The Agris Helmet is a masterpiece of the Celtic art made in the middle of the 4th century A.D. It has been found near Agris, France, in 1981 during an archeological dig under the supervision of José Gomez-de-Soto. This composite object is made of an iron cap covered with bands of bronze. The bronze is itself covered with pure gold leaf, with embedded coral decorations attached using silver rivets. The Helmet was discovered with one cheek guard which had similar materials and designs. Considering this information, the cap would have been used for display rather than worn in battle.
The helmet, discovered in pieces, was restored in Mainz during the 1980th and is now held by the Musée d'Angoulême in Angoulême (France).
The present project led jointly by the AOROC department of the ENS and the C2RMF and supported by PSL, was an opportunity to make complementary work on the Helmet to document it thoroughly.
Our aim was threefold. Firstly, we wanted to document the Helmet in such a way that future studies would be doable even without physical access to the original piece. Then, we were willing to search the best 3D technique and practice for this complex piece. The level of details and the brilliant aspect of the surface were the perfect challenges for our 3D systems. In addition to these technics, we wanted to enhance the knowledge about the object (material and technical identification) and provide new data for archaeology and art history.
We began the study with a complete photographic coverage of the Helmet thus highlighting every single visible detail on its surface. In order to go further concerning the structure of the object itself, we chose to use X-Ray technology. This method allowed us to see behind the surface and understood the organization and construction of the Helmet.
Furthermore, multiple techniques of photogrammetric imaging and 3D digitalization were used to obtain the best set of data. Leadingly, we performed a photogrammetric coverage under artificial lights followed by another under natural light. Thereafter, we used two 3D scanners technologies, one based on structured light and the second based on laser technology. The resulting 3D models were compared in precision and acquisition convenience (simplicity of devices and acquisition time).
While the photogrammetric models were perfect to easily visualize the texture and color of the object, its degree of precision left something to be desired.
On the contrary, the 3D models obtained by scanning technologies were effective for the purpose of getting complete metric and volumetric values.
For the study of the Helmet, the high-resolution models provided information about tools marks and other traces of the production techniques. Moreover, mesh comparing provided new data for the matching of supposedly identical items on the surface. The results of those studies were particularly instructive for the historians of the Celtic period and gave new possibilities to the researches for the work on this precious object.
X-ray imaging is one of the oldest technic used in art analysis. X-Radiography is now completed with tomography. Used on bronze sculptures, it gives unrivalled information on state of conservation or fabrication process. The treasure of Bavay, discovered in 1969 in the north of France, is a hoard constituted by 371 bronze objects dated from the 1st to 3rd century AD. Among objects of very diverse shape and function, an exceptional set of Roman statuettes was discovered. Two Mercury statuettes were studied and, apart that they are hollow, their characteristics are very different. The metal walls are thin and even for the Lysippean Mercury and could only be done by the indirect lost wax casting process. On other hand, Indigenous Mercury still own its inner refractory core made of clay, whose shape is a very simple evocation of the statue. Here the direct lost-wax process was used, with first the core shaping, on which wax was directly carved. In some cases, spatial information is difficult to read on flat 2D radiographies, so complementary images are made thanks to 3D tomography. Reconstruction algorithms generate a 3D object issued from 720X-Ray images, showing the internal and the external surfaces. Virtual cutting can show any part of the object, allowing a detailed visualization of the inner parts. For example, Jupiter statue in tomography showed on the inner surfaces metal infiltration and a secondary casting of a lead-tin. Other analysis can be added to tomographic model: the surface 3D acquired with a scanner can add more precision on external surface representation: it was made for the Jupiter statue. In some cases also, photography for surface color analysis, or X-ray fluorescence chemical maps could also be placed on 3D models for a better materials localization.