Laser cleaning as a conservation technique for corroded metal artifacts

Abstract

Preservation of cultural property for the future is one of the most important activities of our society. The information which we can gather from such materials is a valuable key to understanding the past. Conservation is a process which includes a range of different treatments. Cleaning is one of the critical steps of the conservation process and involves stabilizing the material and the exposure of hidden details on the surface of the artifact in question. As the artifacts are often fragile and irreplaceable it is very important to choose suitable techniques and methods to avoid damage. Research is still continuing to develop new or improved techniques. One of these new techniques is laser cleaning which has shown good potential as a cleaning tool in many material categories in conservation work. This thesis presents an investigation of laser cleaning techniques of metal artifacts using Nd:YAG and TEA CO2 lasers. The work is divided into seven chapters; Chapter 1 The thesis begins with an introduction to the subject of laser cleaning in the context of conservation. The final part of this chapter includes a brief summary of the five published research papers which constitute chapters 3 - 7. Chapter 2 The second chapter describes the ethical questions which must be addressed when scientific research interacts with the field of conservation. The ethical approach of this research work is described and explained in this section. Chapter 3 Paper A: Cleaning of corroded iron artifacts using pulsed TEA CO2 and Nd:YAG lasers This paper presents the results of an experimental program to use CO2 and Nd:YAG lasers to clean corrosion products and organic materials off corroded metallic samples. The Nd:YAG laser produced some positive results and it was demonstrated that the CO2 laser could be an extremely effective cleaning tool when dealing with organic materials (such as wax) and thin layers of corrosion (especially if the surface of the artifact was wet). Chapter 4 Paper B: The removal of layers of corrosion from steel surfaces: A comparison of laser methods and mechanical techniques This paper presents a direct comparison of the efficacy of laser cleaning with mechanical brushing and micro-blasting for removing substantial layers of iron oxide from specially made samples. The conclusions drawn from this work indicated that micro-blasting was slightly more effective than laser cleaning and that Nd: YAG lasers performed better than CO2 lasers. Chapter 5 Paper C: Removal of adhesives and coatings from iron artifacts using pulsed TEA CO2 and Nd:YAG lasers This paper concentrated on the removal of different adhesives and polymer fillers from metal surfaces (as these materials are often present as a result of earlier conservation work). The CO2 laser proved to be an extremely useful tool in this context because its light is absorbed by polymers but reflected by the underlying metal. Chapter 6 Paper D: Experimental study on the effect of wavelength in the laser cleaning of silver threads This paper concentrated on a specific conservation problem involving the cleaning of a fabric made up of thin silver tape wrapped around silk fibres. Three Nd:YAG laser wavelengths were employed: 1,064nm, 532nm and 266nm. The two longer wavelengths were found to damage both the silver and the silk. The 266nm (Green) wavelength cleaned the silver surface without damaging the silk. Chapter 7 Paper E: Cleaning oxides from copper artifacts using a frequency-doubled Nd:YAG Laser This paper presents the results of an experimental program investigating the ablation of oxides from copper alloy surfaces using a frequency doubled Nd:YAG laser. A collection of coins with similar levels of surface oxidation were cleaned using range of laser pulsing parameters on dry, wet and submerged surfaces. It was discovered that laser cleaning was always associated with one of two types of coin surface damage. At higher powers the damage took the form of surface melting and at lower powers the incident laser beam converted the original oxide layer into adherent droplets of molten metal.

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