Trace compounds in Early Medieval Egyptian blue carry information on provenance, manufacture, application, and ageing

Abstract

Only a few scientific evidences for the use of Egyptian blue in Early Medieval wall paintings in Central and Southern Europe have been reported so far. The monochrome blue fragment discussed here belongs to the second church building of St. Peter above Gratsch (South Tyrol, Northern Italy, fifth/sixth century A.D.). Beyond cuprorivaite and carbon black (underpainting), 26 accessory minerals down to trace levels were detected by means of Raman microspectroscopy, providing unprecedented insights into the raw materials blend and conversion reactions during preparation, application, and ageing of the pigment. In conjunction with archaeological evidences for the manufacture of Egyptian blue in Cumae and Liternum and the concordant statements of the antique Roman writers Vitruvius and Pliny the Elder, natural impurities of the quartz sand speak for a pigment produced at the northern Phlegrean Fields (Campania, Southern Italy). Chalcocite (and chalcopyrite) suggest the use of a sulphidic copper ore, and water-insoluble salts a mixed-alkaline flux in the form of plant ash. Not fully reacted quartz crystals partly intergrown with cuprorivaite and only minimal traces of silicate glass portend solid-state reactions predominating the chemical reactions during synthesis, while the melting of the raw materials into glass most likely played a negligible role.

Raman focal point on Roman Egyptian blue elucidates disordered cuprorivaite, green glass phase and trace compounds

“…3 , Fig. S2 ) on the pigment balls unearthed in the ancient Roman cities of Aventicum and Augusta Raurica (Table 1 ), this in consistency with the relevant properties of the Early Medieval Egyptian blue applied in St. Peter above Gratsch studied recently 17 . In Switzerland occurrences of relatively pure quartz sand are concentrated in Rhaetian successions and pockets or basins of the Bean Ore Formation in the Jurassic, in particular in the Basle, Solothurn and Bernese Jura, and in littoral accumulations of the Marine Molasse in the Central Plateau (for example near Benken and Wildensbuch in the Zürcher Weinland).…”

“… 18 for further specifics of the employed instrument and an introduction to Raman microspectroscopic imaging, and Ref. 17 for details on the optimisation of the measurement parameters adopted from the predecessor study on trace compounds in Early Medieval Egyptian blue. As the conditions cannot be adjusted to every mineral individually, the chosen irradiance reflects a compromise between sensitivity and non-destructiveness.…”

“…As the conditions cannot be adjusted to every mineral individually, the chosen irradiance reflects a compromise between sensitivity and non-destructiveness. Therefore, thermal conversion of coloured sulphides and oxysalts cannot be ruled out completely (see the section ‘Contaminations from adherent soil minerals’ below as well as the Supplementary Information of the predecessor study 17 and references therein), which was considered in the interpretation of the results.…”

“…Table S1 in the Supplementary Information provides an overview of the Egyptian blue samples analysed in the present and predecessor paper 17 . The pigment balls under study (Fig.…”

“…Only recently, a study on a monochrome blue mural fragment belonging to the Early Medieval church of St. Peter above Gratsch (South Tyrol, Northern Italy, fifth/sixth century A.D.) by means of area-covering Raman microspectroscopic imaging resulted in the identification of 26 minerals down to the sub-permille level in addition to the chromophoric cuprorivaite CuCaSi 4 O 10 —an assemblage suggestive of type and provenance of the raw materials and of chemical reactions occurring during pigment manufacture and application as well as ageing of the pictorial layer 17 . Especially some accessory minerals attributable to the quartz sand, which survived processing without thermal alterations, were indicative of an import of the Egyptian blue in question from the northern Phlegrean Fields in Campania.…”