Cleaning of metal artefacts using pulsed lasers

Abstract: Preservation of cultural property in the form of historical artefacts for the future is an important interest in our society. The information which can be obtained from these materials is a valuable key to understand the past. One of the aims of any conservation procedure is to preserve both aesthetic and physical conditions of irreplaceable artefacts. Conservation involves a number of different steps. Cleaning is one of the critical steps needed to stabilise the material and to recover hidden details on the surface. It is therefore very important to choose suitable techniques and methods when cleaning to avoid damage to the artefact. Research and development to find more suitable conservation techniques are still going on. One of the new techniques under investigation is laser cleaning which has shown a good potential as a cleaning tool for many material categories met in conservation work. This thesis presents an investigation of the laser cleaning of metal artefacts using Nd:YAG and TEA CO2 lasers. The first part is a general overview of the laser technique applied to metal conservation and an outline of the practical work. The thesis then continues with four papers. Paper A concerns the removal of adhesives and coatings from iron samples using pulsed TEA CO2- and Nd:YAG-lasers. Trial samples were treated with different kinds of adhesives and coatings typically used in metals conservation and then cleaned using the lasers. The results were compared with cleaning using a conventional technique, micro blasting. Comparison of the different laser-cleaned surfaces was also performed by optical microscopy and Raman-spectroscopy. The comparative study showed that the best cleaning result was achieved by the TEA CO2 laser. The coatings were removed entirely without damage to the substrate. Paper B deals with cleaning of corroded iron samples using pulsed TEA CO2- and Nd:YAG-lasers. TEA CO2 lasers were successfully used to remove organic materials and rust. The Nd:YAG laser was able to clean the corroded samples, however, the risk for surface damage was found to be higher than using the TEA CO2 laser. Analysis of the surfaces cleaned by the two types of lasers was carried out by optical- and scanning electron microscopy, X-ray spectrometry and Raman-spectrometry. Paper C is a comparative study of the cleaning of iron samples using TEA CO2- and Nd:YAG lasers. In the first experiment a comparison of Nd:YAG laser and micro blasting for the cleaning of rust was made. The second experiment dealt with the removal of a wax coating from iron samples using a TEA CO2 laser with different power densities. The first experiment shows that the samples cleaned by pulsed laser retain a better surface structure than those cleaned by micro blasting. The second experiment showed how different energy densities affect the same surface. Paper D is an investigation of the cleaning of silver threads in textile using Nd:YAG laser at wavelengths of 1064 nm, 532 nm and 266 nm. The goal of this work was to find a technique to clean the tarnished silver without damaging the underlying silk. This was necessary since the conventional chemical treatment for cleaning tarnished silver is problematic to apply in this specific instance. The laser wavelength of 266 nm was found to be most appropriate for cleaning the silver threads without causing any damages either the silver or the underlying silk. However, the 1064 nm wavelength caused damage such as melting and burning of both the silver and silk.