ROCK ART EXPERIMENT in GAVRINIS: PRESENTATION, METHODS and RESULTS Marie Vourc’H1, Cyrille Chaigneau2, Serge Cassen3, Valentin Grimaud1
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ROCK ART EXPERIMENT IN GAVRINIS: PRESENTATION, METHODS AND RESULTS Marie Vourc’h1, Cyrille Chaigneau2, Serge Cassen3, Valentin Grimaud1 ABSTRACT While the site of Gavrinis (Morbihan, Brittany, France) is known as one of the most beautiful engraved dolmen of European Neolithic, archaeologists do not seem to have examined closely the technology used to peck those carvings. In 2012, we therefore decided to launch an exper- iment in order to better understand engraving techniques, kind of tools, time spent to peck the hundreds of meters of carvings found in the monument and to highlight a possible “chaîne opératoire” of the engraving process in our context. This article introduces the protocol used, recording methods, engraving techniques and tools. We shall also present the first results and their impact on our understanding of Neolithic engravings, especially when considering time factor as a new part of the monumentality. RÉSUMÉ Le site de Gavrinis (Morbihan, Bretagne, France) est considéré depuis longtemps comme l’un des plus beaux dolmens gravés du Néolithique européen Pourtant, aucune étude technique n’a jusqu’à présent été réalisée autour des gravures de cette sépulture Notre expérimentation démarrée en 2012 avait pour objectif de mieux comprendre les techniques de gravure, d’évaluer les types d’outils potentiels, le temps nécessaire au piquetage des centaines de mètres gravés sur les dalles du monument, et de déterminer une possible chaîne opératoire des gravures du site Cet article présente le protocole mis en place, les méthodes d’enregistrement, les outils utilisés et les techniques mises en œuvre Les premiers résultats de cette étude élargissent notre champ de compréhension des gravures néolithiques, notamment par l’introduction du facteur temps comme une dimension nouvelle de la monumentalité Keywords: experiment, rock art, pecked engravings, surface shapes. Mots-clés: expérimentation, art rupestre, gravures piquetées, états de surface 1. Laboratoire de recherches Archéologie et Architectures (LARA), Université de Nantes 2. Musée de Préhistoire James Miln – Zacharie Le Rouzic, Carnac 3. LARA, CNRS/Université de Nantes [email protected] Butlletí Arqueològic, V, 40 (2018), ISSN 16955862 (p 97104) 98 MARIE VOURC’H, CYRILLE CHAIGNEAU, SERGE CASSEN, VALENTIN GRIMAUD Context and aim of study Gavrinis is a Neolithic passage grave, situated in the Vannes and Auray rivers estuary in Brittany (France). The monument has the particularity to have an unusual concentration of engravings. They have been well known for more than 150 years and recently recorded by S. Cassen and his team with new innova- tive methods (lasergrammetry, photogrammetry, rotation light, decorrelation) which give us new possibilities to study carvings closer (depth of impact nega- tives, superimposition, relative chronology) (cassen et al. 2014). This experiment is a joined project between the Laboratory for Archaeolog- ical and Architectural Research (LARA) of Nantes University, the Carnac Mu- seum for Prehistory and the manager of the site (Cie des Ports du Morbihan). The lack of knowledges concerning tools, techniques and gestures used to engrave as well as the time needed, led us to conduct a technical study based on experiment and to reproduce one of the slabs of the monument, L6, with a dual purpose: research and mediation. This article only deals with the research side. It was central to elaborate a fairly strict protocol for the recording of variants. This only allows the verification and reproducibility of our experiment1. The aims of the experiment involved in 2012 were then to determine the tools for engraving on hard rocks such as granite, orthogneiss and quartz (mor- phology and nature of the rocks, possible use of haft), to list the various tech- niques and possible “chaînes opératoires” for engraving and surface preparation, and finally to establish time ratios according to the nature of the rock block and the selected techniques. In 2013, it became clear that we couldn’t ignore a further study of the surface shapes. Indeed, as we progressed in the experiment, this work of preparation of the raw slab became a central point, allowing us to perceive a beginning of “chaîne opératoire” (extraction and transport excepted): almost all of the slabs of the Gavrinis monument have been prepared, if not entirely shaped. Experiment and results The initial aim was the reproduction of the L6 slab, but we had to test dif- ferent material on other rock blocks beforehand. 