Laser-Induced Breakdown Spectroscopy for the Analysis of Archaeological Objects and Artefacts
Anastasia Giakoumaki1,2, Kristalia Melessanaki1, Paraskevi Pouli1, Demetrios Anglos1,*
1Institute of Electronic Structure and Laser,
Foundation of Research and Technology
2Department of Materials Science and Technology,
This chapter aims to provide the reader with a basic understanding of the main physical principles and analytical features of Laser Induced Breakdown Spectroscopy (LIBS) as well as to give an overview of its use in the field of cultural heritage. Laser-Induced Breakdown Spectroscopy is an analytical technique that enables the determination of the elemental composition of materials on the basis of the characteristic atomic fluorescence emitted from a micro-plasma produced by focusing a high-power laser on or in a material [1-6].
A large number of elemental analysis techniques are well established in the field of material science and certainly quite a few of them have enjoyed recognition in archaeometry and artwork analysis providing useful qualitative and/or quantitative information on the composition of materials [7-11]. Among the wide range of existing powerful analytical techniques LIBS is considered a useful and advantageous tool as it offers features that get very important when analysing objects of cultural heritage:
— It is a straightforward and simple analytical technique, which can be employed even by non-specialized users.
— It is a rapid analysis technique providing results practically instantaneously after the analysis.
— It is applicable in situ -that is on the object itself- and under certain conditions is nearly non-destructive.
The simplicity of the technique and its speed permit the analysis of a relatively large number of objects in a short time. For example, archaeological excavations often produce a large number of artefacts and their timely characterization or simple screening is required. The possibility to use the technique in-situ eliminates the need for sampling, a process, which is time consuming and sometimes damaging to the object. This is important because the sensitivity and value of most works of art and archaeological objects often precludes sampling thus preventing the use of analytical techniques, such as for example atomic absorption spectrometry (AAS) or inductively coupled plasma - mass spectrometry (ICP-MS), which require a small quantity of sample that is consumed during the measurement. Obviously, non-destructive techniques are preferred over destructive ones and even though LIBS is not strictly a non-destructive technique it is considered minimally invasive given the very small area of interaction of the laser pulse with the sample surface.
On the basis of these features and research done to date, LIBS appears as a useful alternative to other sophisticated techniques, for obtaining information on the elemental composition of materials in cultural heritage objects. The potential of LIBS in this field is shown by several research papers that have appeared in the last few years, describing its use for the analysis of works of art and objects of archaeological importance [12-34]. Earlier reports can also be found on the use of laser micro-spectral analysis in the determination of the elemental content of metal, pottery and paint samples from different objects [35,36]. The analytical capabilities of LIBS are dealt with in more detail in the following sections and in the examples presented.
LIBS is an analytical technique that enables the determination of the elemental composition of materials on the basis of the characteristic atomic emission from a micro-plasma produced by focusing a high-power laser on or in a material. LIBS has been used in a wide variety of analytical applications for the qualitative, semi-quantitative and quantitative analysis of materials.
The analysis by LIBS starts with the deposition of light energy in a small volume of material (less than 0.1 mm3) and within a short time period (5 - 20 ns). This rapid energy deposition is achieved by focusing a laser pulse on the surface of a solid target (the object analyzed) and results, through a series of processes, to material breakdown and generation of a micro-plasma plume. The plasma consists of electrons, neutral and ionized atoms, small molecules and larger clusters, and is moving away from the solid surface with typical species velocities in the order of 0.5 to 50 km/s. Immediately upon its formation the plasma is characterized by high temperature and electron density and as a result it shows intense broad band emission in the ultraviolet and visible arising from highly excited species. This broadband continuum carries no analytical information, but as the plasma expands in space, the emission evolves with time to a spectrum with sharply peaked features, corresponding to distinct electronic transitions of the different species in the plume. Recording of this emission on a spectrometer produces the LIBS spectrum, which, through a straightforward analysis, yields compositional information about the material examined. More specifically, the characteristic, sharp atomic emission peaks in the spectrum lead to the identification of the elements contained in the minute amount of material ablated, reflecting the local elemental composition of the sample (qualitative analysis). A typical LIBS spectrum is shown in Fig. 4.2.1. The peak intensity or the integrated emission can in principle be associated with the number density of each emitting species in the plume and this, in turn, with the concentration of specific elements in the ablated material (quantitative analysis).
