In the field of diagnostics of cultural heritage, the lead compound plattnerite [β-PbO2
, lead (IV) oxide] is known as a secondary product of lead-based pigments, white lead [2PbCO3
] and red lead (Pb3
), widely employed since medieval times for the realisation of artworks. The tendency towards blackening when the lead pigments are used on wall paintings, with a negative effect on the correct appreciation of the artwork, has been largely studied [1
]. Indeed, several researchers have shown that its formation on mural paintings is closely related to the formation of other lead compounds, such as anglesite (PbSO4
), scrutinyite (α-PbO2
), cerussite (PbCO3
) and lead-magnesium carbonate [PbMg(CO3
]. This phenomenon is linked to different intrinsic and extrinsic factors related to each other, such as the manufacturing process of the pigment [8
] and especially the environmental conditions in which the wall paintings are preserved. The formation of plattnerite has been documented under certain conditions of humidity, light and temperature [9
], and in the presence of an alkaline environment, microorganisms [10
], natural inorganic salts and synthetic salts [6
], acidic pollutants (SOx
, but also CO2
] or chlorine compounds [2
Since it has been mainly studied as a degradation product, plattnerite has received less attention than common lead pigments, and only in one case has it been reported as pigment [14
]. Although plattnerite can be detected even with portable Raman instrumentation [15
], its identification can be difficult compared to common lead pigments, since it presents a very weak Raman scattering with a poorly defined broad main band. Besides, as plattnerite is black, it absorbs most of both the incident and scattered radiation. In addition, it is also highly unstable under the laser due to the heating effect. It has been shown that if the laser power is not kept sufficiently below some threshold, depending on the laser wavelength and the morphology at the analysed point, degradation phenomena may be triggered in plattnerite samples during Raman analyses, with the consequent formation of secondary products [16
]. A colour change in the point of analysis from black to red/orange has also been reported [16
]. For this reason, in some cases incorrect assignments have been made [17
] due to the sensitivity of the plattnerite during Raman analysis, demonstrating the need to pay attention during the analysis of lead compounds [18
]. In the study of De Santis et al. [19
], the presence of plattnerite, coming from white lead, was indirectly demonstrated through its laser degradation to massicot.
In recent years, many studies have been carried out on different metal oxides in order to investigate their behaviour under laser irradiation in different fields of research, ranging from microelectronics to medicine. The investigation of Vila et al. [20
] showed the α-Bi2
phase transformation by laser irradiation in ceramic samples and single crystal nanowires of this oxide, which is of interest to electronic and optoelectronic applications. In this work, the threshold power densities necessary to induce the chemical transformation were determined by micro-Raman spectroscopy.
Another study reported by Camacho-López et al. [21
] showed for the first time the transition threshold for the phase transformation from m
, by means of in-situ micro Raman spectroscopy. On the other hand, a study of the anatase (β-TiO2
) to rutile (α-TiO2
) transition induced by laser irradiation of TiO2
nanoparticles was carried out by Cristian Vásquez et al. [22
]. The kinetics of this process were investigated as a function of the dopant (Al or Fe) on the TiO2
nanoparticles, its concentration, and the irradiation conditions.
One of the most complete studies on the behaviour of the plattnerite under the laser effect of a Raman spectrometer was conducted by Burgio et al. [16
], in which they demonstrated the sensitivity of β-PbO2
when it is irradiated with varying laser power and with different wavelengths. During the Raman analyses, the degradation of plattnerite was demonstrated with the formation of its secondary products, red lead (Pb3
), litharge (α-PbO) and massicot (β-PbO). However, keeping the laser power low enough, it is possible to obtain from the Raman spectrum of β-PbO2
that it is characterised by three bands at 653, 515 and 424 cm−1
, identified as deriving from the B2g
modes, by analogy with the corresponding isostructural SnO2
modes located at 776, 634 and 475 cm−1
. Recently, in one of our works the presence of another band at 159 cm−1
was proposed as belonging to β-PbO2
, since it has been identified both in commercial and in mineral plattnerite using Raman lasers at different excitation wavelengths (632 and 532 nm) [23
According to Burgio et al. [16
], the degradation of β-PbO2
, caused by the effect of the laser power during Raman analyses, occurs following the path: β-PbO2
→ α-PbO → β-PbO. In addition, another lead oxide (PbO1.55
) was identified during the experiments whose spectrum differs from that of the other secondary products (red lead, litharge and massicot) of plattnerite. Previous thermogravimetric studies attempted to characterise compounds of intermediate stoichiometry during the decomposition process from β-PbO2
, defining the temperature at which the phase change occurs as >375 °C. [24
]. However, the results of this research were not considered totally satisfactory due to the influence of atmospheric agents, the heating rate of the sample, and the quenching procedure adopted at any particular temperature to obtain an intermediate phase at room temperature [16
]. The temperature of the phase transition from β-PbO2
was confirmed by Greenwod et al. [25
], according to whom lead dioxide decomposes upon heating in air as follows: β-PbO2
→ PbO, where the first step occurs at 290 °C, the second at 350 °C, the third at 375 °C and the last one at 600 °C.