1. All recording tables relatives to the experiment as well as the previous reports (2012-2017) are available on request to the LARA, Université de Nantes, Nantes. Butlletí Arqueològic, V, 40 (2018), ISSN 16955862 (p 97104) ROCK ART EXPERIMENT IN GAVRINIS: PRESENTATION, METHODS AND RESULTS 99 Tools and techniques selection We had to find hammerstones that could affect the granite (and others hard rock types), common in the region, and represented in archaeological sites, that is mainly quartzite, quartz and quartz of dreikanter type (fig. 1c), naturally shaped by the wind (Leprovost 2009). Stones of other types were also collected and tested to confirm or invalid the assumption of quartz and quartzite cobbles use. For surface preparation, according to our experiment, the only effective technique is direct percussion: the tool is directly thrown onto the block to de- tach flakes (there is no intermediate tool). Three operating modes were tested: “simple” direct percussion (the tool is held in the hand), pendular direct percus- sion and hafted direct percussion (Vourc’h et al. 2014). The efficiency of cobbles between 500 and 800 grams in quartz, quartzite or wind-shaped quartz used with simple direct percussion on horizontal slabs is confirmed by the experiment (fig. 1a, b, c). The stone is lightly handled, using the rebound energy generated during the impact against the rock surface. We have also tested pendulum percussion, a rather heavy system in its implemen- tation, but extremely effective: it allows removing the surface (then vertical) in a record time (fig. 1d). The impact negatives are very large, and the rendering characteristic: lines of engraving are clearly visible due to the pendular system. With hafted direct percussion, the stone tool is much smaller, hafted into a dear antler which is prolonged by a vegetal wood handle; this technique was mainly used for the final stages of the surface preparation (fig. 1e). For the reproduction of the iconographic program of the L6 slab, we tested direct, hafted direct and indirect percussion. In this last engraving technique, the intermediate tool (here simple small quartz pebble or hafted pebble) is in contact with the rock surface and the engraver hits this tool with a soft hammer (hard vegetal wood). This technique has been preferred because more effective, more precise and producing impact negatives that more closely match those of archaeological engravings. The most efficient intermediate tools consist on small wind-shaped pebbles hafted into cervid antler (fig. 1f). Level of preparation and surface shapes After having found a slab for the L6 reproduction, we had to prepare the surface in order to clean up the surface that was sliced and unfit to be engraved (with iconographic program). In 2016, we went further in the study of the surface shapes of a block, resulting from the different levels of surface preparation before engraving of signs. This study, conducted by two different operators, led us to build a first catalog of surface shapes in 3D by photomodelling (Grimaud et al. this vol- Butlletí Arqueològic, V, 40 (2018), ISSN 16955862 (p 97104) 100 MARIE VOURC’H, CYRILLE CHAIGNEAU, SERGE CASSEN, VALENTIN GRIMAUD ume). This work was carried out on a block of fine-grained granite (of Carnac type) and on an orthogneiss block, on square surfaces of 100 cm² to facili- tate the computation of time ratio (fig. 2). Simple quartz cobbles (sometimes wind-shaped quartz) were used as hammerstones. Regarding the determina- tion of the levels of preparation, we took into account the tools used, the hit force, the size of impact negatives, the percussion time and the rendering after the various phases of percussion. The engraver’s size (gender, muscular section) did not prove to be a relevant character in the measurement of time ratios. We were able to distinguish 5 levels of preparation that fall into two distinct stages: cleaning (scouring) and regularization. The time needed for the prepara- tion (last level) is around 30 minutes for a 10 cm square (100 cm²). Levels 1 and 2 correspond to the cleansing of the surface, using direct per- cussion with strong hit (ample gesture) and large hammerstones (cobbles be- tween 500 and 800 g). The aim is to blow up the unstable irregularities of the rock support. The rendering corresponds to large centimetric negatives distrib- uted randomly on the block surface. For level 1, we have no archaeological equivalent. For level 2, there are archaeological parallels, especially in Gavrinis, on the slab on the left just before the entrance to the corridor, which is partially prepared and has very large impact negatives. Levels 3 and 4 correspond to the second stage: regularization in small areas in order to make the surface flatter. The percussion is softer and the rendering corresponds to smaller, millimetric negatives. At level 3, the negatives are still quite large and the slab still has some irregularities. At the end of level 4, the negatives are much smaller, rounder, the slab is completely prepared. The hit is even softer, using direct or hafted direct percus- sion. Level 5 is an additional degree of preparation: abrasion or polishing. The gesture is extremely tiring in a short time (no more than 3 or 4 minutes in a row). This preparation technique was tested because some archaeological slabs presents this surface shape in Gavrinis, and we wanted to understand if abrasion before engraving allowed a better visual rendering of the signs.