The integrated intensity of an emission spectral line from a single species present in the plasma, in a state of local thermodynamic equilibrium, is given in equation (1), as a function which relates the emission line intensity to the number density of the species, relevant spectroscopic parameters and the plasma electron temperature:
is the intensity of emission line at frequency ω corresponding to a transition from the upper quantum state i to the lower k; NS is the number density of species S in the point of observation within the plasma; h is the Planck’s constant, gi the statistical weight of state I, Aik the transition probability for spontaneous emission from energy level i to k, Ei the energy of quantum state i with respect to the ground state of the emitting species, kB the Boltzmann’s constant, T the plasma temperature, and Z(T) the partition function for the quantum states of the emitting species.
On the basis of equation (1), the number density (concentration) NS, for each emitting species can be associated with the intensity of its spectral lines, , thus providing a link between signal intensity and elemental concentration.
The main ingredients of the LIBS analysis include laser induced plasma formation followed by capture of the plasma emission and analysis/interpretation of the spectral data. The instrumentation required for LIBS is straightforward and the basic components are shown in a diagrammatic representation of a typical experimental set-up (Fig. 4.2.2).
The laser source most commonly used is the nanosecond, Q-switched Nd:YAG laser. Excimer lasers have also been used. The wavelength of the laser is a critical parameter that determines the coupling of the irradiation to the material surface. A convergent lens of appropriate focal length, typically between 50 and 500 mm, focuses the laser beam. Typical values of the laser pulse energy lie in the range of 1-30 mJ, which translate to energy density values on the sample surface in the range of 1-50 J/cm2. These values are adjusted either by varying the energy per pulse or by changing the working distance (i.e. lens-to-sample distance) and, therefore, the laser spot size on the sample. The quality of the laser beam is a critical parameter in achieving tight focusing, which is important both for minimizing the affected surface area and for obtaining good spatial resolution.
The collection of the emitted light is done either by using a proper lens or lens system or directly through an optical fibre placed near the plume. Lens systems lead to improved collection efficiency but optical fibres offer simplicity in signal collection. The analysis of the plume emission into its spectral components is done with an imaging spectrograph employing a diffraction grating as the dispersing element. The spectrum is projected at the image plane of the spectrograph and is recorded on the detection system.
Detection nowadays is based solely on intensified diode array or intensified CCD (charge-coupled device) detectors. These detection devices offer high sensitivity with variable gain. They also permit adjustable gating in order to achieve discrimination of the useful atomic emission signal from the broadband continuum background that is present at early times following sample irradiation. A pulse generator is commonly used to control the timing of the measurement while in new generations of detectors software control of the timing parameters is possible. By employing small or medium size standard spectrographs one has the option of recording a broad range of the spectrum (typically 100 to 400 nm) with a medium- to low-resolution gratings or a narrow part of the emission spectrum with the use of high-resolution gratings. Recently significant technological progress has been achieved employing spectrographs based on Echelle gratings, which coupled to two-dimensional CCD type detectors provide wide spectral coverage simultaneously with excellent spectral resolution.
Finally a most important issue relates to the correct interpretation of spectra and reliable identification of elements present. In this case the use of proper software, which provides spectral line data for each element or even reference spectra can be very helpful allowing the user to compare their LIBS spectra against those of the library and identify qualitatively the elemental composition of their samples.
Specific examples involving analysis of real objects are presented to illustrate the application of LIBS to various types of analytical problems encountered in art history, conservation and archaeological analysis.126.96.36.199 Pigment Analysis
Painting has been used in all types of art forms from antiquity to modern times. Easel and wall paintings, wood and metal polychromes, illuminated manuscripts and pottery represent art or craft forms where pigments have been used [37-40]. Determining the identity of pigments is of importance for several reasons. For example, in the case of painted works of art, it can help art historians to characterize the available materials and to understand techniques and effects used by the artist in achieving the result in the final work.
