*3.2. Blue Pigments*

Two main blue pigments were identified in the miniatures, i.e., azurite and ultramarine blue, according to their characteristic absorption bands at 640 and 600 nm, respectively. Two different cases were recognised in the spectra from the blue areas of the *Book of Hours*, each associated to a different blue hue. An exemplary case is reported in Figure 2a: the Virgin's mantle on f. 1r shows bright and dark blue hues employed to paint the drapery.

**Figure 2.** (**a**) Micro-photography (80×) of the mantle of the Virgin on folio 1r. The presence of different blue shades is made more evident by the white border line. (**b**) Selected FORS spectra representative of the different blue shades. The blue line represents spectra collected from darker blue areas within the miniatures and points to the presence of ultramarine, the red line represents spectra collected from lighter blue areas within the miniatures and of the blue areas in the decorations on page borders and capital letters and points to the presence of azurite.

The two different hues revealed by the FORS spectra can be related to the use of different pigments: the spectra from the brighter areas show a broad absorption band centred at about 635–640 nm, thus pointing to the presence of azurite, whereas the band centred at about 610–615 nm in the darker areas suggests the use of ultramarine blue (Figure 2b).

The presence of both pigments, and specifically of ultramarine blue applied over the azurite layer for darkening the colour in the miniatures, was confirmed by micro-Raman by focusing the analysis on grains of the two pigments. This was also was the case for the blue skies, for which the FORS spectra suggested the presence of azurite in the brighter areas and the use of ultramarine blue to darken the colour. The bathochromic shift of the absorption band for ultramarine blue (610–615 nm in place of the expected 600 nm) can be justified by considering the application of this pigment as a thin layer on top of the underlying azurite layer and therefore the final spectrum contained contributions from both pigments. This feature was verified with mock-ups prepared in the laboratory.

The blue decorations on the page borders instead gave FORS spectra that could be attributed to the presence of azurite alone, both in the lighter and darker areas: here the brightness of the colour was adjusted by mixing azurite with a white pigment.

All the XRF spectra collected from the dark and bright areas in the miniatures and from the blue decorations on the page borders showed intense copper signals, as well as signals of barium and zinc; these latter elements were indicated in the literature as possible impurities of natural azurite [11,21], since their presence can be related to the geo-chemical processes of formation of the deposits. A fairly linear relationship between the counts of copper and barium and between those of copper and zinc emerged (Figure 3) and seems to confirm the use of natural azurite throughout the manuscript. More evidence of the natural origin of azurite, as suggested by Aru et al. [22], could be the presence of iron oxides: we did not identify such accessory phases, but the counts of iron in the XRF spectra of the blue areas were on average 4 times higher than in the spectra of parchment.

**Figure 3.** Bivariate plot for XRF signals of copper (Kα line, 8.05 keV) vs. barium (red dots, Lα line, 4.47 keV) and zinc (blue dots, Kα line, 8.64 keV) recorded from blue areas in the miniatures. Regression lines with coefficients R are indicated.

The results obtained by XRF on those areas where ultramarine blue was present deserve some discussion. Portable XRF instruments, that do not operate under vacuum, are generally not able to detect weak XRF signals from light elements such as sodium, aluminium and silicon, which are the main constituents of lazurite—Na3Ca(Al3Si3O12)S i.e., the blue mineral that gives the pigment its rich colour. The intensity of the calcium signals from the dark blue areas did not significantly differ from those obtained from the parchment, where this element is present due to the alkaline treatments with calcium hydroxide that occurred in the production procedures. Signals from sulphur, which is present in the structure of ultramarine blue, were detected from the dark blue areas, but this element cannot be considered as a selective marker for this pigment because it can be present in parchment itself; moreover, it is difficult to separate the S Kα line at 2.31 keV from the Pb Mα line at 2.34 keV and lead is usually present as lead white to tune the blue colour. Nevertheless, the XRF spectra recorded in dark blue areas, in which FORS and spectra Raman indicated the presence of ultramarine in the superficial layer of the miniature, differed from those recorded from the bright areas of pure azurite because of the systematic presence of high signals of potassium. No further indication could be derived from the other analytical techniques employed; therefore, possible explanations for the presence of this element in the ultramarine layers could be related to the phenomenon of vicariance between the sodium ions present in lazurite and the potassium ions present in the environment of formation of the rock (similar to that occurring in pyroxenes between Mg2+ and Mg-vicariant ions [23]), or to the processes adopted for the purification of the pigment (that employed potash according to Cennini [4]) or even to the possible addition of extenders to the precious pigment. Ultimately, the presence of potassium can be considered a potential elemental marker for ultramarine blue in cases where Al, Si and S cannot be clearly detected.

### *3.3. Brown Pigments*

The brown colour was mainly used to depict architectural features, such as the inside of the huts on ff. 24r and 31v (Figure 1c,e). In all cases, the pigments used were iron oxides or iron-rich red earths, according to the features of the FORS spectra and to the dominant presence of iron evidenced by XRF. It was not possible to obtain a better identification by means of Raman spectroscopy. Some of these pigments, however, were probably related to later retouches (see Section 3.11 for further details).
