IR-laser effects on pigments and paint layers

A. Schnell1, L. Goretzki1, Ch. Kaps1

1 Bauhaus-University, Chair 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 discoloration of pigments in paint layers. A test program

was worked out to analyse the chemical and physical background of typical "blackening

effects" for different pigments. The analytical methods ESEM, XRD, DTA, FTIR and

NMR were used to describe the reactions of historically relevant inorganic pigments and

organic binding media caused by laser irradiation at 1064 nm. The discoloration of

pigments and paint layers was documented by colour measurement. Further the

determination of discoloration thresholds of energy density of pigments and paint layers

was a main part of the research.

1. Introduction

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

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

cleaning of polychrome surfaces is currently not suitable without problems due to

the damage 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

[1 - 5]. With 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 causing damage to the contained

pigments and binders.

2. Experimental

methods

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 binders used for the

painting of sandstone samples were linseed oil, casein and gum arabic (resin of

the acacia tree) as organic binders and lime as the only inorganic binder.

The laser effects were tested on pure pigments (pellets of 10 and 20 mm

diameter; pressed under low pressure of 1.. 3 kN/cm²) as well as on paint layers

on sandstone samples. Within this study a Q-switched Nd-YAG laser at 1064 nm

(Quanta System Palladio, customary in trade) at different energy densities was

used for laser treatment of the samples. The beam delivery of the laser system 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 the energy density was

defined. The colour measurement was done by a spectrophotometer (results: CIE-

L*a*b* and specular reflectance). 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.

Table 1.

Analysis techniques and instruments

Analysis technique

Type of instrument

X-Ray Diffraction (XRD) analysis

Seifert XRD 3003 TT

Environmental Scanning Electron Microscope / Energy Philips XL30 ESEM-

Dispersive X-ray analysis (ESEM / EDX)

FEG

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

3. Results and discussion

The analysis of some laser treated inorganic pigments by Environmental Scanning

Electron Microscope showed, that the small pigment particles are partially molten

together at the surface of the pigment. These molten layers are thin, approximately

1 micron or less. The melting of pigment particles at the surface 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 on

the surface of the pigment. EDX analysis of the melted surface of some metal

oxide pigments is pointing at reduced oxygen contents. A phase change as reason

for discoloration was detected by XRD (by grazing incidence diffraction GID) for

the pigment cinnabar only.