Laser parameters optimization for the artifacts silver coated surfaces cleaning

Abstract

In recent decades, lasers have become the devices that came out of research laboratories and are widely used in industry, engineering, medicine, arts, etc. Among the many applications, lasers have found a place in the conservation of cultural heritage as a source of radiation in the modern diagnostic techniques, cleaning and scanning objects of priceless value [1]. Their importance in cleaning of cultural heritage objects is based on the properties of these techniques, such as high sensitivity, nondestructivity, selectivity, flexibility, on-site applicability and others. Laser techniques application in metal artifacts cleaning has to be carefully evaluated due to complex phenomena in laser–metal interaction and specificity of every artifact item. There is a large variety of metal materials and their combinations in cultural heritage. Also, the different organic and inorganic encrustations and corrosion products in different progression stage can be found on artifacts surfaces [2,3]. Therefore, the optimization of laser parameters for safe and effective metal artifacts cleaning process is important step in order to avoid unwanted side-effects such as changes in the color or grater surface damages. A specific field of the metal surface laser cleaning is the cleaning of thin metal foils and coated surfaces. In cultural heritage such examples are metal yarns on the textile embroidery [4, 5]. Often, these yarns are coated with some precious metal as are silver or gold. This paper presents a study of laser cleaning parameters for safe and effective cleaning of silver coated copper metal yarns. In that purpose the preliminary investigation are performed on silver coated copper plate. That investigation involve numerical modeling of laser– silver coated copper plate interaction and experimental irradiation of real sample surface with pulsed nanosecond Nd:YAG laser by changing laser parameters as are wavelength, laser beam energy and number of pulses. The numerical 3D model of the generated heat on the silver coated copper plate was obtained using the COMSOL Multiphysics software package and was carried out with the aim to define the temperature distribution around the irradiated zone and the maximum temperatures. Optical microscopy, SEM and EDX analysis are used for the diagnosis of the morphological and chemical effects of laser irradiation on real sample surface. Obtained results are compared with results of laser irradiation analyses on naturally tarnished metal yarns embroidery from museum sample. This investigation confirms that Nd:YAG lasers can be successfully used for metal yarns cleaning. Application of adequate numerical model can provide an opportunity for a faster and cheaper determination of the cleaning process optimum values range. Some parameters for successfully and safely cleaning of silver coated copper surface were determined.