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.
Table of Contents
1. Abstract
2. Introduction
3. Experimental
4. Results and discussion
4.1. Effects of laser irradiation on pigments
4.2. Effects of laser irradiation on binding media in paint layers
4.2. Discoloration thresholds of laser fluence
4. Conclusions
Research Objectives and Themes
The primary objective of this research is to evaluate the feasibility of using Nd-YAG laser ablation for the cleaning of polychrome natural stone surfaces without causing damage or discoloration to the pigments and binding media contained within the paint layers.
- Analysis of chemical and physical damage mechanisms in pigments (blackening effects).
- Evaluation of laser-induced decomposition of organic binding media.
- Documentation of discoloration thresholds related to laser energy density.
- Development of testing protocols using analytical methods like ESEM, XRD, DTA, FTIR, and NMR.
- Optimizing laser parameters to enable safe cleaning of sensitive artworks.
Excerpt from the Publication
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).
Summary of Chapters
1. Abstract: Provides an overview of the test program designed to analyze the chemical and physical background of pigment blackening effects caused by laser irradiation.
2. Introduction: Outlines the state of laser cleaning for natural stone and states the core problem of pigment damage, establishing the research goal.
3. Experimental: Details the materials selected for testing, including 45 inorganic pigments and various binders like linseed oil, casein, and gum arabic.
4. Results and discussion: Investigates the effects of laser irradiation on pigment morphology, binder decomposition, and establishes discoloration energy thresholds.
4. Conclusions: Summarizes that while Nd-YAG laser cleaning is suitable for polychrome surfaces, it requires strict limitation of energy density based on specific pigment characteristics.
Keywords
Nd-YAG laser, laser ablation, polychrome surfaces, pigment blackening, binding media, laser fluence, discoloration threshold, paint layers, chemical decomposition, sandstone, ESEM, NMR, FTIR, conservation, cultural heritage
Frequently Asked Questions
What is the primary focus of this research?
The research focuses on the risks associated with cleaning polychrome surfaces using Nd-YAG lasers, specifically targeting the damage, discoloration, and chemical degradation of pigments and binding media.
What are the central thematic areas?
The study covers material science in art conservation, laser-matter interaction, analytical chemistry (spectroscopy and microscopy), and the determination of safety thresholds for laser cleaning applications.
What is the main research goal?
The main goal is to identify methods for cleaning polychrome natural stone surfaces using laser ablation without causing irreversible damage or discoloration to the painting materials.
Which scientific methods are employed?
The authors use a variety of analytical techniques, including Environmental Scanning Electron Microscopy (ESEM), X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA), Fourier Transform Infrared analysis (FTIR), and Nuclear Magnetic Resonance (NMR) spectroscopy.
What is covered in the main body of the work?
The work details the experimental setup, presents results regarding pigment melting and binder decomposition, and provides a statistical evaluation of discoloration thresholds for various pigments and binders.
Which keywords best describe this study?
Key terms include Nd-YAG laser, laser ablation, pigment blackening, conservation, cultural heritage, and laser fluence.
Why does zinc white change color under laser irradiation?
The study observes that zinc white changes to brown/grey due to physical melting effects and chemical decomposition processes occurring during exposure to laser energy.
What does the NMR analysis reveal about binding media?
NMR analysis reveals that linseed oil undergoes partial decomposition, emitting gases such as acrolein, which indicates an incomplete combustion process of volatile organic compounds.
How are energy density thresholds categorized?
The research categorizes surfaces into groups, defining "easy to clean" as those with a threshold above 200 mJ/cm², while thresholds below 100 mJ/cm² are considered critical for safe cleaning.
Is laser cleaning universally applicable to polychrome surfaces?
No, the study concludes that some pigments possess very low discoloration thresholds, making it impossible to clean surfaces containing these specific materials using laser ablation safely.
- Quote paper
- Dipl.-Ing. Alexander Schnell (Author), Lothar Goretzki (Author), Christian Kaps (Author), 2003, Nd-YAG laser irradiation of pigments and binders in paint layers, Munich, GRIN Verlag, https://www.grin.com/document/93398
