Surface, samples and cross sections observation
The observations of the cover surface and the cross sections with the digital microscopy under white and UV lights highlighted the decoration strata succession.
Leather was applied on the pasteboard with a thick adhesive and then gilded with gold leaf. It is possible to detect the gold leaf thin regular stratum in all the cross sections and no other layers are visible between the leather and the leaf (Fig. 3).
The above decoration presents floral patterns: firstly, a black ink sign marked out the shapes of the Islamic elements and flowers; secondly, red and green colours filled the grounds and tendrils. Lastly, a thick greenish paste was applied to point out the decoration with reliefs: flower petals, tendrils and the extended central areas of the composition were enhanced with this stratum that, finally, was make precious with golden flowers and black lines (Figs. 4, 5).
The observation of the verso of the sample 5_1869_BNM was useful to understand the greenish paste morphology: it seems to be a combination of fillers—for instance gypsum—and yellow and blue pigments, surely mixed with a binder. Pigments particles are clearly visible through digital microscopy (Fig. 6).
The protecting varnish coat is characterised by a strong, yellow colour and its yellowish fluorescence is visible through UV light suggesting the presence of natural resins such as dammar, mastic, sandarac or colophony [21]. This layer covers the volume external surface as last stratum: it was applied overall the decoration using a brush.
ATR-FTIR spectroscopy
Due to the non-selectivity of IR spectroscopy, the ATR-FTIR signals might arise from molecular groups located everywhere in the sample, even at 2–3 microns below the surface, due to the depth penetration associated to the ATR sampling mode with diamond crystal [22]. Furthermore, peaks of different substances located in the same spectrum area might be overlapped and hidden. As a result, the totality of samples presented similar peaks due to the common presence of the leather ground and of the varnish coating.
Traces of oils and/or resins were found in each sample, due either to the varnish presence, to the colours binders or to the gilding adhesive; therefore, the spectra present sharp broad peaks in the region between 3300 and 2800 cm−1 (C-H stretching vibrations) and in the 1700 cm−1 (C=O stretching vibrations) areas [16,17,18]. These peaks were compatible with those of linseed oil, sandarac, colophony and shellac resin, but the identification must not be considered as definitive for the previously mentioned possible overlapping of the peaks.
Two protein peaks are constantly present around 1642 and 1540 cm−1 (amide I and amide II vibrations, respectively): they mainly refer to the leather [23]. The sample 3_1869_BNM (from the gilded leather; binding, left board) could be considered as an example of the leather and varnish peaks (Fig. 7a).
The sample 5_1869_BNM was the most interesting because of the composition of the thick greenish paste. The recto (Fig. 7b) and the verso (Fig. 7c) showed similar peaks except for the presence of oils and resins in the recto, resulting especially at 2918 and 2851 cm−1. The verso peaks are strikingly delineated. Protein peaks are at 1636 and 1542 cm−1. Other peaks could be observed in the spectrum at 1022 and 779 cm−1: they likely refer to the blue pigment as made of smalt. Finally, the sample’s peaks at 1395 and 679 cm−1 probably refers to a lead-derived pigment, perhaps the lead oxide massicot yellow (PbO) which main peaks are at 1390 and 681 cm−1. The 1395 cm−1 peak could also refer to the calcium carbonate (chalk) presence: despite the absence of the typical calcium carbonate peak (875 cm−1) on the verso, this value seems to be clearly distinguishable on the recto at 879 cm−1 [16,17,18].
The samples 4_1869_BNM (from the red coloured background; binding, left board) and 6_1869_BNM (from the green external frame of the painted decoration; binding, left board) were studied for the red and the green colours compositions, but the weak results suggested to carry out different analysis, as the spot tests.
Raman spectroscopy
The high fluorescence of organic materials when dealing with Raman analysis made the sample spectra useless. In spite of this, the sample 5_1869_BNM gave the only Raman spectrum of the entire research.
The verso of the greenish paste produced the Raman spectrum (Fig. 8) of a lead-based yellow pigment, named massicot yellow, which the band at 147, 289 and 385 cm−1 can be attributed to. The small band at 340 cm−1 could be probably assigned to litharge which has the same composition as massicot (PbO), but a different lattice structure: massicot is orthorhombic, while litharge is tetragonal. The two pigments are often found together since they interconvert into each other due to the laser induced heating [24]. A less probable attribution of this last peak could be to pararealgar, a light induced polymorph of realgar, which was a widely used pigment by Venetian Renaissance artists [25].