Similarly, in the case of ancient painted pottery or frescoes, pigment characterization may lead to increased understanding of the materials and technology available to the craftsmen. This knowledge may even relate to the origin of the materials suggesting local or remote sources that might indicate communication and trade between sites. In addition, pigment identification in painted works of art can often be significant in providing dating information on the basis of the known history of the pigment manufacture or in assessing the state of preservation of a painted work of art in preparation for proper restoration.
The different elemental composition of inorganic pigments is reflected in the corresponding LIBS spectra and this enables their discrimination based on the characteristic atomic emission peaks recorded. The positive identification of a certain pigment results from correlating the spectral data with the colour of the paint analysed. In this respect, information from an art historian or conservator about the possible pigments, anticipated, is essential in the analysis of the work. In many cases, the characteristic elements, can easily lead to determination of the pigment used. For example, the presence of mercury undoubtedly suggests the use of the red pigment vermilion (HgS, also known as cinnabar in its natural mineral form). The presence of lead in a white paint is definitive proof of the use of lead white (Pb(OH)2x2PbCO3). However, in reality, mixtures of pigments are often used by the artist to achieve the desired result in terms of colour and shade. The situation of pigment mixtures gives rise to more complex LIBS spectra but the presence of individual pigments can usually be determined based on the elements found.
A real pigment analysis case is represented by
the study of the miniature (19th century AD,
Examination of the green paint, used extensively on the miniature, shows intense emission due to copper and weak emission due to arsenic (Fig. 4.2.4). This suggests the use of a green pigment based on a copper-arsenic compound, such as either Scheele’s green (Cu(AsO2)2).or emerald green (Cu(CH3COO)2.3Cu(AsO2)2). An alternate possibility could be the use of a mixture composed of the blue pigment azurite (copper compound) and the yellow pigment orpiment (arsenic compound). Additional examination of the paint under the microscope verified the identification as emerald green on the basis of its characteristic appearance (small spherulites).
In the context of pigment analysis, LIBS can also be employed to identify the type of prior restoration performed on paintings by discriminating between the original paint and that used in the restoration. In certain cases, the time a painting was made or an intervention took place can be indirectly estimated on the basis of known dates when synthetic pigments became available. In one case the restoration carried out on several parts of an oil painting (Fig. 4.2.3.b) [12-20] was examined and found to contain mainly titanium white, a modern pigment, in contrast to the original paint, which was composed mainly of lead white (Fig. 4.2.5) [12-20]. This result suggests some retouching, done on the original painting after 1920, as titanium white became commercially available after that time.
Ceramic objects are the most common remnants of ancient life, uncovered in numerous excavations, having been used as storage containers, serving dishes, and votive figurines, among other uses. They are made of clay, which is often decorated by paint depending on the use and quality of the object. In the analysis of ceramic shreds, questions are related to the characterization of pigments, the determination of the elemental composition of the clay and the characterization of surface encrustation. Analysis of clay inclusions can be of importance if they are characteristic of the clay source or the technique employed.
