Nd-YAG laser irradiation of pigments and binders in paint layers

EUROMAT 2003, Symposium P2 - Materials and Conservation of Cultural Heritage, EPFL-Lausanne 2

The laser effects were tested on pure pigments (pellets of 10 and 20 mm diameter; pressed under a low pressure of 1.. 3 kN/cm²) and on paint layers on sandstone samples. Within this study a Q-switched Nd-YAG laser at different energy densities (laser fluence) was used for laser irradiation of the samples. The laser system “Palladio” (Quanta System) works at a wavelength of 1064 nm with a variable pulse repetition rate from 1 to 15 Hz. The beam delivery is realised by a 7 mirror system integrated in an articulated arm. The diameter of the elliptical laser spot is approximately 7 mm.

For each sample the typical discoloration threshold of laser fluence was defined. The colour measurement was done by a MINOLTA spectrophotometer (results: CIE-L*a*b* values and specular reflectance spectra). Various analysis techniques were further used to describe the reactions of pigments and organic binding media caused by laser irradiation. A general view of the applied techniques and instruments is given in table 1.

Tab. 1: Analysis techniques and instruments

4. Results and discussion

4.1. Effects of laser irradiation on pigments

The analysis of laser treated inorganic pigments by Environmental Scanning Electron Microscope showed, that the small pigment particles are partially melted together. These melted surface layers are thin, usually 1 micron or less. The melting of pigment particles at the surface (micro molten areas) could be observed on almost all tested pigments including pigments with high melting points. Examples are given in Figure 1 and 2 with the pigments titanium white (TiO 2 ; melting point at 1855 °C) and zinc white (ZnO; melting point at 1975 °C). The colour of zinc white was changing to brown / grey, titanium white turned to a blueish grey. The discoloration can be caused by the chemical decomposition of these metal oxides (example: 3 TiO 2 (white) → Ti 3 O 5 (blue, grey) + ½ O 2 ) or by physical effects

(increasing particle size caused by the melting). The precise analysis of the discoloured material is difficult due to the thin modified layers. EDX analysis of the melted surface of some metal oxide pigments is pointing at reduced oxygen contents. Only in case of the pigment cinnabar a phase change as reason for discoloration was detected by XRD (by grazing incidence diffraction GID).

EUROMAT 2003, Symposium P2 - Materials and Conservation of Cultural Heritage, EPFL-Lausanne 3

Fig. 1 and 2. Surface of pigment pellets after laser irradiation at 600 mJ/cm² (left: titanium white at 10000x; right: zinc white at 16000x)

4.2. Effects of laser irradiation on binding media in paint layers

The gas phase developed during laser irradiation of different paint layers was analysed by NMR spectroscopy. For this tests, linseed oil paint layers containing pigments with low discoloration thresholds have been laser irradiated at high energy densities (~ 600 mJ/cm²) in an enclosed system. The reaction products were collected in a cooling trap, which was mounted to the system. Deuterized chloroform was used as solvent for H-NMR sample preparation. The proton spectra in Figure 3 shows, that glycerin, alkenes, alkanes and some aldehydes were detected in the gas phase. The signal at 9,6 ppm is evidently originated from acrolein. This is confirmed by a crosspeak pattern in TOCSY spectra (total correlation spectroscopy). Further many low-molecular reaction products are emitted, but have not been clearly detected yet. The presence of acrolein and other aldehydes points on a incomplete “combustion” of the volatile compounds.

Fig. 3. H-NMR proton spectra of solved gas phase emitted from paint layer containing linseed oil as binding media