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Although frequently negligible, an increase of potassium related to the paper support was detected. This increase may possibly be due to the thinness of the painting layer and, tentatively, can be associated with the practice of making lake pigments by adding potash alum (AlK(SO4)2∙12H2O); these are well-documented in European pigment recipes (Kirby, Spring, and Higgitt 2005) and also reported in pre-Columbian historical references (Dupey García 2015, 153; 2016). As previously noted, this cochineal-based painting material seems to have been quite liquid, since it bleeds through the paper sheet more than any other color in the manuscript.

Yellows, oranges, and browns

Two different types of yellow compounds have been found in the Codex Mendoza. The first is arsenic-based, as indicated by XRF performed on some yellow areas (folio 2r: central shield and Tenoch’s icpalli and folio 37r: headdress of the left warrior, gourd, and mat box). The compound is most probably orpiment (As2S3), as identified by Raman spectroscopy in other codices (Buti et al. 2018; Domenici et al. 2017; Brunetti et al. 2016) The second yellow (foli 63r: astronomer’s seat and woman’s face in third register and folio 64r: costume of the central lower warrior) is an organic dye; we concluded it was organic in these areas because XRF analysis did not reveal any key element characteristic of inorganic pigments. Neither XRF nor FTIR provided indication of an inorganic support for the dye: XRF analyses showed a negligible amount of potassium (although this could be due to the low layer thickness) and FTIR showed the absence of any infrared signal of clay, although the hindering effect of the cellulosic support cannot be overlooked. These observations suggest that the organic color was not used in these areas as a lake pigment or as a hybrid pigment (Buti 2012; Domenici et al. 2017).