Documentacion y análisis de un cepo de ancla romano y sus elementos iconográficos y epigráficos sellados

Abstract

Resumen: El objetivo de este trabajo es presentar la metodología de documentación y análisis llevada a cabo sobre un cepo de plomo de época antigua perteneciente a la colección de cepos del Museo de Cádiz (Andalucía, España). La eliminación de los diferentes estratos de alteración que cubría la superficie durante los tratamientos de conservación y restauración dejó al descubierto un conjunto iconográfico y epigráfico inédito, así como posibles huellas de fabricación y uso. Sin embargo, el estado de conservación deficiente de la superficie original no permitía visualizar los detalles en su conjunto mediante técnicas convencionales, por lo que se decidió el empleo de la documentación fotogramétrica y posterior tratamiento de los modelos mediante diferentes herramientas de análisis en Sistemas de Información Geográfica (SIG) como QGIS y procesamiento de nubes de puntos (CloudCompare). El análisis tridimensional (3D) viene siendo empleado con éxito en la documentación de detalles superficiales sobre objetos arqueológicos. La combinación de los métodos tradicionales con las ventajas de la fotogrametría ha demostrado una mejora sustancial del método. Los resultados obtenidos gracias a la metodología empleada sobre la pieza objeto de esta investigación han revelado detalles microsuperficiales del conjunto iconográfico y epigráfico imposibles de apreciar mediante técnicas de documentación convencionales. El bajo coste y la facilidad de aplicación, así como las posibilidades de procesamiento posterior de los datos obtenidos con diferentes programas, hacen de esta metodología una herramienta muy útil frente a otras técnicas. Los datos microsuperficiales obtenidos, junto con los obtenidos mediante la reconstrucción 3D, aportan una valiosa información para la interpretación del objeto. Las tareas llevadas a cabo también plantean una metodología de estudio eficaz para el análisis de otros cepos de plomo, una tipología de material muy abundante en colecciones de museos a lo largo de todo el Mediterráneo y parte del Atlántico y cuyo estudio permitirá conocer más sobre las anclas de época antigua, sus características morfológicas, procesos de fabricación, elementos epigráficos e iconográficos, mecanismos de alteración, etc. Abstract:This paper aims to present the documentation and analysis methodology carried out on a lead trap from the ancient period, which belongs to the collection of traps in the Museum of Cádiz (Andalusia, Spain). The anchor stock had some interesting characteristics for this research: on the one hand, from the point of view of conservation and restoration, due to the alterations it presented; on the other hand, from a historical and archaeological point of view, it showed signs of reliefs on its surface hidden under the alteration products. After removing different layers of alteration that covered the surface during conservation and restoration treatments, an unpublished iconographic and epigraphic programme, as well as possible marks of use and manufacture were revealed. The poor state of conservation of the original surface made it impossible to visualise the details as a whole, so the authors applied photogrammetric methods, and subsequently processed models using various GIS analysis and point cloud processing software. Two photogrammetric models (in Agisoft PhotoScan) were made to document the trap in general: one prior to the conservation and restoration process; plus a second three-dimensional (3D) model once the surface had been cleaned. The purpose of the second model was to visualise the reliefs programme in general, as well as the various surface details. The first complete 3D model of the object was used to perform a virtual reconstruction of the anchor including the elements that did not preserve, using a 3D modelling program (Blender). Nine areas of the stock surface were selected for the analyses of the various iconographic and epigraphic features, which were documented and processed in Agisoft PhotoScan. The Digital Elevation Model (DEM) and point cloud models were then processed with different analyses tools in Geographic Information System (GIS), such as QGIS also including point cloud processing software (CloudCompare). Our results document a piece of highly interesting information from its surface consisting of reliefs of four dolphins; there are at least four rectangular stamps: two of them with possible inscriptions, plus an anthropomorphic figure. Thanks to the comparative data, one conclusion is that the four dolphins were made with the same stamp during the stock manufacturing process. Furthermore, the authors were able to reconstruct the dolphin stamp, partially preserved in each of the reliefs, by unifying the 3D models, thus revealing the original set. This system of stamping by means of reusable dies is well known in other elements such as amphorae, but has not been studied in the specific case of lead traps. Regarding the epigraphic elements, the 3D documentation methodology revealed numerous micro-surface details, not visible under conventional documentation techniques, which could help specialists to interpret these inscriptions. Although they have not been analysed in this research, its documentation has facilitated observing surface details that could refer to the manufacturing processes (moulds and tools) or the traces of use, providing historical information on this object. Moreover, the virtual reconstruction of the anchor has promoted the formation of hypotheses on the anchor dimensions and original appearance. The different tools used, such as raster analysis through shadow mapping and point cloud alignment, proved to be very effective. They have fulfilled the established objectives and have helped to design a possible analysis methodology for future lead traps with decorative elements. These types of artefacts recovered from underwater sites are very common in museum collections. In many cases, their state of conservation and the difficulty in handling them due to their size and weight make it difficult to document surface details. In this case, the multidisciplinary work of conservation and 3D documentation allows for high-quality documentation that is easy to access and exchange between researchers. The combined use of photogrammetric techniques with virtual RTI (V-RTI) provides a non-invasive method for the object, low cost and easy processing compared to other conventional methods.