For a Digital Archaeology of Coins? A RTI project on Celtic coins kept at the BnF

Lire ce billet en français : Pour une archéologie numérique des monnaies ? Un projet RTI sur les collections celtiques de la BnF

Foreword, by Dominique Hollard (BnF curator of Celtic Coins)

The research project initiated by A. Baron Tacla from Universidade Federal Fluminense (Rio de Janeiro) immediately caught our attention. The technology used (RTI) seems indeed particularly well adapted to the Celtic coinages. They are as marked by a rather complex iconography as many non figurative types. Blanks are usually smaller than engraved dies employed as punches. The use of the RTI thus appears as a privileged way to better understand these objects, particularly those kept at the  BnF – the most important Celtic coinages collection in the world, 11,000 coins – available to be used as a material for the development of this promising technology.

Stater, billon, Curiosolites, 6,40g (BnF, MMA, 6586)

In November-December 2017, I had a stay at the BnF in order to collect data for my current project on Iron Age coinages in the digital era. This project is based on the application of Reflectance Transformation Imaging (RTI) to Iron Age coins that belong to the Brazilian National Historical Museum (MHN). I started the project asking myself whether, besides considering the usage of new techniques, one can look at Iron Age Coinage under a new light. Digital visualization, and in particular 3D technologies offer us the means to not only register, preserve, and display artefacts and images, but also to analyse, compare and restore them. Contrary to what some scholars believed1, 3D technologies do not delude us, but rather help us to test working hypotheses and lead us to a better understanding of archaeological finds, both from a cognitive point of view (ie. from perception) and from its production.

It would be naive to consider that 3D models reconstruct the past or constitute an exact replica of past structures, artefacts or environment. Those are in fact interpretations of the past based on data extracted from primary and secondary sources2, as it is usually done in any historical or archaeological research. The level of detail and type of design that is considered in a model directly depends on the research objectives. For example, 3D models can be used as tools for historians and archaeologists to understand possibilities regarding sensorial perception of space, with varying simulations of movement and visual perspectives – used even to understand the interaction between artefacts and structures in certain places. Digitized artefacts can then be viewed, presented and preserved with a level of detail never seen before. For objects that are corroded, 3D technologies can contribute to their preservation, as well as to the study of their use and wear along time. Moreover, digitized objects can be ‘handled’ in different ways. Not only can they be turned and seen in different positions, but also with different light incidence, which can, for instance, highlight details on the objects’ surface that cannot be usually perceived. These technologies can even unveil new shapes in the design that are important for the comprehension of past craftworks and patterns of consumption.  For such cases, RTI is a valuable tool.

H-RTI at the BnF

Along my stay at the BnF, I have been interested in its Celtic coin collection. Here, I have been working with Highlighted RTI (H-RTI), a method that was developed by Mark Mudge, Marlin Lum e Carla Schroer3 from the Cultural Heritage Imaging. H-RTI is a very accessible method, which requires basic photographic equipment – i.e. DSLR camera, flash, radio flash, remote control, Kodak Grey card 18%, and tripod – a piece of string and a black or red reflexive ball.

H-RTI setup at the département des Monnaies, médailles et antiques, BnF

The photos were taken from a stationary position and I moved the flash around the object to obtain different light incidence. In my research, I decided to work with 48 images per coin side taken from 12 different positions around the objects at the angles of approximately 0°, 30°,65°, and 90° degrees. The photos were obtained in RAW format; they were then converted to JPEG for processing in the RTIBuilder software, which compiles the image data into a PTM or RTI file.  After processing, the image can be viewed in the RTIViewer software. The next step will be to annotate the images in CHER-OB, a software created by the Yale computer graphics team. All the software is free and results from academic research in the area of computer science applied to cultural heritage.

The RTI method interests

RTI, a method of computational photography, was created by Tom Malzbender and Dan Gelb, researchers at the HP Laboratory in 2001. Initially called Polynomial Texture Mapping (PTM), RTI captures an object image and colour in different light positions. It enables one to create interactive object re-illumination and run mathematical simulations of surface enhancement and colour attributes, which emphasize colour and texture for a better visualization of the artefact’s surface and for apprehension of its geometry.