XRF spectroscopy
Aiming to determine the heterogeneous composition of the greenish paste, the sample 5_1869_BNM was analysed by XRF spectroscopy both in the recto and in the verso, with different settings of the X-ray tube. The spectra showed a large quantity of chemical elements, which intensity values in some cases change in the recto and the verso (Fig. 9).
Firstly, the using of blue smalt is clearly depicted by the presence of potassium, cobalt, nickel, arsenic, bismuth and silicon (and possibly iron) [26].
Secondly, regarding the characterisation of the yellow pigment, the large variety of lead–tin–antimony yellows used in the artworks during the 16th century makes the identification difficult [27].
Nevertheless, the high content of lead seems to confirm the use of a lead-based yellow, perhaps the massicot lead oxide (PbO) previously resulted by the Raman spectroscopy.
Owing to the absence of traces of tin in the results, lead–tin yellow type I (Pb2SnO4) and lead–tin yellow type II (PbSnO3 or PbSn1-xSixO3) could be apparently excluded from the possible used pigments. Moreover, the absence of antimony in the spectra may exclude the use of the lead antimonite, also called Naples yellow (Pb2Sb2O7) [28].
On the other hand, the presence of elements as arsenic and zinc could suggest either the use of different typologies of yellow pigments or the addition of such materials due to artistic techniques or practices. The arsenic, which may refer to the smalt composition only, is known for its arsenic sulphides pigments (invisible to the IR spectroscopy), as the orpiment and the pararealgar.
Unfortunately, as the molybdenum of the X-ray tube anode (2.9 keV) hides the typical fluorescence emission values of the sulphur (2.3 keV), it is not possible to verify its presence in the sample. Moreover, zinc-based substances as zinc sulphate (white vitriol) or zinc oxide were frequently added to pigments as driers or preservatives [29, 30].
Another element found in the sample is calcium, surely related to the calcium-based filler either in form of gypsum (calcium sulphate) or in chalk (calcium carbonate).
The presence of pure gold is due to the pictorial decoration of the paste surface, made with shell gold applied by brush: the difference between its recto/verso measurement values confirms its presence in the recto only.
Iron and copper are also visible in the spectra, probably referring to the composition of an iron-gallic ink: they were produced by mixing tannins (vegetal extracts obtained by gall-nuts) with inorganic salts called vitriols, consisting of iron sulphates with other metal sulphates such as manganese, copper, zinc, lead and others [31].
Spot tests
Pigments: red and green colours and black ink
The sample 4_BNM_1869 was investigated to detect the nature of the red colour [19]. Initially, the procedure aimed to discover the presence of iron(III) ion-derived pigments, like red ochre; however, results were negative due to the absence of the final blue precipitate, the Prussian blue. The research on sample 4_BNM_1869 followed as a second hypothesis the presence of lead(II) ion-derived pigments, like red lead (or minium). Consequently, the spot test sensibility to lead ion was positive, producing a final yellow lead iodide precipitate.
The green substance on sample 6_1869_BNM was supposed to be a copper-based pigment [19]. Spot test analysis followed the research methodology for copper(II) ion-derived substance, but the final colour of the precipitating solution was not red-brown as it should be. Nevertheless, the solution responded to the test with the formation a blue precipitate, the Prussian blue, thus indicating that the green colour contains an iron-derived pigment.
Sulphates
By ATR-FTIR and XRF analyses, sample 5_1869_BNM (Fig. 6) was thought to be composed by a mixture of gypsum, chalk, animal glue and pigments, with the final effect of a relief greenish paste.
The presence of sulphates was investigated by spot tests: after the procedure, the final presence of a white precipitate in the solution and its turbidity indicated the presence of sulphates, probably related to gypsum. Finally, it was detected by the Quantofix® Sulfate test strips. The presence of sulphates could also depends on the use of pigments such as arsenic sulphides pigments, zinc sulphate (white vitriol) and lithopone.