Jan Marczak

Varnish Ablation Control by Optical Coherence Tomography

Preliminary results of the application of optical coherence tomography (OCT), in particular in its spectral mode (SOCT) to the control of a varnish ablation process, are presented. Examination of the ablation craters made with an Er:YAG laser allows optimization of the laser emission parameters controlling fluence with respect to efficiency and safety of the ablation process. Results of measurements of ablation crater depth as a function of the number of pulses for a given fluence are presented for selected resins. This validates the applicability of SOCT to calibration of ablation conditions for the particular laser-varnish-paint layer combinations, and of its usage in planning the varnish ablation procedure. These results also imply that a review of conventional ablation thresholds is called for. Applicability of the SOCT technique to contemporaneous in situ monitoring of the range of varnish ablation is discussed.

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.

Investigation into the removal of graffiti on mineral supports: Comparison of nano-second Nd:YAG laser cleaning with traditional mechanical and chemical methods

Abstract This contribution discusses the analysis of polymeric paint layers and the removal of graffiti from historical buildings and sculptures. The main objective of the research is to compare the effectiveness of nano-second (ns) laser ablation with traditional cleaning procedures for the removal of painted graffiti from the surface of monuments. The vast number of various aerosol paints used for graffiti which contain different binders, solvents, and pigments does not allow the use of a single or universal cleaning method. In this study the efficacy of laser ablation was compared with chemical and mechanical cleaning methods. The laser system used for cleaning was a ns Nd:YAG, irradiating at 1064, 532, and 355 nm. Laser cleaning tests were performed on samples of limestone, sandstone, plaster, and brick covered with black, white, and red graffiti paints based on nitrocellulose, acrylic, and alkyd resins. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) were used to characterize paint binders. The morphology of treated surfaces was examined by optical microscopy and SEM. The results indicated the high efficiency of laser cleaning compared to traditional graffiti removal methods. A strong dependence of the efficacy of laser ablation on the paint composition and the porosity of the substrate was observed.

Depth-Resolved Multilayer Pigment Identification in Paintings: Combined Use of Laser-Induced Breakdown Spectroscopy (LIBS) and Optical Coherence Tomography (OCT)

A detailed feasibility study on the combined use of laser-induced breakdown spectroscopy with optical coherence tomography (LIBS/OCT), aiming at a realistic depth-resolved elemental analysis of multilayer stratigraphies in paintings, is presented. Merging a high spectral resolution LIBS system with a high spatial resolution spectral OCT instrument significantly enhances the quality and accuracy of stratigraphic analysis. First, OCT mapping is employed prior to LIBS analysis in order to assist the selection of specific areas of interest on the painting surface to be examined in detail. Then, intertwined with LIBS, the OCT instrument is used as a precise profilometer for the online determination of the depth of the ablation crater formed by individual laser pulses during LIBS depth-profile analysis. This approach is novel and enables (i) the precise in-depth scaling of elemental concentration profiles, and (ii) the recognition of layer boundaries by estimating the corresponding differences in material ablation rate. Additionally, the latter is supported, within the transparency of the object, by analysis of the OCT cross-sectional views. The potential of this method is illustrated by presenting results on the detailed analysis of the structure of an historic painting on canvas performed to aid planned restoration of the artwork.

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.

From medical to art diagnostics OCT: a novel tool for varnish ablation control

Since many medical diagnostic methods are non-invasive and even non-contact, they are well suited for inspection of fragile and unique objects of art. In art conservation it is a need for convenient and non-invasive method for monitoring of removal of the varnish layer from paintings - one of the most crucial operations in their restoration. In this study we present application of the Spectral Optical Coherence Tomography (SOCT) for in-situ monitoring of the laser ablation of the varnish layer. The examination of the ablation craters made with Er:YAG laser permits for the optimization of the laser emission parameters like fluency and working regime, with respect to efficiency and safety of the ablation process. Frames from the SOCT movies obtained during real time monitoring of the burning of the ablation crater are shown for the first time.

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.

Numerical modeling of laser-matter interaction

Laser radiation is often used in cleaning and conservation of artworks. Interaction of laser radiation with matter is so sophisticated process that analytical solutions rarely bring the valuable formulas. Even numerical methods seldom give quantitative insight into the physics of processes. Chemical and physical properties of surface impurity layers may change from point to point within the same sample. Absorption coefficient of the layers depends on such factors as weather or air humidity. In spite of this, theoretical description of laser cleaning is necessary, as it allows to explain some characteristic features of processes under investigation. In this work we present a model for laser pulse interaction with graphite layer placed on aluminium substrate. The model is limited to one dimensional hydrodynamic equations. The following phenomena are included into the model: absorption and reflection of laser radiation, heat conductivity, radiation transport in grey body approximation, shock waves, ionisation and the elastic properties of the media. Numerical calculations and experimental results give a qualitative agreement.

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.