Quinarius, silver, Sequani, type Q. DOCI SAN.F, 1,97g (BnF, MMA, 5410)

RTI became rather popular among archaeologists, and particularly numismatists, since the method is especially interesting for working with almost flat surfaces, such as coins, and it does the best surface mapping. Nonetheless, RTI does not create 3D images per se, but a 3D simulation, based on the calculation of the variation of light incidence per pixel in each photo4

This method has drawn much attention, for it allows better understanding and unique visualization of the form, of the materials used, besides the decoration, colouring and drawing applied to the objects. In fact, it highlights any type of surface changes, what makes it particularly interesting for observing repairs and further alterations in design. Often these are changes and details that cannot be observed in photographs and might be relevant for analysis. In other words, it allows one to see the changes made on the objects along their lives; this is of great interest for understanding the processes of destruction of images and inscriptions, for example; in a sense, the objects biography.

Applications

Our use of 3D technologies must be guided by our research questions. For instance, digital technologies can help us further understand ancient craftworks. In numismatics, RTI has contributed for the further development of traditional research focuses, such as:

  • Identification and classification of monetary types;
  • Die studies;
  • Reconstruction of series;
  • Reconstruction of broken pieces;

RTI has also been widely used to digitize collections that are being made available to the public, such as that of Pallazzo Blu. Additionally, RTI is equally suitable for corrosion studies, since one can easily observe the state of wear and corrosion of coin image and surface, as well as the identification of surface damages. I am particularly interested in that, as well as on the analysis of visuality on those pieces. RTI can actually largely contribute to the further understanding of image compositions and techniques in Iron Age Coinage, such as in the use of rabatment5 or in the case of 3D illusions found on Curiosolitae monetary types.

Adriene BARON TACLA

Citer ce billet : abarontacla, "For a Digital Archaeology of Coins? A RTI project on Celtic coins kept at the BnF", in L’Antiquité à la BnF, 19/02/2018, https://antiquitebnf.hypotheses.org/1746, [consulté le 26/05/2018].

 

  1. EITELJORG, H. 1998.  Photorealistic Visualizations May Be Too Good. Computer Technologies for Archaeologists and Architectural Historians (CSA) Newsletter 11 (2). Last access: 08/04/15. []
  2. STANLEY, D. J. 2003. Computers, Visualization, and History: how new technology will transform our understanding of the past. Armonk, New York: M.E. Sharpe. []
  3. MUDGE, M.; MALZBENDER T.; SCHROER, C.; LUM, M. 2006. New reflection transformation imaging methods for rock art and multiple viewpoint display. In: VAST’06: Proc.7th Int. Symp. Virtual Reality, Archaeology and Cultural Heritage (Cyprus, 2006).   Eurographics Association, p. 195–202; DUFFY, S.; BRYAN, P.; EARL, G.; BEALE, G.; PAGI, H.; KOTOULA, E. 2013. Multi-light Imaging for Heritage Applications. Swindon: English Heritage. Acessed in: 10/01/2017. []
  4. Bentkowska-Kafel, A. ; del Hoyo Melendez, J. M. ; MacDonald, L. W. ; Mathys, A. ; de Almeida, V.M. 2016. Colour and Space in Cultural Heritage in 6Ds : The Interdisciplinary Connections. In: Campana, S.; Scopigno, R.; Carpentiero, G.; Cirillo, m. (eds) CAA2015: Keep the Revolution Going: Proceedings of the 43rd Annual Conference on Computer Applications and Quantitative Methods in Archaeology. Oxford: ArchaeoPress, p. 958-959. []
  5. OLIVIER, L. 2014. Les codes de représentation visuelle dans l´art celtique ancien. In:  GOSDEN, C.; CRAWFORD, S.; ULMSCHNEIDER, K. (Eds.) Celtic Art in Europe Making Connections. Oxford: Oxbow Books, p. 39-55. []

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