On the other hand, some studies show that the chromatic variation of pigments due to the laser effect on easel and wall paintings can be opportunely used for laser cleaning operations, although the presence of a binder and the mixtures of pigments may affect the result [26
]. Concerning the chromatic reconversion of lead-based pigments, preliminary studies on both pure plattnerite powder samples and darkened red lead paint samples were proposed for the restoration of darkened red lead-containing paintings by a laser-induced photo-thermal reduction [30
In this work, analyses varying the laser power were performed on a sample from a late medieval pictorial cycle preserved in the church of Santo Stefano of Montani (Bolzano, Italy), which consists of a mixture of plattnerite and scrutynite, present as a degradation product coming from red lead. During Raman analyses, the formation of degradation products coming from β-PbO2
, generated by laser power, was noted to be similar to what is reported in the literature about the behaviour of the commercial product. Therefore, to better understand the behaviour of plattnerite and of lead-based pigments in general under laser excitation, analyses were performed by Raman spectroscopy, varying the incident light power on powders of commercial plattnerite and lead oxides (red lead, massicot and litharge), and these were compared with those from the wall paintings samples. Subsequently, commercial plattnerite was analysed in the form of pellets using a temperature-controlled stage (Linkam THMS600-Renishaw, Linkam Scientific Instruments Ltd., Waterfield, UK), with the aim of determining the temperature range at which the phase transition of plattnerite occurs. The temperature-controlled stage was used in order to show the gradual degradation of plattnerite and the formation of the secondary products during the Raman analyses. This methodology has been applied in other investigations for the study of materials belonging to cultural heritage [31
], and has been used on lead compounds [33
] in which stability with temperature was required during Raman analyses.
The Raman spectrum of plattnerite can be collected by maintaining very low laser power, in order not to cause any thermally induced degradation phenomena that could cause a wrong interpretation of the results, especially when they are present as a degradation product over wall paintings. Therefore, in the presence of plattnerite, some precautions regarding the instrumental parameters to be used, in particular the laser power, should be taken if Raman spectroscopy is the chosen diagnostic analytical tool.
Raman analyses, performed with increasing laser power on historical painting samples of plattnerite and on commercial plattnerite samples, emphasised the photo/thermal instability of β-PbO2. However, the Raman signal obtained could be recognised from those of other lead-based compounds such as red lead, massicot and litharge, which arise from plattnerite degradation.
The same sensitivity was found for red lead, massicot and litharge, formed as laser-induced degradation products from plattnerite. In particular, test varying the laser power showed a shift of the main bands of about 20–25 cm−1, depending on the temperature. Such behaviour was not found for commercial samples of lead oxide (massicot, litharge and red lead), which showed a good stability with increasing temperature (laser power) when they were analysed with both 633 nm and 785 nm. For them, in fact, there was evidence of neither a widening of the band nor a shift towards lower wavelengths. At maximum laser power, the phase transition was seen only for the red lead. These characteristics could allow us to recognise their presence as secondary products, generated by the decomposition of plattnerite, or as pigments voluntarily used when they are found on wall paintings.
Experiments carried out using a temperature-controlled stage showed the progressive degradation of β-PbO2, confirming that it followed the path of Pb3O4, α-PbO and β-PbO. However, the degradation process was reversible, since the only contribution of the laser beam, even if used at maximum laser power, was not enough to obtain a stable form of massicot. Indeed, the intermediate products of the decomposition of plattnerite were evident when the sample was analysed again at low laser power after cooling.
New phases of lead oxides were formed during the Raman analyses of commercial plattnerite, and they were identified through XRD analyses. They were generated by the effect of the laser power due to the high thermal sensitivity of lead dioxide compounds during Raman analysis.
Only thanks to the use of the temperature-controlled stage was it possible to recognise the gradual degradation of the plattnerite. It allowed us to see the Raman spectra belonging to the different phase transitions, which it had not been possible obtain by same point analysis via varying the laser power.
These findings indicate that, in the presence of lead compounds, it is very important to use a low laser power during the Raman measurements, in order not to induce structural modification and to obtain an exact characterisation of the materials, especially when they are found on mural paintings’ surfaces.
However, the difference in behaviour found between pigments from historical painting samples and commercial pigments under the effect of the laser power could be determined by the type of painting technique used for the realization of the artwork under study; in our case, the painting was made with the fresco technique. Thus, it would be interesting to extend the study to other type of paintings in order to verify whether the behaviour of plattnerite found in wall paintings could also be influenced by the presence of other minerals present in the surface and in the matrix. In particular, it could be interesting to test the behaviour of lead oxide and its degradation products in tempera painting, to understand the influence of the binder when the substances are subjected to a degradation process induced by laser power during Raman analysis.