Marek Strzelec

Comparative Laser Spectroscopy Diagnostics for Ancient Metallic Artefacts Exposed to Environmental Pollution

Metal artworks are subjected to corrosion and oxidation processes due to reactive agents present in the air, water and in the ground that these objects have been in contact with for hundreds of years. This is the case for archaeological metals that are recovered from excavation sites, as well as artefacts exposed to polluted air. Stabilization of the conservation state of these objects needs precise diagnostics of the accrued surface layers and identification of original, historical materials before further protective treatments, including safe laser cleaning of unwanted layers. This paper presents analyses of the chemical composition and stratigraphy of corrosion products with the use of laser induced breakdown spectroscopy (LIBS) and Raman spectroscopy. The discussion of the results is supported by material studies (SEM-EDS, XRF, ion-analyses). The tests were performed on several samples taken from original objects, including copper roofing from Wilanów Palace in Warsaw and Karol Poznański Palace in ŁódŸ, bronze decorative figures from the Wilanów Palace gardens, and four archaeological examples of old jewellery (different copper alloys). Work has been performed as a part of the MATLAS project in the frames of EEA and Norway Grants (www.matlas.eu) and the results enable the comparison of the methodology and to elaborate the joint diagnostic procedures of the three project partner independent laboratories.

Characterization of Laser Cleaning of Artworks

The main tasks of conservators of artworks and monuments are the estimation and analysis of damages (present condition), object conservation (cleaning process), and the protection of an object against further degradation. One of the physical methods that is becoming more and more popular for dirt removal is the laser cleaning method. This method is non-contact, selective, local, controlled, self-limiting, gives immediate feedback and preserves even the gentlest of relief - the trace of a paintbrush. Paper presents application of different, selected physical sensing methods to characterize condition of works of art as well as laser cleaning process itself. It includes, tested in our laboratories, optical surface measurements (e.g. colorimetry, scatterometry, interferometry), infrared thermography, optical coherent tomography and acoustic measurements for “on-line” evaluation of cleaning progress. Results of laser spectrometry analyses (LIBS, Raman) will illustrate identification and dating of objects superficial layers.

Application of laser radiation in decoration and marking of ceramic products

In cooperation with the Institute of Optoelectronics MUT, the Institute of Ceramics and Building Materials conducts work on laser decoration of ceramic products. Two methods are under development: laser activation and laser sintering. The activation method is based on change of color of specially prepared ceramic material due only to illumination by laser beam. Laser sintering is a deposition welding process in which a layer of ceramic powder is deposited on the substrate material, and the two ceramic materials are fused through the application of laser beam, in turn creating any desired color pattern. The paper describes the influence of some physical phenomena on the progress of the laser process as well as sample experimental results.

Interferometric measurements of acoustic waves generated during laser cleaning of works of art

At the beginning, the ranges of laser-matter interaction in dependence eon the radiation power density are shortly discussed. Next, the suitable hydrodynamic model is described and some results of numerical calculations of interaction phenomena are presented. Finally, the measurements of amplitude of elastic wave generated inside object by interaction with pulse, Nd:YAG, Q-switched laser radiation are proposed. Measurements are conducted using Michelson interferometer.

Laser damage thresholds of bone objects

The main aim of laser cleaning is removal of encrustation without damage of original artwork substrate material. Art object are often made of fragile and breakage vulnerable materials, for which laser cleaning is an irreplaceable technique. The same applies to figures and sculptures made of different kinds of bones, so it is needed to determine damage threshold values of laser fluency. The paper, therefore, presents the results of investigations of pulse laser radiation interaction with bovine bones, ivory and bear tusk, utilizing different harmonic wavelengths of Nd:YAG laser in a wide range of fluences. It includes fundamental wavelength of 1064 nm with maximum energy of 500 mJ and harmonics: 532 nm (250 mJ) and 355 nm (90 mJ), generated by ReNOVALaser 5 system. Laser fluency has been controlled by means of direct energy variations or by radiation focusing at the object (from 8 mm to 300 &mgr;m), using constant pulse duration of 15 ns. Exact determination of bones threshold damage was based on microscopic investigation of the results of laser pulse irradiation.

