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Chapter 4.2 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,
Contents 4.2.4.2
Analysis of Metal Objects 4.2.4.3
Monitoring of Leaser Cleaning 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:
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,
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.
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.
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).
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 [15]. 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.
As
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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4.2.6.2 Providers and Useful Websites Commercially available general-purpose LIBS
instruments (may not be appropriate for analysis of Artworks): http://www.globalspec.com/FeaturedProducts/Detail/OceanOptics/LIBSELITE/21115/1 http://www.lla.de/english/laser-plasma-spectrometer-artikel10.html http://www.marwan-technology.com/english/schede_prodotti/modi.htm http://www.appliedphotonics.co.uk/Products/LIBSCAN/libscan.html IESL-FORTH has developed a laboratory (LMnt I) and a portable (LMnt
II) LIBS instrument designed specifically for artwork
analysis.
Anastasia
Giakoumaki Institute
of Electronic Structure and Laser (IESL) Foundation
of Research and Technology GR
- 71110 Heraklion and Department
of Materials Science and Technology Heraklion Kristalia
Melessanaki Institute
of Electronic Structure and Laser (IESL) Foundation
of Research and Technology GR
- 71110 Heraklion Paraskevi
Pouli Demetrios Anglos
Institute
of Electronic Structure and Laser (IESL) Foundation
of Research and Technology GR
- 71110 Heraklion |
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