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identifiable in the walls running north to south representing
the earliest currently identified phase of construction. A second
phase of construction is seen in the walls running east to
west that have the same mortar, added after the initial construction
of the structure. A third phase of repair and redivision
of the space is clear in the analysis, both chemically and
visually, in the walls on both orientations. A fourth and possibly
final phase can be identified by the addition of an apse to
the southeast corner of the complex, either built concurrently
with the previous phase of repair using a different mortar
mix or more likely as a separate aggrandizing renovation.
Overall, this aligns with our understanding of the monumentalization
of the site, with the bath complex growing, changing,
and adapting through the centuries. Through this analysis,
when paired with our other research, we can achieve a
clearer picture of the construction occurring on site, even
when all the materials are sourced locally and appear similar
in visual analysis.
Bucchero Pottery
Previous studies that included bucchero chemical composition
measurements identified many of the elements identified
in our study although with some variation depending on analytical
technique, most commonly XRF,25,26 and research
focus such as trace elements only.27 Considering some elements
identified in these Orvieto bucchero sherds, namely
Si, Al, Mg, K, and O, are more challenging or impossible to
detect using XRF, this LIBS-based study adds a new view to
prior published work in the field.
A PCA of the data set was performed with the resulting
three-dimensional score plot in Fig. 4. This model accounted
for 80.1% of the variance in the data within the first three
PCs. Three conclusions were made from the examination
of the score plot. First, the data points for the Crocifisso
del Tufo bucchero in the upper left of the plot form a cluster
separate from those of the Cavità 254 data points at the bottom
right of the plot. This indicates ceramics from the two
sites are chemically different and likely produced from different
raw material sources. The LIBS spectral overlay in Fig. 5
illustrates this difference, showing higher levels of Ca and Sr
in the Cavità 254 bucchero compared to those from
Crocifisso del Tufo, and higher levels of Fe and Mg in the latter
compared to the former. These chemical differences are
an interesting finding considering the proximity of Cavità 254
within the walled city of Orvieto to Crocifisso del Tufo just
outside Orvieto along the base of the wall. Secondly, no distinct
separation of data points is observed for gray versus
black bucchero from Crocifisso del Tufo suggesting all ceramics
from this site were produced using the same clay source
and possibly by the same workshop; the visual color differences
therefore are not attributed to chemical composition,
but rather choices made during the production of the ceramics.
The color of bucchero results from the red iron oxide
clay used in production being reduced to ferrous oxide
upon firing in an airtight kiln under low-oxygen conditions
with organic material present within the kiln.28 Shades of
black and gray likely were achieved by varying firing conditions
with elemental carbon from the organic matter as the
main source of the black color.29 A tendency toward lightercolored
bucchero over time has been observed with suggestions
that color can be used for dating, although this is an
oversimplification and may be further complicated by color
changes caused by long-term burial in soil.30 Lastly, looking
instead at the Cavità 254 data points, all gray bucchero
whether inscribed or not inscribed form one overlapping
group at the top right of the score plot indicating the same
raw material source likely was used for their production.
Interestingly, about half of the Cavità 254 black bucchero
data points overlap with this Cavità 254 gray bucchero cluster
whereas the other half of the Cavità 254 black bucchero
data points instead group separately toward the bottom of
the score plot. These chemically different Cavità 254 black
bucchero sherds may have been produced by a separate
workshop, possibly for a different purpose. Etruscan bucchero
was produced for several purposes, including utilitarian,
votive, and funerary.31 It is possible that separate
pottery workshops existed in Orvieto (Etruscan Velzna),
each producing products for different uses. More elaborate
pieces, unusual shapes, and inscriptions are often associated
with votive and funerary bucchero.31 Considering all analyzed
Cavità 254 black bucchero were small fragments, the original
forms are not known and thus no confident conclusions can
be drawn regarding the purpose for the select black bucchero
samples that correspond to the data points at the bottom
of the score plot.
Examination of the PCA loading plot revealed that Ca and
Mn are markers for the combined Cavità 254 gray and black
bucchero whereas Sr is a marker for the separate Cavità 254
black bucchero. Looking instead at the Crocifisso del Tufo bucchero,
Mg and O are markers for both gray and black. A number
of these differences are evident upon examination of Fig. 5.
A two-way analysis of variance of sites and marker elements
statistically supports a chemical difference between Cavità 254
bucchero and Crocifisso del Tufo bucchero (p< .001).
Conclusion
The utility of handheld LIBS for field archaeology is clearly
demonstrated through the two applications presented in
this paper. LIBS can be used as a complement to XRF as
shown in the Roman mortars study, or as a singular analytical
method from which to draw conclusions as was the case with
the Etruscan bucchero sample. Adding this type of analysis to
other forms of analysis strengthens the conclusion and
increases the speed at which research questions can be
answered in the field. Future handheld LIBS work will include
additional analyses of bucchero from different sites and of different
colors to better understand observed chemical
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