second half of the 13th century. Materials research with particular use of portable X-ray fluorescence, Raman micro-spectroscopy and powder X-ray micro-diffraction revealed the presence of rare pigments. In Ku?ívody, it is only a second identification of intentionally used yellow mineral crocoite (PbCrO_4) in European art. Its identification is facilitated by providing a very good Raman scattering, even when present in small amounts in fragmentarily preserved colour layers. Light yellow mimetite (Pb_5(AsO_4)_3Cl) was never before mentioned as intentionally used pigment in Europe. Its finding in Ku?ívody, however, corresponds more likely with undesirable physical-chemical conditions causing its formation by alteration of orpiment (As_2S_3) and minium (Pb_3O_4). Obtained results highlight the importance of Raman spectroscopy for direct identification of mineral pigments in low concentrations, which may be crucial for interpreting cultural heritage objects in historical context. By materials, the almost forgotten paintings in Ku?ívody can be seen as outstanding and rare example of ancient artistic tradition that has spread to Europe from Mediterranean in early Middle Ages. After all, mineral crocoite was already used by ancient Egyptians to paint sarcophagi and degraded orpiment decorates the walls of the Nefertari's tomb in Thebes.