Excursions with DAVID
The base station of DAVID and its humble operator Loes is Amsterdam, where the scanning of the experimental typeset, produced by colleague Caroline Jeffra, takes place. The 3D models of the experimental ceramics can be found in the TPW online reference collection of wheel-fashioned pottery. For the archaeological material, DAVID has been traveling around to scan ceramics of the Aegean Bronze Age for the Tracing the Potter’s Wheel project. But DAVID has traveled to other places as well, within Amsterdam and Europe, to boldly explore where no DAVID has gone before: from bacteria in Amsterdam to large objects in Italy.
Encouraged by the success of scanning and recording tiny traces on the surfaces of ceramic sherds, Loes (and DAVID) accepted an invitation by Dr Nils Meiresonne of the Swammerdam Institute of Life Sciences (Faculty of Science, UvA). His challenge? To apply DAVID with Loes’s methodology to scanning bacteria colonies because the recording of colony structures may have valuable future research uses, not only for these colonies but ultimately cell behavior too.
Scanning at such a scale was quite a challenge but the pilot exceeded all expectations: a resolution of 0.00341 mm (vertex spacing) was reached, despite the maximum resolution of the DAVID given as 0.04 mm. This is a promising result, not only for bacterial colonies and other microbiological features but also for expanding the use of 3D scanning within archaeological applications that depend upon recognition and recording of extremely fine or faint traces (use wear or textile analysis). What helped is that the conditions for 3D scanning were excellent at Science Park: a modern concrete building, no public transport vibrations or ambient light, as opposed to the old building with wooden floors in the city center where the archaeology department is housed (ACASA), amidst tram and metro lines.
Scanning results of the bacteria colony in April 2019, Loes was also invited by Prof Marijke Gnade (University of Amsterdam) to scan significantly larger objects in the Museo Nazionale Etrusco di Villa Giulia (Rome). These fragments of architectural terracottas, in the shape of mythical creatures measuring up to 60 cm in height, once protected and decorated the late Archaic Temple of Mater Matuta from Satricum (ca. 500 BC). Some of these terracotta objects were on display and could not be taken out from their showcase, so an ad hoc solution had to be found - to scan with a device from a fixed position – and once again, Loes and DAVID proved a successful team. The resulting 3D models of the terracottas can now be placed back virtually in their original context as part of the 3D reconstruction of the temple (also developed by Loes, a project that started in 2016 and will be published in the edited book Tracing Technology in Autumn 2021).
Scanning small and large objects
This section explains how this type of objects were scanned and offers a solution to scan other artefacts in a similar fashion. One would expect that in order to scan very small objects such as a bacillus subtilis, or, as I tried last week, a malleus (auditory bone) of 2 mm, the 30mm calibration pattern provided by DAVID should be the right choice. But it rarely isn’t. It’s often better to use the 60mm pattern because it’s extremely tedious to obtain a sharp calibration with the 30 mm pattern. For the bacteria measuring 0.5x0.5mm, the scanning option ‘automatic turntable’ with a sequence of 12 scans was selected. With Profile set to Quality (34 patterns) it took 8 minutes to scan. For the bacteria colony I was eventually able to calibrate with the smallest 30 mm pattern. This increased scanning time to almost 16 minutes. Alignment proceeded automatically without problems and it was fused on maximum resolution (vertex spacing), the bacillus reaching 0.00341mm and the colony 0.00259mm.
The large terracotta antefixes (I used the same procedure for large jars and amphoras) were calibrated on the 240mm pattern and scanned manually, per single scan. Calibrating on this size and not a larger size improves also the resolution (more distance between projector and cameras decreases projection strength and camera vision range). Scan Profile was set to Quality. Because the terracottas were bigger than the 240mm, they were scanned in 3 batches of 15 to 22 scans each, lower, middle and upper part of the object. Once the first batch completely covered the surface of the object, for example the lower part, I simply adjusted the scan angle of the bar on which the camera and projector to project on the middle part of the object (but never move the tripod or change the camera-projector distance!), and repeat this until the object’s surface is fully covered by the scans. Make sure enough overlap remains between the scanned sections, in order to manually align them later. Each scanned section should cover, but not exceed, about the size of the calibration pattern, in this case 240 mm.
Scanning in several parts with a 240 mm targeted calibration
One terracotta couldn’t leave the showcase, so DAVID had to move to the showcase instead. As DAVID is designed to operate to scan from a fixed position, this was a challenge. 14 scans were made from different positions, with the tripod dangling on a folding chair and other improvised supports. However, the distance between the object and the scanner was carefully maintained using a measuring tape. As such, distortion was prevented as much as possible. You can inspect the (decimated) resulting 3D model here. Lastly, the aim for scanning these terracottas was very different from that of TPW or the biologist: they would form the basis of a 3D reconstruction and virtual restoration; therefore, an incredible level of detail was not required. That being said, I always try to reach the best quality as possible because you never know who could use the 3D models for future research, to for example forming techniques or use-wear analysis.