Historical buildings are prone to deterioration due to various reasons including environmental conditions, humidity and structural failures. The main factors of their degradation are moisture and salt activity. Salt weathering affects the appearance of the monuments but also causes chemical and mechanical degradation. The effect of salts in building deterioration is well-known, with several laboratory-based studies focusing on understanding the formation mechanisms. Here, we introduce a new methodology for the non-invasive monitoring and identification of moisture and salts following a complementary remote sensing approach. The study is based on ground-based remote short-wave infrared (SWIR) imaging and remote Raman spectroscopy from standoff distances of 3 to 15m [1]. The remote SWIR spectral imaging system covers the spectral range between 1 and 2.5 μm, with a spectral resolution of 5.5 nm and spatial resolution of 150 μm at a distance of 3m. The in-house developed mobile standoff Raman system operates with a continuous-wave (CW) excitation laser source at 780 nm. The laser beam can be focused at different distances resulting in a spot size of ~1 mm on the target. In our approach, SWIR imaging is used for scanning large wall surfaces. The post-processing of the acquired spectral imaging data using our novel machine learning-based clustering methods highlights the material variations across the wall. The detailed examination of the mean SWIR spectra for each cluster, allows a primary identification of moisture and salts, indicating also variations in volume concentration. The salts identification is then confirmed by remote Raman spectroscopy. The new method is presented through the examination of the historical building in Fort Brockhurst, an English Heritage monument in Portsmouth, UK.
This study sets out to analyse the artistic materials used in the maritime Southeast Asian manuscript collection at the British Library. To gain a full understanding of how artistic practises may have developed over time and changed between regions, it is necessary to perform large scale scientific analysis. Visible/NIR spectral imaging is an efficient method of collecting spectral reflectance data which can be used to distinguish different materials. Recent advancements in automatic data collection have meant that the volume of data collected has greatly increased, making traditional approaches to data analysis impossible to perform in a timely manner. Machine learning provides a viable solution to this as it can be used to automatically cluster millions of spectra into smaller, more manageable numbers of distinct spectral groups. Self-organising Maps are used as the building blocks of an algorithm which can perform clustering of large collections of spectral imaging data. Spectral reflectance alone is often not enough to perform pigment identification, consequently other complementary techniques are required. Advances in spectral imaging mean that each of these complementary techniques has a corresponding imaging modality. The machine learning approach developed in this project can be adapted to allow for the clustering of multimodal spectral imaging data including VIS/NIR hyperspectral imaging, macro-X-Ray fluorescence mapping, macro-Raman mapping, and Fourier transform infrared mapping. For multimodal clustering, each modality can be clustered individually and then brought together to produce a single cluster map which is a more refined representation of the material distribution than that produced from any individual spectral imaging modality. A visualisation tool has also been developed for the easy interpretation and interrogation of spectral imaging data cubes and cluster maps for entire collections. Both the visualisation tool and clustering method will be made accessible to the cultural heritage community through an online DIGILAB platform.
Limoges enamels along with a lot of glass-based objects suffer from deterioration that affects the condition, structure and longevity of the pieces affected. Understanding the mechanisms and causality of this deterioration can aid in conserving and preserving the wares in an optimal manner. Limoges enamels are composed of multiple layers of glass over a metal substrate, multiple colours and levels of opacity are used with gildings and paillons to create vibrancy and decoration within the artwork. A main concern is the exposure of the glassware to moisture and pollutants that can cause crizzling, cracking and spalling of the enamel and cause detrimental and irreversible damage. Detection and analysis of the moisture content, progression of the gel layer (a compositionally modified layer formed on the surface of the glass due to the attack from moisture or pollutants), and overall subsurface condition of the enamels can help give insight into the level of deterioration. Furthermore, some enamels and certain colours show worse levels of deterioration. Is this a product of the environmental conditions of storage or the composition of the enamels? Non-invasive analytical techniques are a priority in analysis of cultural objects; therefore, multiple non-invasive techniques were employed to survey and analyse eight Limoges enamels from the sixteenth century in the British Museum collection. These techniques were 810nm Ultra-High-Resolution Optical Coherence Tomography (UHR OCT), 400-2300nm Fibre Optical Reflectance Spectroscopy (FORS), X-Ray Fluorescence (XRF), VIS/NIR (400nm-850nm) spectral imaging (PRISMS) and Shortwave Infrared (1000 – 2500nm) spectral imaging (SWIR). UHR OCT images the structure and level of deterioration throughout the enamel by showing the extent of the gel layer in the glass, as well as the structure and condition throughout the top layer. SWIR can be used to give information on the level of hydration in the glass across the entire enamel, this is mapped in 2D over the entire enamel for analysis. XRF has been used widely in glass analysis and can help identify the composition of the glass. PRISMS results give information on the spectra in the visible/NIR region complementing elemental analysis by XRF, it is useful in this investigation as a standardised way to analyse spectra and categorise the spectra and colour of an enamel. By using a combination of all of these techniques, investigations were conducted to understand if there is a link between the composition and deterioration of the selected sixteenth century Limoges enamels. Results using the OCT and SWIR analytical techniques have already shown a strong link between the level of hydration and the level of structural deterioration in these Limoges enamels, further analysis showed there was a correlation between the depth of the gel layer and the level of hydration. Furthermore, when incorporating all the techniques, initial findings have shown interesting insights into how the composition can affect the deterioration of these enamels.
