X-ray tomography of a soil block: a useful tool for the restoration of archaeological finds

X-ray imaging provided important contributions to archaeology, especially in the recent
years, both through radiography 1]-4] and tomography 5]. X-ray computed tomography (CT) has been used in many cases because of its potential
to visualise inner and invisible parts of an object, providing information in a completely
non-invasive way. Thanks to their availability medical CT scanners have often been
used in the archaeological field: to investigate mummies and related materials 6]-8], to discover the content of a pot before opening it 9],10] or simply to study the inner and outer of an object in high detail 11]. CT proved to be a very powerful tool even in the related field of anthropology 12],13]. Synchrotron radiation can also be used for some special cases 14]; an example was presented of a dedicated transportable instrument developed specifically
to analyse fossils 15].

The analysis of soil blocks from excavations is one of the most recent applications
of CT in the field of archaeology. Medical 16] and industrial 17],18] scanners have been employed so far to scan the blocks in the search for artefacts
of different nature. In this paper we would like to present the application of a CT
instrument 19] specifically designed for the analysis of Cultural Heritage materials. This instrument
has been developed within the neu_ART project 20] and installed in the Centre for Conservation and Restoration “La Venaria Reale”:
although it has already demonstrated its reliability and usefulness for the analysis
of large artworks 21], however, thanks to its versatility both in energies and geometry, it proved suitable
for the analysis of artefacts from an archaeological excavation too.

The archaeological recovery and restoration

The soil block we analysed was extracted in the necropolis of Villalfonsina, discovered
in the province of Chieti (Abruzzo, Italy) and probably dating back to the VI-IV century
B.C. 22]. It is one among several other blocks excavated in the same and surrounding areas
and containing artefacts of very different use (weapons, finely decorated objects,
clothing accessories). This block contained a decorative, elegant and wide bronze
belt for men, worn by the deceased. It is composed of two engraved buckles and a continuous
metal belt strap, perforated on both upper and lower edges to fasten a leather strip.

If generally small and well preserved objects are directly recovered during an archaeological
excavation, this time an indirect extraction has been carried out: as the find, part
of the grave-good, was quite complex and fragile, a large soil block with dimensions
40?×?15?×?10 cm³ has been removed (Figure 1). This method of extraction allows safe transport and therefore the micro-excavation
can take place in a controlled environment: this helps to preserve even small fragments
of the object, delicate materials such as textiles remains and, possibly, to record
traces of food offerings produced during the burial (e.g. seeds and pollens). Due
to the complexity and fragmentation of the contents of the block, already visible
in a preliminary radiograph (Figure 2), a full CT scan has been planned. For this measurement the soil block has been firstly
enveloped in a plastic film with low radiopacity and then kept in a vertical position
by means of a custom-made plaster support. Thanks to the availability of the dedicated
CT equipment at the Conservation and Restoration Centre, the scan has been performed
by scientists together with conservators: thus, results have been promptly discussed
and the micro excavation and the restoration of the bronze belt have been speeded
up and simplified. The CT data helped the recovery of the object, a difficult task
because of both the metal composition and the burying manner: in fact the artefact,
placed around the body of the deceased, was exposed both to the chemical effects of
organic decomposition, that in an anaerobic underground environment produces acids
and substances dangerous for the conservation of bronze 23],24], and to the pressure of the burial soil. As these stresses affected the curvature
of the belt until it became flat; the heavy corrosion resulted in flaking and splintering
of the metal into small fragments. The restoration process allowed to recreate better
conservation conditions by arresting or delaying the existing degradation processes
and improving the aesthetics of the artefact. Specifically, we proceeded with a stratigraphic
excavation that led to the recovery of all the pieces and archaeological relevant
information. The artefact has been cleaned by using a micro-drill in order to eliminate
the soil concretions and dangerous corrosion products; finally, the object was reconstructed
using pigmented epoxy resin and has been protected with acrylic resins and microcrystalline
waxes.

Figure 1. The soil block. The appearance of the soil block as arrived at the restoration laboratory after its
extraction in the archaeological excavation.

Figure 2. Radiographs. A picture (a) and two radiographs (b-c) of the soil block viewed from different angles.

Experimental setup

The tomographic system used for this measurement is described in detail in Ref. 19]: it is composed of an X-ray tube, a rotating platform and a linear X-ray detector
which scans the projection plane thanks to a high precision mechanical system. The
equipment is installed in a shielded area and it operates remotely through a fully
automated acquisition procedure. For this measurement 540 projections have been acquired:
each one has been obtained by means of a horizontal translation of the detector of
26 cm (at a speed of 2 m/min) and a following rotation of the object (up to a final
rotation of 270°, with one image acquired each 0.5°). The X-ray source has been used
at its maximum tube voltage and power, 200 kV and 900 W respectively, and, to limit
the beam-hardening effects 25], a 2 mm thick aluminium slab has been introduced to absorb the softer X-rays. The
air-cooled tube has been used with cycles of 40 minutes of irradiation and 20 minutes
of cooling to avoid overheating.

A summary of the parameters of the experimental setup is shown in Table 1.

Table 1. Experimental setup

To minimize the penumbra effect due to the focal spot size, the object-detector distance
was chosen as the minimum achievable, while the source-detector distance was set large
enough to obtain good resolution while keeping a reasonable signal intensity.

A non-rotating marker was added in the image field, allowing to check and refine the
horizontal alignment of all the projections. Moreover, several open beam and dark
projections have been acquired for later use in the image-processing stage. In Figure 2 two radiographs acquired at different angles are shown (already corrected using the
open beam and the dark images). The CT reconstruction was performed with a non-commercial
software-utility developed by Dan Schneberk of the Lawrence Livermore National Laboratory
(USA), using the approximation of fan beam geometry and the filtered back-projection
algorithm 25]. The commercial software VGStudio MAX 2.2 of Volume Graphics was used both to visualise
the 3D rendering and to perform the segmentation of the data. Some examples of three-dimensional
rendering are shown in Figure 3.

Figure 3. CT 3D rendering. 3D rendering of the CT volume of the soil block: (a) complete volume; (b) transparency effect of the earth; (c) segmentation of the metallic parts inside the soil block.