Therefore, powdered commercial bone black pigments were analyzed by infrared spectroscopy in transmission and reflection mode.
The infrared spectrum profiles recorded from two blue areas (M_01 and M_08) and from a brown area of the musician's trousers (Figure 3) highlight the diagnostic signal of the bone black at the 2013 [cm.sup.-1] in both investigated colors.
However, it is important to emphasize that the bone black is still visible even in the presence of barium sulphate (Figure 4) used by the artist as white pigment.
In Figure 5, the infrared spectrum collected in the apse on a blue hue clearly display the characteristic CN stretching at 2094 [cm.sup.-1] of the Prussian blue [18] and the diagnostic signal of the bone black at the 2013 [cm.sup.-1] band.
In Figure 7, the bone black pigment and the hydroxyapatite transmission infrared spectra are compared.
The hydroxyapatite component is clearly underlined in the infrared spectra of bone black pigment commercial sample showing the characteristic phosphate group bands: the [v.sub.3] [(P[O.sub.4]).sub.3-] at 1087 and 1038 [cm.sup.-1], the ]1 [(P[O.sub.4]).sup.3-] at 875 and 962 [cm.sup.-1], the [v.sub.4] [(P[O.sub.4]).sup.3-] at 630, 604, and 567 [cm.sup.-1], and the [v.sub.2] [(P[O.sub.4]).sup.3-] at 469 [cm.sup.-1] (Figure 7(b)).