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Applied Spectroscopy Practica 1(1)
An obvious disadvantage of LIBS is the minimally destructive
nature of the technique, not ideal for one-of-a-kind objects of
significant value. With careful choice of analysis location and
instrument settings, however, LIBS can successfully be applied
to many sample types with no visible damage or permissibly
minimal damage. Recent studies using LIBS for elemental characterization
of wall murals,9-11 glass,12 coins,13
construction
materials,14 and a large copper alloy assemblage15 demonstrate
the potential of the technique for archaeological materials.
The handheld LIBS studies reported in this paper focus on
elemental characterization of two different material types,
Roman wall mortars and Etruscan ceramics. In both cases
the materials are man-made with their compositions related
to the raw materials and processes used in their production,
thus providing an opportunity to establish artifact groups
based on chemical similarities. All analyses were performed
during one campaign season conducted in the Orvieto
(Umbria), Italy, geographic region. The Roman wall mortars
were discovered at Coriglia, Castel Viscardo, an archaeological
site that is centered on several natural springs and associated
structures, most notably a bath complex. The site was
first occupied by the Etruscans around the sixth century
BCE, reached its peak during the Roman period, and continued
to be inhabited through the medieval era. The mortared
walls form a series of north-facing terraces and were previously
studied both in situ and ex situ using handheld X-ray
fluorescence (XRF) spectrometry.16 The Etruscan bucchero
sherds are from two locations, Cavità 254, which dates to
approximately the fifth century BCE, and Crocifisso del Tufo,
which dates to at least the sixth century BCE. Cavità 254 is
a human-made pyramidal cave located under the city of
Orvieto that is believed to have been used as a quarry. The
space was filled in at the end of the fifthcenturyBCEwithvarious
materials associated with the Etruscan occupation of
Orvieto, including both gray and black bucchero, a type of typical
Etruscan ceramic.17,18 Crocifisso del Tufo is an Etruscan
necropolis consisting of chamber-type tombs organized in a
street-like system with family names inscribed on each
tomb's lintel. Objects such as vases, pots, jewelry, and urns
were placed in the tombs along with the dead. Both of these
assemblages have been the focus of research due to the complexity
and volume of the material, including this study.
The goals of the two studies were threefold: (i) evaluate
the practical utility of handheld LIBS for field archaeology;
(ii) analyze the LIBS data using multivariate statistics to determine
possible associations or groupings among samples to
draw conclusions regarding the archaeology; and (iii) compare
the conclusions from the LIBS work to those from previously
reported studies.
Methods and Materials
The mortar on a total of 23 loci was examined using handheld
LIBS. Locus (singular) and loci (plural) are archaeological
terms used to designate discrete, definable actions, such as
structures or deposits, uncovered at an archaeological site.
Loci are numbered as they are unearthed. Some loci may
appear visually to be part of a singular structure, a wall for
example, but because they were excavated or constructed
at separate times, they are indicated by different locus labels,
in this case, numbers. Trench, another archaeological term, is
used to designate a specific section of a site being excavated;
trenches are commonly designated by letters in alphabetical
order from earliest to most recently opened area. Mortar
on each wall locus within Trench C was analyzed at 12 to
40 different locations distributed as evenly as possible
across the wall's length and height, with duplicate analyses
at each location. A total of 752 mortar analyses were conducted
over the course of 13 days. Figure 1a is a digital
image of Locus 22 with two scale arrows indicating where
handheld LIBS analyses were conducted. Previously
reported handheld XRF results are available for comparison
to the handheld LIBS data for eight of these loci analyzed ex
situ and for 15 loci analyzed in situ.16
In a separate study, 58 bucchero pottery sherds were
investigated using handheld LIBS of which 24 were identified
as gray by visual inspection and 34 were identified as black.
For the gray samples, four were from the Crocifisso del
Tufo excavation site and 20 were from Cavità 254. Seven of
the gray Cavità 254 sherds have inscriptions. For the black
samples, 25 were from Crocifisso del Tufo and nine were
from Cavità 254. Digital images of select sherds, one gray
and one black, are provided in Fig. 1b. Each sherd was analyzed
at three separate locations along cut edges; three samples that
were more intact and had bases, as well as bowls, were analyzed
at more than three locations. Cut edges were selected
as the preferred locations for analyses considering the
decreased visibility of the resulting small laser craters compared
to the inner and outer surface areas of the pottery.
A total of 185 sherd analyses were conducted over two days.
The handheld LIBS system used in both studies was a
SciAps Z-500ER (Massachusetts, USA). This instrument has
multiple modes of operation. Geochem mode, designed for
small analysis areas and based on an average spectral signal
calculated for a multilocation grid, was used for this work.
The pulsed laser in the LIBS was operated at 5-6 mJ/pulse
with a 50 Hz frequency and a 1 ns pulsewidth. The instrument
is capable of monitoring wavelengths using four different
spectrometers with the following ranges: 180-255,
255-315, 315-420, and 420-860 nm. Measurements with
the Z-500ER can be conducted with an argon purge gas,
which increases the instrument's sensitivity, but due to the
limited argon supply transported to the excavation site, this
study was conducted without argon purge. Data for each
analysis location were collected in a 4 × 3 grid pattern
with four cleaning shots followed by three analysis shots at
each of the 12 spots in the grid; these data then were
averaged to produce one spectrum per analysis location.
Each analysis required about 6 s and was approximately
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