Egyptian blue can be synthesised by heating a raw material mixture consisting of quartz sand, limestone, copper ore and a flux (soda or plant ash) to about 950 °C. The use of mankind’s first artificial pigment became widespread in the Fourth Dynasty of Egypt. In the 1st century BC and 1st century AD, Roman sources report that a certain Vestorius transferred the production technology from Alexandria to Pozzuoli. In fact, archaeological evidence confirms production sites in the northern Phlegraean Fields near Naples (Campania, Southern Italy) 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 during the Middle Ages.
Art technologist Dr Petra Dariz and analytical chemist Dr Thomas Schmid identified Egyptian blue on a monochrome blue mural fragment (5th/6th century AD), which was excavated in the church of St Peter above Gratsch (South Tyrol, Northern Italy) in the 1970s. The two researchers, who originally came from South Tyrol, conducted Raman spectroscopic analyses in the laboratories of the School of Analytical Sciences Adlershof (SALSA) at Humboldt-Universität zu Berlin and the Bundesanstalt für Materialforschung und -prüfung (BAM), Berlin (Germany).
Since the rediscovery of Egyptian blue during Napoleon’s Egyptian campaign 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 have petrographic investigations been included with the aim of characterising and differentiating possible production sites. Conventional analytical approaches applied so far were limited to components with contents of more than 1 %, because finely distributed minerals in powdered samples behave like the proverbial needle in a haystack.
In earlier studies on historical mortar materials, the team employed Raman microscopy as a 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 micrometre and the spectroscopic identification of minerals at each individual measurement spot ensured that even the smallest information carriers could be found.
The results were beyond all expectations. In 166,477 individual measurements, 28 different minerals with contents from the percent range down to 100 ppm were identified. Inclusion of knowledge from neighbouring disciplines made it 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.