Alice Clinch, Cornell
Jari Pakkanen, Royal Holloway University of London
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Standard techniques of photogrammetry can be used to produce 3D models of archaeological objects relatively quickly and at a low cost, requiring only digital images, photogrammetry software and a computer. Focus-stacking macro photogrammetry requires in addition a macro lens, further software and quite a bit more time, yet promises to deliver more detailed models than standard photogrammetry. Previous case studies have demonstrated the benefits of the method for certain materials. However, its potential for the study of archaeological plasters remains unexplored. This paper investigates the documentation and analysis of fragmentarily preserved ancient plasters: when does the additional investment in equipment, software and time bring sufficient benefits to be justified?
In focus-stacking, an image used in modelling is stitched together from a number of photographs in which only a small part of the object and background is in focus. The depth of field of a photograph can be increased by reducing the aperture of the lens: this technique is often enough to keep most of the object in focus and produce a good 3D photogrammetry model. However, if more details are required, a macro lens with a very shallow depth of field is needed. Focus-stacking can then be used to produce a highly detailed composite image which is fully in focus. The equipment used include a mirrorless camera with a cropped sensor and a full-frame DSLR tested with both an old manual and a current autofocus macro lens. The main aim of archaeological 3D documentation should be to produce models for the research of the objects and for the wider public dissemination of the topic.
The composition and durability of archaeological plaster fragments vary. They are often fragile and handling them too often should be avoided. 3D documentation using photogrammetry can be used to reduce the number of occasions the pieces are physically handled in macroscopic studies, and focus-stacking offers an opportunity to produce a more detailed model in the first instance. Compositional layers can be analysed using the digital models and their dimensions can be more accurately determined than by taking manual measurements. The plaster fragments analysed in this paper are from 18th-dynasty contexts at Amarna and from a fifth-century BCE residential building at Naxos in Sicily. The difference in chronology and context demonstrates how photogrammetry can capture the differing trends in plaster technology. The plasters presented in this study are housed at the Petrie Museum of Egyptian Archaeology at University College London, and the storerooms at the Giardini-Naxos Archaeological Park in Sicily, where lighting conditions are variable and this paper deals with the challenges of such settings.