Since 2007, more than 450 pieces of inscribed medieval slateplates have been found to date during the still ongoing excavation of the Archaeological Zone Cologne. Most of these slateplates come from the fire debris layers of the so-called plague pogrom of 1349. The mostly Hebrew script and images were scratched onto the surface with a pointed stylus. In the process, the fine incised lines are sometimes so pale that they can hardly be seen with the naked eye or only under obliqued light.
In April 2017, a project took place in cooperation with the University of Leuven for the purpose of scanning these boards using the Portable Light Dome System (PLD).¹ The aim was to make the contents of the panels more visible and readable.
Among the systems of digital finds documentation, PLD scanning is relatively new in the field of historical data processing, so that the implementation of the project is to be understood as a test of the technology.
A detailed introduction to the different recording methods can be found in the report with the case studies.
The aim of the project is scanning the slateplates and their subsequent inclusion in the database for the inventory of the excavation finds. In this way, the PLD system can be used in a completely new way to help scientists with the digital archiving of historical finds. It is to be tested whether illustrations and the like can be seen with the naked eye. The aim is to test whether images and the like, which were not visible to the naked eye, become recognisable through the scan.
In order to be able to use the PLD system, three members of the Archaeological Zone team underwent a full day of training by a member of staff from the University of Leuven. The PLD system was rented by the university for a limited period of time.
The PLD system consists of two main components: a Portable Light Dome Scanner and a PC with the necessary software.
Portable Light Dome (light scanner):
The PLD scanner consists of a special camera, a USB controller, a so-called light dome and a laptop or PC.
As a source of illumination, the inside of the dome is equipped with 2282 small LED lamps with a colour temperature of 4000°. They provide a neutral white colour during the recordings. The numerous individual lights are needed to illuminate the panels from different angles and to make the fine incised lines visible through the cast shadows. In this way, individual photos are taken of each panel with different exposures. By adding the individual photos together on the computer later, the writing can be optimally worked out. The result is better than what can be seen with the naked eye.
A total of 160,000 slateplates have been found (as of 2017). Most of them are uninscribed. 296 pieces were examined in the project. These are the pieces with graffiti that were known up to the start of the project. In the meantime, many more pieces have been added.
The fragments are mostly the remains of a larger panel, though some pieces are also complete. They are usually roof slateplates that were reused in secondary applications as writing material.
Basically, the slateplates can be divided into three subject areas in terms of content:
1. Inscriptions: Hebrew characters. Some of them were writing exercises of the Hebrew alphabet, but others were lists of names or texts from business life.
2. Figurative representations: Depictions of animals or caricatures.
3. Various other graffito: Scribbles or tally marks that cannot be defined in more detail.
The panels were scanned on both sides, which means that a PLD file contains on average two scans per panel. In addition, these scans were subjected to different image processing filters to further clarify the results. In order to make the representation of the different lens settings tangible, these were combined into a total of eleven levels.
The more decisive factor for the final result is rather the surface condition of the object. A smooth surface basically requires a different resolution than a roughened one in order to achieve the same result.
Glossy surfaces make any photogrammetric image more difficult due to the mirror effect.
Particularly important for the result of the scans is the depth of the respective scribes. This means that the processing of the slateplate surface is decisive for the quality of the evaluation. The deeper the scribing has been worked into the surface, the more clearly the images make the scribing visible.
The surface is illuminated by the PLD system from different angles up to a total of 180°, thus the carvings are recorded with different shadow effects. As a result, an object can be depicted with different effects or resolutions.
In principle, deeper carvings can be better represented in the scan than in conventional photographs.
Conversely, this means that shallower incisions are often more visible in photographs than in the scan.
The results have shown that in this case there is a kind of chalk effect. The grinding marks on the panels suggest that they have been used several times.
Particularly in the case of figurative representations, it has also been shown that the PLD method was able to clearly bring out images / incisions that were previously almost impossible to identify with the naked eye or with conventional photography.
Basically, the results of the scans show that it is worthwhile to subject the objects to graphical processing. Once the evaluation has been completed, the system allows scientists greater data mobility and easier access to the data.
A virtual working field is established in which it is easier to operate with the finds already recorded and, if necessary, to create individual evaluations.
Thanks to the scans, the scientific processing of the slateplates will be much easier. This is currently being done in a scientific project at the University of Frankfurt under the direction of Elisabeth Hollender. The scans not only make the carvings more visible, but are also much easier to access than the delicate originals.
(This text was originally published in German on MiQua: https://miqua.blog/2022/05/05/die-beleuchtung-der-geschichte-pld-scan%c2%b9-der-judischen-schiefertafeln/)
Notes: RTI-Scan (Reflectance Transformation Imaging). https://www.wikiwand.com/en/Polynomial_texture_mapping  University of Leuven: Onderzoeksgroep Nabije Oosten Studies, Blijde-Inkomststraat 21, 3000 Leuven (Belgium)  E. Özcan M.A., U. Schikowski M.A., E. Wojewoda B.A.