Nano and microparticles emission during laser cleaning of stone

Air contaminants which emerge during laser ablation often cause health risks if released in the workplace and decrease laser cleaning efficiency if redeposited at the material surface. In addition, ultra-fine particles are generated if short laser pulses are applied. Consequently, a description of the nano and microparticle aerosol generation and the influence of the laser parameters, such as fluence and pulse energy, and type of material surface on the particle size distribution is given in the presented paper. The conducted experiments have shown that for applied laser fluences almost 80% of all emitted particles are in the nanoparticle size range of 30 - 100 nm. The high respirability of such particles can pose health risks, so suitable capture systems near to the processing zone or personal protective equipment such as respiratory masks are required.

Laser micro-structuring of surfaces for applications in materials and biomedical science

Laser radiation is used, among others, for surface treatment of various materials. At the Institute of Optoelectronics, under the direction of the late Professor Jan Marczak, a number of works in the field of laser materials processing were performed. Among them special recognition deserves flagship work of Professor Jan Marczak: implementation in Poland laser cleaning method of artworks. Another big project involved the direct method of laser interference lithography. These two projects have already been widely discussed in many national and international scientific conferences. They will also be discussed at SLT2016. In addition to these two projects in the Laboratory of Lasers Applications many other works have been carried out, some of which will be separately presented at the SLT2016 Conference. These included laser decorating of ceramics and glass (three projects completed in cooperation with the Institute of Ceramics and Building Materials), interference structuring medical implants (together with the Warsaw University of Technology), testing the adhesion of thin layers (project implemented together with IFTR PAS), structuring layers of DLC for growing endothelial cells (together with IMMS PAS), engraving glass for microfluidic applications, metal marking, sapphire cutting and finally the production of microsieves for separating of blood cells.

The influence of environment on corrosion of cast iron and carbon steel representing samples of outdoor metal technical heritage

This paper presents the results of annual measurements of the corrosion progress at test samples of cast iron and carbon steel placed in different natural environments. Comparative tests were performed in two outdoor stations, one at the Railway Museum in central Warsaw and one at the location of a Railway Museum in the small town of Sochaczew, 50 km west of Warsaw. The influence of surface roughness on the development of corrosion was determined by two kinds of treatment of all sample surfaces − metal brush or grinding. Stratigraphy and composition of corrosion products in quarterly periods were analyzed with laser-induced breakdown spectroscopy (LIBS) and Raman laser spectroscopy. Comparative tests were performed using a scanning electron microscopy (SEM) system equipped with energy dispersive spectrometer (EDS) and micro-chemical analytical methods. The corrosion layers on carbon steel have proven to be thicker on average than on cast iron, and thicker on the brushed parts of both materials. Furthermore, a thicker corrosion layer was found on the cast iron test samples exposed in Sochaczew than in Warsaw. Different iron oxides, namely lepidocrocite, goethite, hematite and magnetite were identified in the surface Raman spectra of corrosion layers, the last compound only in the sample from Sochaczew. SEM EDS measurements of surface elemental concentrations showed a higher concentration of sulfur in all samples from Sochaczew. Registered LIBS spectra have been additionally analyzed with statistical approach, using Factorial Analysis (FA). Results generally confirmed conclusions drawn from SEM/Raman/LIBS results.

Activation of colour changes in ceramic glazes by means of the Nd:YAG picosecond laser

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Preliminary optoacoustic measurements of air pollution generated during laser cleaning of artworks

In the presented preliminary experiments, a simple glass cup enables to isolate the cleaned surface and to introduce a laser beam and a measuring probe. A sensitive opto-acoustic measurement system was able to detect the time averaged changes of concentrations of benzene, carbon monoxide, carbon dioxide, methane, sulphur dioxide and water vapours at the level of ppm. Final concentrations of carbon monoxide and sulphur dioxide (well known as main pollutants of the atmosphere) can significantly increase during laser cleaning in closed areas. Moreover, concentrations of measured atmosphere contaminants can be considerably different in the case of ablation of encrustation on different substrates (wood, stone, plaster, parchment etc.). The measured values of gas concentrations emitted during laser cleaning showed large changes from point to point, depending on local properties of soiling, its quantity and type.