Remote standoff sensing in the service of murals examination
-Cathedral Church of St. Barnabas-
The Blessed Sacrament Chapel
Remote Standoff Spectral Imaging System (PRISMS)
The Unity Chapel
The Blessed Sacrament Chapel
Traditionally for the analysis of large wall paintings, scaffolding is used to lift the mobile instruments and operators to the painted surface, which is inconvenient, dangerous, costly, and unstable for sensitive scientific measurements. To address this issue, the ground- based PRISMS visible/near infrared spectral imaging system has been developed by the ISAAC lab and successfully deployed on heritage sites for the past decade to image wall paintings from a distance of tens of metres. However, where overpainting is an issue, PRISMS and other non-destructive methods of analysis are not the most suitable techniques for investigating the painting scheme. Although it might be possible to analyse an underlying paint deposit using non-destructive methods, this would be dependent on the thickness, opacity and completeness of the top paint layer, and is unlikely to give a full characterisation of multiple overpainted layers.
The aim of this project was to assess the use of new remote standoff Raman and LIBS systems developed by the ISAAC Research Centre, which can be used at distances of 3-15 metres from an object or surface. LIBS (Laser Induced Breakdown Spectroscopy) is a micro destructive analysis technique which uses a pulsed laser to ablate a surface, removing small portions of the material with each pulse, and providing elemental compositions of the material it ablates. It is therefore capable of analysing different successive layers of overpainting. Raman spectroscopy can provide a diagnostic “fingerprint” spectrum for the materials present in the analysed area and can therefore be used for confirmation of specific pigments present per layer.
Addressing the challenge
The Cathedral Church of St. Barnabas (Nottingham, UK), is a fine example of Gothic Revival Architecture, designed by the renowned architect Augustus Welby Pugin (1812–1852), who also designed the decoration of the Houses of Parliament in London. Analysis focused on two Chapels: the Blessed Sacrament Chapel and the Unity Chapel.
Analysis in the Blessed Sacrament Chapel
The Blessed Sacrament Chapel has extensive murals on the walls, meaning that PRISMS, remote standoff Raman for heights up to 15m and handheld XRF at accessible heights can provide direct, non-destructive data collection and pigment identification. These techniques, used together, were able to identify many of the painting materials. Red areas of the mural were investigated and found to have a reflectance spectrum similar to both vermillion and cadmium red. Both pigments shared similar reflectance spectral features in the visible spectral range, which makes it impossible to distinguish between them solely by spectral reflectance. Raman data obtained using the remote standoff Raman system showed two strong peaks which are diagnostic of vermillion. Portable XRF measurements of various areas at accessible height did not find cadmium and therefore there is no evidence to support the presence of cadmium red.
Remote standoff investigation of a red area in the Blessed Sacrament chapel: (a) colour image of part of a mural next to a stained glass window (b) reflectance spectra collected with the remote spectral imaging system PRISMS of the red area (black filled squares) indicated by the red arrow in (a) compared with PRISMS spectra of a reference sample of vermilion (red dots) and cadmium red (blue triangle) oil paints; (c) the raw remote standoff Raman spectrum of the same red area (red), and the background spectrum collected over the same integration time with the laser off (blue) showing typical absorption bands of the solar spectrum (ie. from daylight); (d) the processed spectrum after subtraction of daylight.
Analysis in the Unity Chapel
Within the Unity Chapel, areas of the original wall painting scheme have been covered by successive redecoration, and some areas are currently whitewashed.
The remote standoff Raman/LIBS system was set up at a 7m distance from the East wall paintings. Measurements were performed across the area, but to minimise the impact of the analysis, LIBS measurements were performed near the edges of already damaged areas. Titanium signals dominated the analysis of the whitewash layer, but underneath this, during successive ablations, signals for other layers became apparent.
The Unity Chapel
The Unity Chapel showing overpainted area on the left
The remote stand-off LIBS sytem
The Unity Chapel
Revealing a green under-layer using LIBS analysis. Left photos- laser ablation on the surface of the wall to reveal underlying pigment;
Middle- LIBS spectrum showing high levels of Chromium where the green layer is revealed. Right- spectral reflectance on the revealed green pigment (blue line) showing a computer fit (red line) suggesting a mix of Viridian Green (a Chromium Oxide pigment), Prussian Blue and Chromium Yellow
The information obtained suggests that the combined Raman/ LIBS system is a very useful addition to the ISAAC lab toolkit, allowing accurate pigment identification and safe elemental analysis of large heritage structures at remote standoff distances of tens of metres. Used together, the techniques enable the investigation of pigment layering and overpainting with only a minimally destructive impact. This suite of equipment would also potentially have applications to industrial buildings and structures.
Li, Y., Cheung C. S., Kogou, S. Hogg, A., Liang, H. and Evans, S. 2021. Standoff Laser Spectroscopy for Wall Paintings, Monuments and Architectural Interiors. In Transcending Boundaries: Integrated Approaches to Conservation. ICOM-CC 19th Triennial Conference Preprints, Beijing, 17–21 May 2021, ed. J. Bridgland. Paris: International Council of Museums.
Li, Y., Cheung C. S., Kogou, S., Liggins, F., and Liang, H., 2019. Standoff Raman Spectroscopy for Architectural Interiors from 3-15m Distances. Optics Express, 27(22), 31338-31347.