Cultural HeritageDigital preservation and dissemination of ancient lithic technology with modern micro-CT
Graphical abstract
“Virtual” online museums created using X-ray micro-CT will widen scientific and public access to valuable, rare and fragile cultural artefacts, such as ancient stone tools.
Highlights
► Ancient flint stone tools are valuable for studying the evolution of human culture and cognition. ► Access to some museum collections is restricted because flint tools are rare and fragile. ► “Virtual” tools (including missing flakes from refits) can be created using micro-CT and rendered in three-dimensional perspective. ► “Virtual” tools are completely interactive and can be viewed and measured as though in the hand. ► Museums could widen scientific and public access to rare cultural artefacts.
Introduction
Lithic technology refers to a broad array of methods used to produce usable tools from various types of stone. Stone tools are commonly made from flint via the process of hard hammer percussion. A flint core is repeatedly struck with a harder stone to break off sharp flakes (Fig. 1). The flakes are often retouched to shape, sharpen or blunt the edges for improved function. Although some cultures still use stone tools today, most lithic technologies are associated with prehistoric societies or cultures and extinct species related to modern humans. Understanding how lithic technology was conceived and developed can help reveal when and how modern human cognition and culture evolved [1], [2], [3], [4]. Hence, museum collections of lithic artefacts have great historical, social and scientific significance. Flint is a rather stable, but somewhat brittle, material. Its handling and replication are likely to alter or remove microscopic use-traces and residues, which are critical for understanding how the tools were used, whereas the artefacts can potentially be damaged during their transport between institutions. Thus, it is difficult to compare collections stored at different institutions.
Artefacts have traditionally been illustrated with drawings [5] and/or photographs [6] alongside written descriptions. Drawings can be subjective and open to interpretation [7]. Photography is more objective, but it does not always show the key technological features of the artefact. Broadening the access to lithic collections would support the analysis of temporal, geographical and species-specific variations in morphology and technology. One way to accomplish this would be to create computerised “virtual” tools that could be shared, thereby widening the access while maintaining the safe-keeping of the original artefacts. A suitable virtual technique must be able to record the size and shape of a flint and, in particular, its surface topology. The cracks, fissures and ripples on the surface of a flake provide a record of the flaking process and can be used to determine whether a stone was intentionally modified by humans (i.e., an artefact) or created naturally (i.e., a geofact) [8]. The virtual technique must also be non-destructive [9] and non-contact. The resulting data set must be fully three-dimensional and should provide the means to manipulate, view and measure a virtual specimen as though it was held in the hands.
A number of techniques have been applied to record virtual data in cultural heritage contexts (see Table 1 for some examples) [10]. Almost all the listed techniques are non-destructive, except serial grinding [11], [12] and casting. Casting is likely to leave residues on the convoluted surfaces of an artefact and might also damage the flint when removing the specimen from the mould. Similarly, contact measurement with calipers or a micro-scribe may also damage artefacts. Drawing, microscopy and photography require little handling but do not produce fully three-dimensional (3D) data sets that can be manipulated on a computer. It is possible to visualise objects in 3D by generating stereo-anaglyphs or digital elevation models, but this can only be done for one surface at a time. The only systems capable of producing true 3D models of specimens are laser (stereo-) photogrammetry and volumetric scanners, such as magnetic resonance imaging (MRI) and computed tomography (CT). Laser scanners emit light and detect the reflections to map the surface of an object as a point cloud. Flint tools are not suited to this technique because the surfaces are reflective and translucent (see [13]). Reflective surfaces create noisy point clouds, and translucency causes subsurface scattering that degrades image quality, particularly near sharp edges. Photogrammetry refers to the use of two-dimensional (2D) photography for reconstructing computerised 3D surface maps [14] of substrates [15], buildings [16] or objects [17]. Taking a photograph projects 3D objects into 2D, and photogrammetry reverses this process. Flint is well suited to the technique, which can produce realistic colour by mapping the photos onto the 3D computerised reconstruction [18], [19], [20]. However, where cracks, fissures or flake scars infold the laser and photogrammetric instruments cannot penetrate the flint to create an image. Hence, convolutions create gaps in the reconstructed 3D surface and can preclude the effective scanning of some artefacts.
Furthermore, photogrammetric and laser techniques cannot record the internal features of a refitted group. Refitting is the process of reconstructing a flint block by replacing flakes onto the block in the order in which they were removed [21], [22]. Particular pieces may be missing from reconstructed blocks, and they may have been carried away from the location where knapping took place. With volumetric scanners, it should be possible to make 3D reconstructions of voids and thus create virtual models of missing pieces. While the most probable forms of the missing pieces can be loosely inferred from the shapes of any voids and the orientations of the flakes that preceded the missing material, it is impossible to be sure of the exact form of the missing pieces without modelling them. For instance, the orientation of preparatory flakes preceding a missing piece might suggest that methods had been deliberately employed to produce pointed forms. Modelling the missing flakes allows the determination of whether particular types of end product were intentionally produced and used or whether the attributes of the missing pieces were more variable. Such observations have implications for understanding the behavioural and cognitive nature of particular technological strategies.
The MRI technique, using magnets to image hydrogen content, is not suitable for dense stones because of their low water content. In contrast to laser, photogrammetric and MRI scanning, CT uses penetrating radiation to image an object and can therefore reconstruct objects that are reflective and translucent and have a convoluted infolding surface and a high density. To date, numerous studies have employed CT to image rocks, stones, fossils, bone and teeth [23], [24], [25], [26], [27].
Micro-CT is a powerful non-destructive imaging modality for the full-volume visualisation and inspection of an object in 3D [28], [29], [30]. The present study aimed to determine whether CT can be used to create “virtual” artefacts and whether the data can be used to provide wider access for scientists and the public. In particular, this study assesses (a) whether CT could capture the fine surface topology created by the knapping process and (b) if flakes missing from refits can be visualised. The findings are discussed within the wider context of museum preservation and research strategies.
Section snippets
Sample
The efficacy of CT for visualising flint artefacts was tested using a single flake and a refitting group of flakes. The single flake (Fig. 1a) was selected from a >0.78-million-year-old (mya) assemblage excavated at Happisburgh (Norfolk, UK). The assemblage comprises simple flakes, flake tools and cores, assigned to a ‘flake and core’ or Mode 1 technology. The earliest appearance of Mode 1 technology was the Oldowan Industry ≈2.6 mya in East Africa. By ≈1.7 mya, Mode 1 assemblages also existed in
Results and discussion
Stone artefacts provide valuable insight into the evolution of human behaviour and cognition [2]. Using CT scans to provide 3D rotatable models of stone tools opens the access to information about stone tool technology and typology and allows the critical assessment of the human or natural processes involved. Furthermore, CT scans can be an important research tool to investigate the nature of missing pieces from refitting groups.
The Happisburgh flake (Fig. 1a) has a proximal striking platform
Acknowledgements
The authors would like to thank the British Museum for funding the Happisburgh excavations. We would also like to thank the following people: C. Stockton and S. Stockton for their practical support; B. Farrow and P. Frew (North Norfolk District Council), E. Couzens, the Lomax family (local landowners) and M. Kerby (North Norfolk Coastal Concern Group) for permitting and facilitating the excavation; J. Roylance and M. Appleford for the plant hire and machining and Rob Kruszynski for arranging
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