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Such non-invasive analyses provide previously unthinkable data in the scientific study of rare and precious manuscripts that cannot be sampled for obvious reasons. However, it is also important to stress that such techniques also have some inherent limitations. For example, the employed techniques can reveal the chemical composition in a certain color area but not their absolute amount; moreover, when dealing with organic colorants, it is easier to characterize blue and red dyes than yellow and orange ones.

The following spectroscopic methods and imaging techniques were used in situ in a complementary fashion: X-ray fluorescence (XRF), reflection mid-FTIR, UV-Vis reflection and emission, micro-Raman, digital microscopy, and digital NIR imaging. Experimental details can be found in a previous publication (Domenici et al. 2017).

Results

White areas (paper)

The mid-FTIR spectra collected on the white ground show the signals related to the cellulose paper, as well as small and variable amounts of Ca, Fe, S, and K detected by XRF. The absence of any significant signal confirms what was readily detactable by naked-eye observation: most of the white areas of the images were simply left unpainted. Unfortunately, none of the rare areas where a white painted color was employed were measured. Nevertheless, the fact that calcium carbonate was detected as a component of a grey area (see below) strongly suggest that the white color of the Codex Mendoza painters’ palette was indeed calcium carbonate (called tizatl in colonial Nahuatl texts) (Dupey García 2016), as suggested by Gómez Tejada (2012).