Roman Ostrowski

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.

Absolute LIBS stratigraphy with optical coherence tomography

In this contribution preliminary studies on the application of Optical Coherence Tomography (OCT) to absolute depth calibration of Laser Induced Breakdown Spectroscopy (LIBS) data in application to revealing stratigraphy of easel paintings are presented. The procedure of in-situ monitoring of LIBS by means of OCT is described. Numerical method developed for precise extraction of the depth of the LIBS ablation crater is explained. Results obtained with model paintings are discussed.

Picosecond laser ablation system with process control by optical coherence tomography

In this contribution we describe an apparatus for precise laser ablation of delicate layers, like varnish on pictures. This specific case is very demanding. First of all any changes in colour of remaining varnish layer as well as underneath paint layers are unacceptable. This effect may be induced photochemically or thermically. In the first case strong absorption of the radiation used will eliminate its influence on underlying strata. The thermal effect is limited to so called heat affected zone (HAZ). In addition to colour change, a mechanical damage caused by overheating of the structure adjacent to ablated region should be considered also. All kinds of treads must be carefully eliminated in order to make laser ablation of varnish commonly accepted alternative to chemical and/or mechanical treatments [1]. Since the varnish ablation process is obviously irreversible its effective monitoring is very important to make it safe and trusted. As we showed previously [2-6] optical coherence tomography (OCT) originated from medicine diagnostic method for examination and imaging of cross-sections of weakly absorbing objects can be used for this task. OCT utilises infrared light for non-invasive structure examination and has been under consideration for the examining of objects of art since 2004 [7-10]. In this case the in-depth (axial) resolution is obtained by means of interference of light of high spatial (to ensure sensitivity) and very low temporal coherence (to ensure high axial resolution). In practice, IR sources of bandwidths from 25 to 150 nm are utilised. Resolutions obtained range from 15 down to 2 μm in the media of refracting index equal 1.5. In this contribution we expand application of OCT to space resolved determination of ablation rates, separately for every point of examined area. Such data help in better understanding of the ablation process, fine tuning the laser and finally permit increase of the safety of the ablation process.

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.

Microstructure of thin films: correlation with laser damage threshold

Microstructure and laser damage resistance of dielectric optical thin-film coatings was determined by several experimental techniques. These are film growth and test parameters as well as thin-film characteristics, e.g. absorption, structure and reflective index. The oxide coatings (HfO2, Nb2O5, Ta2O5, TiO2, and ZrO2) were prepared with varying technology parameters by reactive e-beam evaporation in Leybold A1100 batch coater. The structure of the films was found to be amorphous for very thin films with tendency to columnar growth for thicker films. X-ray diffraction (XRD) and reflectivity (GIXR) were used to determine the structural properties of the films. The surface morphology of oxide layers was determined using atomic force microscopy (AFM). The optical properties were investigated in UV-IR ranges by spectrophotometer measurements. The changes in the shape of absorption edge are found to be related to the structure of thin films. Laser damage threshold for the coatings was measured at wavelength of pulsed Nd:YAG laser. Correlation between the observed film properties and laser damage threshold is discussed.

Microstructure and properties of laser interference crystallized amorphous FeSiB ribbon

Abstract The influence of Q-switched pulsed Nd:YAG laser interference heating, using 120 mJ of pulse energy and a variable number of consecutive pulses, on the microstructure and magnetic properties of amorphous Fe80Si11B9 alloy was examined. Microstructural analysis, using light, scanning and transmission electron microscopy, was complemented by results of Mössbauer spectroscopy and measurement of magnetic properties (vibrating sample magnetometer). Periodically distributed crystallized micro-areas, ∼10 μm in diameter, in an amorphous matrix were produced by the treatments. Magnetization measurements showed that the as-cast ribbon and laser light irradiated samples are magnetically soft materials. The results lead to the conclusion that the dots corresponding to the laser modified regions exhibit a perpendicular magnetic anisotropy.

LIBS, Raman spectroscopy, and optical microscopy analyses of superficial encrustations on ancient tesserae in Lebanon

The aim of research was determination of composition and nature of superficial deposits, cumulated at the selected mosaic’s tesserae from Lebanon. Selected were three series of objects from different locations, namely from the seaside and mountain archaeological sites as well as from the mosaics exposed in the city center. Analyzed were stone and ceramic tesserae. The selection of objects was dictated by wide diversification of factors influencing the state of preservation and composition of deposits in given location. Investigations were performed including LIBS, FT-IR, Raman spectroscopy and optical 3D microscopy. The experimental results included composition and kind of deposit at the tesserae surfaces, and composition of tesserae itself. Compounds in the superficial deposits were identified. Confirmed was occurrence of different encrustations in dependence on geographic localization of a given sample. The interpretation of results was supported by multivariate statistical techniques, especially by the factor analysis. Performed analyses constitute the pioneer realization in terms of determination of deposits composition at the surface of mosaics from the Lebanon territory.

Analysis of deformation of aluminum plates under the influence of nano- and microsecond laser pulses

The paper presents numerical modeling of interaction of strong laser radiation with conventional aluminum sheets, similar to those used in military technology. The theoretical model uses equations of continuum mechanics (equations of hydrodynamics and the equations of mechanics of solid bodies in a cylindrical coordinates r, z), enriched with equations describing the typical effects of high temperature, such as absorption of laser radiation within the Al shield, electronic and radiative thermal conductivity, and energy loss on phase transitions (melting, evaporation, ionization). Semiempirical equations of state were used to describe the properties of material in the conditions of large deformation and the Johnson-Cook’s model. The equations were solved using the method of free points developed by one of the authors. Two kinds od laser pulses were considered: microsecond pulse with duration of 200 μs and a low peak power of 10 kW/cm2 (CW laser), and nanosecond pulse with duration of 10 ns and high peak power of 5 GW/cm2 (pulsed laser). The aim of this study was to determine the shapes and temperatures of Al plates under the influence of these pulses for the comparison of the numerical results with future experiments and to verify the possibility to determine the distribution of the energy density of the laser beam on the basis of the plate deformation.

LIBS spectra of multi-component Al, Fe, Cu alloys and composite materials used for selected elements of armament and munition

Spectral investigations in the UV-VIS range of selected Al, Cu, and Fe alloys and composite materials were performed using LIBS technique. The investigated objects were typical rifle cartridges, mortars, rocket launchers and samples of different type steel, Cu and Al alloys, as well as composite materials of special chemical composition. Two Nd:YAG lasers were applied: a short 4 ns, 60 mJ Brio Quantel/BigSky laser (1064 nm) and a long pulse 200/400 (up to 1000) µs (~ 2/4 up to 10 J) laser (1064 nm) constructed at the Institute of Optoelectronics MUT. This spectrochemical analysis was possible for Al, Cu, and Fe alloys objects for both lasers, and in case of composites only if the samples were irradiated by short laser pulse since in the experiment with the long pulse, all composite materials spectra, in general, were very similar to each other – they imitated a grey/black body spectra. For metal alloys in experiments with a short laser pulse only atomic spectra were observed while for long microsecond laser pulses molecular transitions have been registered for Al alloys. Electron temperatures of plasma created on different materials for short and long laser pulses were found on the base of Boltzmann plots. Temperatures are clearly higher for plasmas generated with a short laser pulse which results from much higher laser power density on the sample surface for short pulse and not from fluence which is 20-40 times larger for long pulse.