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.
Table of Contents
1. Introduction
2. Experimental methods
3. Results and discussion
4. Conclusions
Research Objectives and Topics
The primary objective of this research is to evaluate the feasibility of cleaning polychrome natural stone surfaces using Nd-YAG laser ablation without inducing damage to the pigments and organic binders. The study addresses the chemical and physical changes, such as discoloration and decomposition, that occur in pigments and binders under laser irradiation at 1064 nm.
- Chemical and physical analysis of pigment discoloration mechanisms
- Investigation of thermal effects and melting on pigment surfaces
- Identification of decomposition products from organic binders via NMR spectroscopy
- Determination of energy density thresholds for safe laser cleaning
Excerpt from the Publication
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.
Summary of Chapters
1. Introduction: This chapter outlines the problem of pigment discoloration during laser cleaning of natural stone and establishes the research goal of developing a safe cleaning method.
2. Experimental methods: The section details the test program, including the variety of pigments, binders, and analytical techniques like ESEM, XRD, and NMR used to evaluate laser effects.
3. Results and discussion: This part presents the experimental findings regarding pigment melting, chemical decomposition of metal oxides, and the analysis of volatile reaction products from organic binders.
4. Conclusions: The final chapter summarizes that laser cleaning is generally suitable for polychrome surfaces, provided energy densities are strictly controlled to avoid discoloration.
Keywords
Laser cleaning, Polychrome surfaces, Nd-YAG laser, Pigment discoloration, Thermal effects, Binder decomposition, NMR spectroscopy, Energy density threshold, Inorganic pigments, Chemical decomposition, Surface melting, Cultural heritage conservation
Frequently Asked Questions
What is the core subject of this research paper?
The paper investigates the effects of Nd-YAG laser irradiation on the stability of pigments and organic binders used in historical polychrome artworks during the cleaning process of natural stone.
What are the primary themes discussed in the paper?
Key themes include the thermal effects of laser ablation, the chemical mechanisms behind pigment "blackening," the analysis of gas-phase reaction products from binders, and the establishment of safe energy density limits for cleaning.
What is the central research question?
The central question is how to clean polychrome natural stone surfaces with laser ablation without causing physical or chemical damage, such as discoloration or decomposition, to the historical pigments and binders.
Which scientific methods are employed in this study?
The researchers utilized a combination of analytical techniques, including ESEM/EDX, XRD, DTA, FTIR, and NMR spectroscopy, alongside colour measurement via spectrophotometry to document changes.
What is covered in the main body of the paper?
The main body details the experimental setup using various pigments and binders, presents SEM findings on particle melting, discusses the decomposition of binders detected via NMR, and provides statistical thresholds for safe laser energy.
Which keywords characterize this research?
The research is characterized by terms such as laser cleaning, polychrome surfaces, energy density thresholds, pigment discoloration, and conservation science.
Why does zinc white change colour under laser irradiation?
The paper suggests that discoloration in metal oxide pigments like zinc white is often caused by chemical decomposition or physical effects, such as the melting and subsequent increase in particle size on the surface.
What role does the organic binder play during laser cleaning?
The study found that while organic binders are less sensitive to laser irradiation than certain pigments, they can partially decompose, releasing volatile compounds such as glycerin and acrolein, as detected by NMR.
Is laser cleaning suitable for all types of polychrome surfaces?
According to the conclusions, laser cleaning is generally suitable, but it is not recommended for surfaces containing pigments with very low discoloration thresholds, as they cannot be cleaned without damage.
- Quote paper
- Dipl.-Ing. Alexander Schnell (Author), Lothar Goretzki (Author), Christian Kaps (Author), 2003, IR-laser effects on pigments and paint layers, Munich, GRIN Verlag, https://www.grin.com/document/93399
