Hyperspectral Imaging for Monument Preservation

Hyperspectral imagery for monument preservation offers a non-destructive examination of existing surface materials and mapping of the extend of foreign materials from previous restauration efforts. These imaging techniques can aid in the conservation and restoration efforts for monuments of historic importance, such as the baroque hereditary burial site in this example from a study by the German Research Centre for Geosciences Potsdam, GFZ shows.

The baroque hereditary burial from the 17th century is located in the Eastern wall of St. Mary’s Church in Frankfurt (Oder), Germany. This particular burial site was walled in and only gradually re-appeared during repairs of the St. Mary’s Church after severe damage from 1945. Apart from damage sustained in the war period, the destruction of the Church’s roof and decades of weathering and moisture penetration led to severe losses of surface design. Changes in microclimate caused by opening and construction measures after 1982 caused salt crystallization processes and decomposition of the surface to occur.

The different elements of the burial site are shown in Figure 1. It contains inscription panels framed in fine stucco decoration. The sides are lined by pilasters with Corinthian capitals and the parapet zone below the panels is decorated with stucco motifs of transience such as skulls and bones. Above the pilasters sits a stuccoed crowning group with angels, hourglass and a skull, topped by a calotte with an 18th century mural painting depicting biblical scenes.

Figure 1: Overview of the monuments’ dimensions and elements with details of the mural paintings. N. Köllner¹.

The first conservation and restoration measures were carried out in 1983. Restorative measures included various adhesives, backfilling of hollowed out plaster, acrylate dispersions to fill defects in various aggregates and glass-fiber-reinforced epoxy resin to repair metal defects². Different approaches to qualify these restauration measures and the overall state of the monument were tested in Köllner et al, 2021¹. Two of them will be highlighted here.

The epitaph was scanned using cameras from the HySpex Classic and Mjolnir series operating in the VNIR (400-1000nm) and SWIR (970-2500nm) spectral regions with a spectral sampling of 3.3nm and 5.5nm, respectively. They were operated from a motorized moving stage, scanning line-by-line in a rotational manner (Fig. 2). Five construction spotlights totaling 5200W were distributed over the entire length of the site at a distance of 5m. Four gray reference plates of different reflectivity values were placed in the scene for reflectance retrieval. Distinguishable materials on the surface were also sampled with a point spectrometer and compiled into a reference spectral library. More details on the scanning and correction of the illumination can be found in Köllner et al, 2021¹. The data analysis was performed by the GFZ hyperspectral geology group around Nicole Köllner in cooperation with the University of Applied Science Potsdam in the Department of Conservation and Restoration.

Figure 2: Setup of the HySpex hyperspectral cameras, lamps and showing the field of view and rotational movement of the cameras¹. Image: N. Köllner

Figure 3. RGB representation of the PCA from components 1-5-8¹. Image: N. Köllner

Figure 4: Representation of the spectral fingerprints by the depth and position of their respective absorption feature in the shortwave infrared. Left: Stucco elements, Right: Gypsum¹. Feature adapted after Nicole Köllner¹.