Ancient Man-made Copper Silicate Pigments Studied by Raman Microscopy

Abstract

This article describes the application of Raman microscopy (micro-Raman spectroscopy) to the characterization and identification of alkaline earth copper silicates, which were used in ancient times as blue and purple pigments and were man-made. Thus, Egyptian Blue (CaCuSi₄O₁₀), Chinese Blue (BaCuSi₄O₁₀) and Chinese Purple (BaCuSi₂O₆) give rise to well-defined Raman spectra (excitation 514 nm), which allowed the identification of Egyptian Blue in 11 original Egyptian samples from the VIth Dynasty to Roman times and that of Chinese Purple in six ancient Chinese samples dated from the Warring States period to the Han Dynasty (479 BC to 220 AD). One greenish-blue Egyptian sample turned out not to contain Egyptian Blue but rather a mixture of cupro-wollastonite and libethenite. Some of the Egyptian Blue samples showed only minor admixtures of CaSO₄ or silica. Four of the Chinese samples were colour sticks. In one original Chinese stick sample Chinese Blue was recognized as an additional pigment component. One colour stick was revealed to be highly contaminated with Sa-silicates, BaCO₃, CaCO₃, PbSO₄, BaSO₄, PbO, and SiO₂. The investigated two pigment layer samples of the Terracotta Army, Xi'an, China were analysed by Raman spectroscopy and were found to contain, in addition to Chinese Purple, azurite, vermilion, iron oxide red, PbCO₃, PbO, and BaSO₄, Chinese Blue was not found.