Early Medieval Egyptian blue in laser light

SALSA School of Analytical Sciences Adlershof | Highlight: Early Medieval Egyptian blue

Early Medieval Egyptian blue in laser light

Petra Dariz, Thomas Schmid, Scientific Reports 11 (2021) 11296.

Egyptian blue, the first artificial pigment of mankind, can be synthesised by heating a raw material mixture consisting of quartz sand, limestone, copper ore and a flux (soda or plant ash) to around 950°C. At the turn of the eras, Roman sources report that a certain Vestorius transferred the originally Egyptian technology to Pozzuoli near Naples (Campania, Southern Italy). In fact, archaeological evidences confirm production sites in the Phlegraean Fields and seem to indicate a monopoly in the manufacture and trade of pigment spheres. Due to its almost exclusive use, Egyptian blue is the blue pigment par excellence of Roman antiquity; its art technological traces vanish in the course of the Middle Ages.

Art technologist Dr. Petra Dariz and analytical chemist Dr. Thomas Schmid identified Egyptian blue on a monochrome blue mural fragment, which was excavated in the little church of St. Peter above Gratsch (South Tyrol, Northern Italy) in the 1970s. The fragment was provided by the office for archaeology of the autonomous province of South Tyrol as a reference material for age determination within the frame of a research project on Early Medieval stucco fragments of this church, co-funded by the South Tyrolean provincial museums. The two researchers originally stemming from South Tyrol conducted the investigations in the laboratories of the School of Analytical Sciences Adlershof (SALSA) at Humboldt Universität zu Berlin and Bundesanstalt für Materialforschung und -prüfung (BAM) in Berlin (Germany). The results were published in the Nature Research journal Scientific Reports.

Photo and Raman microscopy image of a patch of the pictorial layer of the mural fragment.
(adapted from Sci. Rep. 11 (2021) 11296).

Finding each needle in a haystack

Since the rediscovery of Egyptian blue in the context of the Egyptian expedition and the excavations in Pompeii and Herculaneum around 200 years ago the pigment has exerted an unbroken fascination, triggering numerous subsequent research works. Only within the last decade, petrographic investigations were included with the aim to characterise and differentiate possible production sites. Conventional analytical approaches applied so far, were limited to components with contents of more than one percent, because finely distributed minerals in powdered samples behave like the proverbial needle in a haystack. In earlier studies on historical mortar materials by the team, Raman microscopy was employed as new method to reconstruct technical process parameters and the origin of the raw materials. Based on these experiences, the application of this technique appeared promising for the detection of potential trace compounds in Egyptian blue. Extensive scanning of the paint layer with a laser beam focused to around a thousandth of a millimetre and conduction of a non-destructive material analysis at each individual measurement spot ensured to find even the smallest information carriers.

The individual “biography” of a colourant

The results were beyond all expectations. In 166,477 individual measurements, 28 different minerals with contents from the percent range down to 0.1 permille were identified. Inclusion of knowledge from neighbouring disciplines made possible to read out the information about the type and provenance of the raw materials, synthesis and application of the pigment and ageing of the paint layer preserved in the trace components, and thus to reconstruct the individual "biography" of the Egyptian blue from St. Peter. This multifaceted insight represents a paradigm shift in the research history, at the same time raising new research questions. Particularly noteworthy are minerals associated with volcanic activity, which according to the composition of beach sands at the Gulf of Gaeta, indicate a production of the pigment in the northern Phlegraean Fields. Furthermore, indicators for a sulphidic copper ore (instead of often-mentioned metallic copper or bronze) and plant ash as flux in the raw material mixture were found. Comparable Raman microscopic analyses of Egyptian blue in Roman and Medieval wall paintings and of pigment spheres could finally provide a sound scientific evidence for the assumed manufacturing monopoly in Pozzuoli surviving over centuries after the fall of the Western Roman Empire.

Within a follow-up project the two researchers will apply this new analytical strategy to pigment spheres and wall painting fragments from the ancient Roman cities Augusta Raurica and Aventicum in Switzerland.

P. Dariz, T. Schmid, Trace compounds in Early Medieval Egyptian blue carry information on provenance, manufacture, application, and ageing. Scientific Reports 2021, 11, 11296. DOI: 10.1038/s41598-021-90759-6.