While heritage science in museums usually focuses on the analysis of a single or a few iconic objects, material analysis of archival collections often deals with documents at mass-scale. In this setting, automation of data acquisition and data processing becomes particularly relevant. The analysis of a collection of Tudor maps of Ireland from the National Archives, London, serves as a case study to both test the applicability, and to optimise the methodology, of AI-based spectral clustering on data extracted from a commercial multiband imaging system, used typically for single-band and colour RGB imaging of archival documents, manuscripts and artworks on paper. The clusters revealed important information about the maps’ materiality, such as highlighting the different composition of painted areas within and across maps that appeared very similar in colour with the naked eye or differentiating original ink from later annotations, for instance. The clusters guided the selection of areas for further spectroscopic point analysis, both to check the performance of the algorithm and to interpret the material composition of the different clusters, with the ultimate goal of shading light on Tudor mapmaking practices. Strengths and aspects to be optimised in the methodology are finally discussed.
Material analysis is important to the study of architectural interiors and wall paintings in order to inform the research in history and to monitor the state of conservation. Multimodal spectral analysis is increasingly used in mobile lab campaigns conducted in situ at historical sites. Some challenges specific to the investigation of immovable cultural heritage arise from the inaccessible heights and remoteness of the sites. Therefore, complementary spectroscopic techniques that can be conducted from the ground at a large distance (> 3 m) are required.
The Imaging and Sensing for Archaeology, Art history and Conservation (ISAAC) Mobile Lab routinely employs remote spectral imaging to record the spectral reflectance in the visible and near infrared of wall paintings at high spatial resolution per pixel. Raman spectroscopy identifies molecular structural fingerprints by observing the spectral shift from the excitation laser wavelength resulting from molecular vibrations. A by-product of Raman spectroscopy is laser induced fluorescence spectroscopy (LIF). Laser-induced breakdown spectroscopy (LIBS) detects characteristic lines for different elements from the plasma created by high power laser pulses. The combination of Raman, LIF, LIBS and spectral reflectance can provide complementary material information about the artworks: molecular structure and elemental composition. Assisted with a computer-controlled telescope mount, small area remote spectroscopic mapping (2D scanning) with Raman and LIF is also achieved to complement long range remote visible and near infrared spectral imaging.
In this work, we present the developments of a combined long range mobile remote spectroscopy system for working in the range from 3m to 15m, and its recent applications in remote material identifications on wall paintings.
The complementary use of X-ray fluorescence (XRF) mapping, spectral imaging, and Raman mapping, allows for the analysis and identification of important artistic materials used in the production and illustration of illuminated manuscripts. This project uses combined non-invasive imaging techniques to analyse 17th – 19th century manuscripts from the British Library’s Southeast Asia Collections so that more can be understood about the adoption and evolution of artistic materials and techniques used in Maritime Southeast Asia. Using multiple different imaging techniques has shown to provide positive results, however, a consequence of this is the collection of large amounts of data, necessitating the automatic and unsupervised analytical techniques used in machine learning. Data collected in-situ at the British Library using macro-XRF mapping, macro-Raman mapping, and Spectral Imaging, will be analysed using a range of machine learning techniques to cluster pixel information representing materials used in southeast Asian manuscripts.
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