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

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Nd-YAG laser irradiation of pigments and binders in paint

layers

Alexander Schnell, Lothar Goretzki, Christian Kaps

Bauhaus-University, Department of Building Chemistry, Coudraystr. 13c, 99423 Weimar,

Germany

1. Abstract

Laser cleaning of polychrome surfaces is currently problematic due to the fact, that laser

irradiation can cause damage of the paint layers. A test program was worked out to

analyse the chemical and physical background of these typical "pigment blackening

effects". The analytical methods ESEM, XRD, DTA, FTIR and NMR were used to

describe the reactions of inorganic pigments and organic binding media caused by laser

irradiation. The used laser system is commercially available, is currently used for the

cleaning of natural stone surfaces and works at a wavelength of 1064 nm. The

discoloration of pigments and paint layers was documented by colour measurement.

Further the measurement of discoloration thresholds of energy density (of pigments and

paint layers) was a main part of the research.

2. Introduction

The use of laser technique for the cleaning of natural stone surfaces has been established

for several years. Nd-YAG laser ablation is mainly based on thermal effects (infrared laser

light). The cleaning of polychrome surfaces is currently not suitable without problems due

to the damage and/or discoloration of many pigments in paint layers, that can be caused by

laser irradiation. In most cases the colour of these pigments is changing to black or grey.

Known reasons are phase changes and different decomposition reactions. At some

pigments these effects can be observed already at low energy densities. The objective of

the research project is the cleaning of polychrome natural stone surfaces by laser ablation

without any damage of the contained pigments and binders.

3. Experimental

Based on past investigations a test program was worked out to analyse the chemical and

physical background of typical reactions for different pigments. Primarily historically

relevant pigments and binders were chosen for the tests. Most of the 45 pigments were of

inorganic origin. The binding media used for the painting of sandstone samples were

linseed oil, casein and gum arabic as organic binders and lime as the only inorganic binder.

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

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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

Analysis technique

Type of instrument

X-Ray Diffraction (XRD) analysis

Seifert XRD 3003 TT

Environmental Scanning Electron Microscope and

Philips XL30 ESEM-FEG

Energy Dispersive X-ray analysis (ESEM / EDX)

Differential Thermal Analysis (DTA)

Setaram Setsys 16 / 18

Fourier Transform Infrared analysis (FTIR)

Biorad FTS 175 L

Nuclear Magnetic Resonance spectroscopy (NMR)

Bruker Avance 400

Colour measurement by spectrophotometer

Minolta CM 2600-d

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 (TiO2; 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 TiO2

(white)

Ti3O5

(blue, grey)

+ ½ O2) 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).