The Laser Laboratory for Heritage Science (LLHS) offers new analytical strategies specifically tailored for Heritage Science applications based on the use of laser methods. It includes techniques and systems for non-invasive analysis and advanced laser cleaning methodologies. Capabilities include expert knowledge based on almost two decades of research activity by the team, and in-house developed equipment for:
- The laser removal of unwanted layers constituted by materials of diverse origin (organic and inorganic), assembled in mixtures (e.g., thick pollution, burial accumulations) or in layers (e.g., multiple protective, metallic, dirt and/or overpaint layers) on weathered, even fragile, original surfaces. The laser action is based on the process of ablation or vaporization and it offers unique possibilities in surface cleaning as it entails precise control, material selectivity and immediate feedback. These attributes are particularly important in Heritage conservation and within the last twenty years, lasers have evolved as exceptionally practical, and at the same time, delicate cleaning tools. The laboratory operates a variety of pulsed laser sources emitting in the ultraviolet, visible and infrared spectral ranges with pulse duration in the nanosecond and femtosecond regimes.
- The non-invasive determination of the elemental and molecular composition of materials of artworks and heritage substrates based on the combined application of laser spectroscopies, i.e. laser-induced breakdown spectroscopy, laser-induced fluorescence and Raman. These techniques are well established for compositional analysis of artworks and heritage materials, substrates and objects and to provide information of physicochemical transformations associated with ageing, degradation or restoration.
- The morphological, structural and chemical non-destructive characterization in all dimensions with micrometre accuracy, allowing identification of multilayers and determination of thickness and composition as a function of surface position and depth, using the non-linear optical microscopy (NLOM) imaging techniques of Multi‐Photon Excited Fluorescence, and Second or Third Harmonic Generation. NLOM is based on the excitation of the non-linear optical response that any material can generate upon excitation with laser pulses of very short duration, in the range of femtoseconds.
Laser cleaning
The laser removal of unwanted layers constituted by materials of diverse origin (organic and inorganic), assembled in mixtures (e.g., thick pollution, burial accumulations) or in layers (e.g., multiple protective, metallic, dirt and/or overpaint layers) on weathered, even fragile, original surfaces is based on the process of ablation or vaporization and it offers unique possibilities in surface cleaning. This process entails precise control, material selectivity and immediate feedback, attributes that are particularly important in Heritage conservation. The installations at the LLHS operate a variety of pulsed laser sources emitting in the ultraviolet, visible and infrared spectral ranges with pulse duration in the nanosecond and femtosecond regimes. Laser cleaning investigations are carried out on a large variety of heritage materials, including varnished paintings, polychromes on wood or stone, heritage stone with pollution and/or biodegradation crusts, metal substrates with corrosion layers, paper and parchment based documents and other materials. Studies aim at determining the most adequate laser parameters and the most convenient methodologies (choice of laser wavelength, dual irradiation schemes, combination with other types of treatments, etc.) for a safe laser cleaning treatment according with material properties and state of conservation.
Different laser sources are available for laser cleaning investigations, including Q-switched Nd:YAG lasers (with variable repetition rate) and excimer lasers with pulse duration in the nanosecond domain and a continuous CO2 laser. The characterization of chemical and physical effects induced upon laser treatment are assessed using different microscopies (conventional and non-linear optical modalities), laser based spectroscopies and other complementary analytical techniques aiming at the formulation, for each type of substrate and overlayer, of optimization procedures (elimination without damage), to the identification of side effects and to the design of mitigation strategies.
SEM-BSE images of Verrucaria Nigrescens lichens colonizing Redueña quarry dolostone after sequential IR–UV laser treatment. The thin arrow points to detachments of the upper cortex and the wide arrow marks the position of exposed unprotected algae. The asterisks mark exposed algal cells that have experienced loss of some of their content.
Laser-based spectroscopies for analysis of Cultural Heritage
Together with laser cleaning, we apply and develop laser-based methods for analysis of Cultural Heritage objects. We combine laser spectroscopies, as laser-induced breakdown spectroscopy (LIBS), laser-induced fluorescence (LIF) and Raman for compositional analysis and to monitor physicochemical transformations occurring during laser cleaning or as a result of degradation or ageing.
We have developed a laboratory, hybrid system based on the pulsed laser excitation of Raman, LIF and LIBS signals based on a nanosecond Q-switched Nd:YAG laser operating at its second (532 nm), third (355 nm) and fourth (266 nm) harmonics and a spectrograph coupled to a time-gated intensified charge coupled device for spectral analysis allowing detection with temporal resolution.
Hybrid Raman-LIF-LIBS set-up
The following figure illustrates the case of a multianalytical photonic study of an historical set of Late Roman glasses.
Compositional analysis of an historical set of Late Roman glasses
Non Linear Optical Microscopy (NLOM)
More recently, we are applying NLOM in imaging mode, using an ultrafast femtosecond excitation laser, exploiting several nonlinear optical effects. Using Multi‐Photon Excited Fluorescence (MPEF) and Second or Third Harmonic Generation (SHG, THG), we aim at obtaining high contrast imaging of samples for the non‐destructive accurate determination of thickness within multi‐layer samples and objects and of the composition as a function of depth.
NLOM is based on the excitation of the non-linear optical response that any material can generate upon excitation with laser pulses of very short duration, in the range of femtoseconds. The combined use of the different NLOM modalities derives information, in a totally non-invasive way, on the presence of layers of different chemical nature, their thickness or their crystalline or hierarchical internal organization. It is possible to obtain highly contrasted 3D images at the micrometer scale, without any preparation or sampling, of the object under study. Lateral and axial resolutions are in the micrometre range and the penetration depth can reach up to 1 mm, depending on the sample transparency. The technique can be applied to substrates that are transparent in the IR region, such as varnishes, painting layers, corrosion layers on metal substrates, parchments and others.
The NLOM system, developed in-house, consists in an upright microscope which uses an ultrashort laser system as excitation light source. The laser is a mode-locked Ti:Sapphire oscillator emitting at 800 nm, delivering 70 femtosecond pulses at a repetition rate of 80 MHz.
Nonlinear optical microscope
Our activity in the field of Heritage Science is supported at national level by participation the CSIC Interdisciplinary Platform Open Heritage, research and Society (PTI-PAIS) and by Comunidad de Madrid project TOP-Heritage-CM. We are also participating as partners in EU H2020 projects IPERION CH (Integrated Platform for the European Research Infrastructure ON Cultural Heritage), PARTHENOS (Pooling Activities, Resources and Tools for Heritage E-Research Networking, Optimization and Synergies), E-RIHS-PP (The European Research Infrastructure for Heritage Science, Preparation Phase) and IPERION HS (continuation of IPERION CH).
Further information is available at:
Tecnologías en Ciencias del Patrimonio (TOP HERITAGE-CM)
http://www.parthenos-project.eu/
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