Vitruvius-like synthesis of Egyptian Blue using Roman coins as copper source

Abstract

The modern synthesis of Egyptian Blue has been attempted - mostly with success- by many researchers, since the scientific community has been working on this experimentation for more than a century. The aim is that of reconstructing the chaîne opératoire used in the past to produce this pigment, among the first synthetic ones in the history, whose colour is due to the blue copper silicate cuprorivaite. Almost all the experiments carried out up to now in this sense have used malachite or commercial CuO as copper source in the receipt, contrarily to what stated by Vitruvius who described the procedure as using copper filings, probably obtained from scrap metal. The aim of this paper is that of filling this gap in the literature in the direction of the use of alloys filings. For this reason, copper filings from two Roman coins of private collection were used to synthesize Egyptian Blue, and the results were compared with different attempts made with malachite and with two ancient samples of archaeological origin. We also aimed to evaluate the role of Egyptian Blue grains added to the preparations to act as seeds during the reactions. Preliminary investigations with optical microscopy, spectrocolorimetry and Visible Induced Luminesce (VIL) were used to discard the products less compatible with the archaeological Egyptian Blue samples. Then, Raman spectroscopy and X-ray Diffraction (XRD) were employed to analyse the most closely related products. While the use of a coin with arsenic impurities caused the synthesis of copper arsenate as blue phase, the test with a purer coin and Egyptian Blue nucleating agents was successful, giving rise to cuprorivaite crystallization. Raman spectroscopy was also used to monitor in situ the transformations during the firing, which took place in a heating stage positioned under the spectrometer's microscope; these experiments highlighted the difficulty of following the structural evolution in situ at high temperatures in heterogeneous mixtures that sometimes pass through liquid phases.

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