Crocoite PbCrO₄ and mimetite Pb₅(AsO₄)₃Cl: rare minerals in highly degraded mediaeval murals in Northern Bohemia

Abstract

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 Kuřívody, 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 Kuřívody, 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 ) 3 Cl) was never before mentioned as intentionally used pigment in Europe. Its finding in Kuřívody, however, corresponds more likely with undesirable physical-chemical conditions causing its formation by alteration of orpiment (As 2 S 3 ) and minium (Pb 3 O 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 Kuřívody 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.

On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments

“…The reaction between red lead and orpiment produced galena, which could further degrade into sulphates, and lead arsenate species [241]. Mimetite Pb 5 (AsO 4 ) 3 Cl was as well identified as a reaction product of minium and orpiment [186]. Moreover, it seems that minium promotes the blackening of leadwhite, when the pigments are mixed [204].…”

“…Relative humidity alone seems to have no effect, while degradation is observed when both humidity and light are present [64]. Light exposure causes discolouration [30,128,182,185,186]. Orpiment darkens upon heating, it burns in air producing white arsenolite (As 2 O 3 ) and volatile SO 2 , and it decomposes in water (the artificial product having smaller particles, dissolves faster).…”

Raman identification of cuneiform tablet pigments: emphasis and colour technology in ancient Mesopotamian mid-third millennium

“…It is worth noting that TiO2 was not spread all over the clay but within the engraving sign in the E point. Finally, Raman spectra collected in the point F (second row col. 2–side A) revealed the presence of Lead carbonate in the cerussite phase (intense band ad 1053 cm −1 and medium at 830 cm −1 ), also known as Lead White pigment [23]. Yellow pigment : the vibrational bands at 355 and 844 cm −1 identify the mineral Crocoite, often associated to the Chrome Yellow pigment [24]. This pigment was detected in the second row of column 1 (side A), especially in the sign “ dingir ”.…”

“…Yellow pigment : the vibrational bands at 355 and 844 cm −1 identify the mineral Crocoite, often associated to the Chrome Yellow pigment [24]. This pigment was detected in the second row of column 1 (side A), especially in the sign “ dingir ”.…”

“…However, the application of this mineral as yellow pigment was found, for example, in precious funerary relics belonging to Egypt (1600 BC) [35, 36]. Moreover the use of Lead- based minerals as pigments, especially Lead Chromate, has been conducted during the centuries (Crocoite and Mimetite − Palmyra 200 BC [37]; Crocoite − North Bohemia 1300 BC [24]; Crocoite, Lead Tin yellow, Lead antimonate − Renaissance [38]) until the modern age when the synthetic production was developed [32]. The evidences here reported indicate that Sumerian people did know lead based mineral compounds and used them to colour their clay tablets.…”