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 1064nm. 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.
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 20mm diameter; pressed under low pressure of 1.. 3kN/cm²) as well as on paint layers on sandstone samples. Within this study a Q-switched Nd-YAG laser at 1064nm (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 7mm.
- 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