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| Compound | Selected spectral bands (cm -1 ) | Assignment |
|---|---|---|
| Cu 3 (CO 3 ) 2 (OH) 2 (Azurite) | 1493 | CO 3 2- asymmetric stretch |
| Silicate based blue pigments | 1035 | Si-O stretching |
| 2PbCO 3 ·Pb(OH) 2 (White lead) | 1399 | CO 3 2- asymmetric stretch |
| A natural ferruginous aluminum silicate red pigment (Bole) | 3697 | OH stretching |
| CaSO 4 ·2H 2 O (Gypsum) | 1109 | SO 4 2- asymmetric stretch |
The deep blue layer 3 corresponds to azurite and the light blue paint layer 2 to a mixture of silicate based blue pigments and white lead. Although beyond the ATR crystal’s spatial resolution limit of 20 µm, the absorption of bole was detected by the characteristic triple absorption bands of 3697, 3651, and 3619 cm -1 as seen in spectrum d of [link] . The white layer 0 was identified as gypsum.
To identify the binding material, the KBr embedded sample proved to be more effective than the polyester resin. This was due in part to the overwhelming IR absorbance of gypsum in the same spectral range (1700-1600 cm -1 ) as a characteristic stretch of the binding as well as some contaminant absorption due to the polyester embedding resin.
To spatially locate specific pigments and binding media, ATR mapping was performed on the area highlighted with a box in [link] . The false color images alongside each spectrum in [link] indicate the relative presence of the compound corresponding to each spectrum in the boxed area. ATR mapping was achieved by taking 108 spectra across the 220x160 µm area and selecting for each identified compound by its characteristic vibrational band.
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