Characteristics of the Raman spectra of archaeological Malachites

Abstract

Archaeological malachites, represented by the malachites found on the ancient Chinese and Vietnamese copper/bronze coins, may also incorporate those on other archaeological objects. The Raman spectra with Ar laser of these malachites differ slightly from those of the natural malachites found in mines. In this study, 120 measurements of the malachites on 40 coins identified 26 bands, while only around 18 of them are frequently observed. The wavenumbers (cm −1 ), shifts (±)and relative intensities (in parentheses) of the 18 common bands read, respectively: 153±4 (0‐vs), 179±7 (m‐vs), 217±8 (m‐vs), 274±7 (0‐vs), 355±5 (0‐m), 431±4 (0‐vs), 514±3 (0‐m), 533±5 (0‐s), 566±3 (0‐m), 599±2 (0‐m), 718±6 (0‐m), 754±2 (0‐m), 1061±7 (0‐m), 1093±10 (0‐m), 1365±9 (0‐m), 1491±7 (0‐vs), 3321±11 (0‐vs) and 3380±7 (0‐vs). In comparison with those of the 105 measurements on the natural malachites in five mines, the Raman spectra of the archaeological malachites tend to show less bands, higher backgrounds and greater shifts in the wavenumber position. The weakening or loss of bands is in the order of the OH stretch (3300 cm −1 ) (most severe), CO 3 (600–1500 cm −1 ) and CuO (<600 cm −1 ) (less severe) groups, indicating successive stages of corrosion. The malachites on the coins from three climate zones show their own characteristics. Several coins may have experienced two or more climatic or geologic episodes and show complex Raman spectra different from those of the natural malachites. Copyright © 2012 John Wiley & Sons, Ltd.