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Non-destructive investigation of large wooden statues by means of X-ray Computed Tomography

Team Leader: Prof. Franco Casali.

University of Bologna Research Group: Franco Casali, Maria Pia Morigi, Matteo Bettuzzi, Rosa Brancaccio, Vincenzo D’Errico.

Partner/Collaborations: UniBo, INFN (Turin) Conservation and Restoration Center "La Venaria Reale" (Turin) University of Bologna.

Context and objectives

Tomography comes from the Greek word tomos, which means "section".

This kind of non-destructive analysis is used to "virtually" cut an object and see inside it using penetrating radiation, such as X-rays, gamma rays, neutrons or ultrasounds. This method, commonly called CT (Computed Tomography), is used in medicine, archaeology, biology, geophysics and other sciences.

In case of wooden artworks X-ray CT is able to give many information on the technique used to make them and on their conservation status. For example, if the work of art is composed by different pieces of wood, it is possible to understand how they were assembled, their total number, their orientation with respect to growth rings and consequently the stress forces acting on them.

In the following we will focus the attention on the CT analysis of two Japanese wooden statues, carried out on-site at the Conservation and Restoration Center "La Venaria Reale" near Turin.

The first statue is called Kongo Rikishi (Fig. 1 - Gallery), which means "the Guardian of the Temple", and is over 2 meters tall. It dates back to the Kamakura Period (XIII century) and was realized by assembling many pieces of japanese cypress wood (hinoki) by means of a complex technique called yosegi-zukuri. The second statue, called Tamon Ten (Fig. 2 - Gallery), dates back to the EDO period (XVII century) and symbolizes the "Guardian of the North".

Methodologies and equipment

The CT analysis of large works of art represent a challenge that can be overcome by means of dedicated equipments, capable to perform on-site investigations. Several years ago our research group developed a transportable CT system, expressly conceived for the investigation of large objects.

The system is composed by:

  • a transportable X-ray tube (200 kVp), movable on a vertical translation axis;
  • a rotating platform, where the object is positioned;
  • a detector which can be moved both horizontally and vertically, by means of two translation axis. The detector is a CCD-based system, with a Field of View (FOV) of 450×450 mm2.

If the FOV of our detector is smaller than the size of the object to be investigated, the so called tile scanning technique is adopted. Tile scanning is the term experts use when a CT is performed step by step by moving the detector to cover the object projection with a certain number of different frames.

Results

The CT analysis of the two statues has given very interesting information on the technique used to assemble the different pieces of wood and on previous restoration works. In fact the CT images put clearly in evidence the types of joints as well as several discontinuities between the wood pieces and the presence of putties and metallic elements, such as nails and screws, used in previous restoration works to assemble the pieces with damaged joints.

It has also been possible to assess the depth of the cracks visible on the surface and to notice the presence of the pith in the bearing element of the statue, that is an inner wooden upright of rectangular cross-section which holds up the structure from the right shoulder to the heel of the right foot. In Fig.s 3, 4 and 5 several CT slices are reported together with 3D reconstructions of different portions of Kongo Rikishi.

The CT images of Tamon Ten head, shown in Fig. 6, furnish very interesting information. For example, from the axial and sagittal sections in Figs. 7 and 8, it is possible to notice that the face is probably made with a different kind of wood, in fact its radiopacity is clearly different from that of the other pieces of wood. Moreover it is the only element that has been attacked by woodworms.