An indicative example is shown in Figs. 4.2.6a and 6b where LIBS spectra from two different types of microscopic inclusions are shown. The dark inclusion appears to be based on clay with a high content of iron (possibly from magnetite, Fe3O4, a black mineral), while the white inclusion shows a high content of calcium, most likely in the form of calcite. Quantitative elemental analysis of pottery shreds is important in differentiating between various types of clay from the same or different excavation sites, aiding archaeologists in classifying objects so allowing them to draw conclusions about materials and techniques, which relate to the socio-economic status of populations.188.8.131.52 Analysis of Metal Objects
Objects made of metal or alloys, including sculpture tools, weapons, home utensils and jewellery have been widely used for different purposes since metallurgy was invented. The main materials used include copper and bronze - that is copper-tin alloys (in the Bronze Age) - and later iron. Other metals used include lead and tin while precious metals such as silver or gold alloys have been used in jewellery and for decorating different objects. Also, metal alloys have been extensively used in coinage. The first analytical aim concerning a metal object is to identify the type of metal or metals in an alloy. This information could be enough for an initial classification of an object, for example to distinguish between copper and bronze (copper-tin alloy). Furthermore accurate quantitative analysis of the various metals and of any trace elements can lead to a more complete characterization of materials, yielding information about objects regarding period or technology of manufacture and possibly provenance of raw materials. Examples of spectra collected from various metal objects including an ancient bronze tool, a piece of ancient gold jewellery, and a 20th century coin are shown (Fig. 4.2.7).
184.108.40.206 Monitoring of Laser Cleaning
A major breakthrough in art conservation has been brought about in the last two decades by the introduction of laser based techniques for cleaning art objects ranging from marble, stone, metal sculpture and stained glass to paintings, icons and paper [41-42]. The cleaning process relies on the controlled removal of contaminants or other unwanted layers from the surface of the object. This is effected by means of laser ablation due to the interaction of focused nanosecond laser pulses with the material (see Chapters on Laser cleaning).
The process depends strongly on the material properties (absorptivity, surface roughness, mechanical stability) and irradiation parameters (wavelength, energy density, pulse duration). There have been extensive studies on the use of laser ablation cleaning, which are beyond the scope of this chapter, however one critical question regarding the success of the cleaning process relates to LIBS. It is very important, when carrying out any type of cleaning methodology, to know where to stop the process. That is, to be able to assess reliably to what extent the contamination layer has been removed. Such control of laser cleaning can, in certain cases, be achieved by monitoring the optical emission resulting from material ablation. In essence, LIBS measurement is carried out simultaneously with the laser cleaning. If distinct differences between the LIBS spectra of the contamination layer to be removed and the cleaned surface exist, then it is, in principle, possible to control the process of cleaning by a simple algorithm implemented on a computer. This is a delicate approach and careful preliminary tests of working parameters, with respect to the specific case in hand, have to be done beforehand in order to define properly the end point of the cleaning.
Such control of laser cleaning has been demonstrated in several cases including the removal of overpaint from frescos [13,42] or encrustation from marble [14,24,42,43] or glass . An example is shown in Fig. 4.2.8 indicating clear spectral changes as the overpaint layer is removed, which allow effective, on-line control of the process. Cleaning is then stopped before exposing the original paint layer to the laser irradiation in order to avoid any pigment modification.
briefly outlined, LIBS features several unique advantages, which make
its use in archaeological and artwork analysis attractive. First of
all, it requires no sample removal from the object. The analysis can be
performed in situ and requires only optical contact with the object.
Material loss in a typical LIBS measurement is minimal and any damage
to the sample surface is practically invisible to the naked eye. Thus
LIBS can be considered as a nearly non-destructive technique. The
absence of sampling and sample preparation, in combination with single
laser pulse measurement being complete in less than a second, offer
unparalleled speed to the technique. The spatial resolution achieved by
LIBS across a surface is nearly microscopic. In addition, the technique
has the capability of providing depth profiling information if spectra
from successive laser pulses delivered at the same point are recorded
individually. Finally, the equipment used is compact and can be
“packaged” in a portable or transportable unit.
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Commercially available general-purpose LIBS instruments (may not be appropriate for analysis of Artworks):
IESL-FORTH has developed a laboratory (LMnt I) and a portable (LMnt II) LIBS instrument designed specifically for artwork analysis.
Institute of Electronic Structure and Laser (IESL)
of Research and Technology
GR - 71110 Heraklion
Department of Materials Science and Technology
Institute of Electronic Structure and Laser (IESL)
of Research and Technology
GR - 71110 Heraklion
Institute of Electronic Structure and Laser (IESL)
of Research and Technology
GR - 71110 Heraklion