3.1.4. Blue

The analysis of hyperspectral data suggests the use of Prussian blue in plate 9. The characteristic spectral curve in blue regions (Figure 9), in particular the local minimum around 675 nm and the inflection point around 554 nm, are consistent with characteristic spectra for Prussian blue published in other identifications of Prussian blue [43–45]. Prussian blue did not become commercially available until 1724, seven years after Merian's death, confirming that the coloring could not have been applied during Merian's lifetime [29].

Although Prussian blue is rich in iron, an element that can be detected with XRF, it has been observed that in practice Prussian blue requires relatively little pigment to produce strong coloration and thus it can be challenging to detect this pigment with XRF [25,46]. In our case we observe high pixel intensities in MA-XRF maps for iron in regions corresponding to darker shades of blue in the butterfly at the upper right of plate 9 (Figure 9). All blue regions of the butterfly's blue wings in hyperspectral images of plate 9 demonstrate the characteristic reflectance spectrum of Prussian blue, which together with the iron map allows us to confirm the identification of Prussian blue.

**Figure 9.** Details from MA-XRF map for iron (**top left**), blue endmember map (**bottom left**) and characteristic spectra (**right**) used to identify Prussian blue in plate 9.

#### 3.1.5. Green and Yellow

We were unable to propose the presence of specific colorants in the case of a number of likely-organic green and yellow pigments. The hyperspectral data treatment was able to extract characteristic spectra for green regions, but these curves did not correspond to the curves for inorganic pigments [47]. XRF data in these regions did not register elements typically associated with green pigments (such as copper) or inorganic yellows (such as iron). This lack of association along with the identification of barium, sulphur, zinc, and, in some green regions, iron with varying pixel brightness throughout suggests the use of unidentified organic pigments or mixtures. Sampling and invasive analysis would be necessary to further specify these pigments.

#### 3.1.6. Comparisons between Prints

The recurrence of several pigments (lithopone, barium-rich and cadmium reds, and brown umber) identified on the basis of XRF data and, in the case of cadmium red and Prussian blue, hyperspectral imaging suggest that there is a common palette between the three prints. This is supported by the close correspondence in hyperspectral data and characteristic spectra between prints. Characteristic spectra for red regions of plate 54 and plate 55, which conform to published spectra for cadmium red (Figure 6), support the identification of the same pigment in both prints [30,32]. Although we are unable to precisely identify which pigment they correspond to, characteristic spectra for the green regions in each print are nearly identical between the three plates (Figure 6), suggesting the repeated use of one green pigment between all three. The use of the same pigment between prints suggests that they were colored together by the same hand or with a systematic application of color between the three.

#### *3.2. Printed Lines*

Normal maps and comparison with other printed editions of *Metamorphosis* were used to determine whether lines that appeared to be printed were in fact the product of an eighteenth century intaglio printing process such as copperplate engraving, or if they were outright forgeries or reproductions.
