Scientific analysis of carved stones is increasingly contributing to carved stone study in multiple ways including conservation techniques, surface analysis and stone sourcing. Stone is vulnerable to deterioration and decay under the influence of a variety of physical and chemical agencies. 'Weathering' encapsulates a range of processes, driven by moisture movement, driving rain, freeze-thaw cycles, salt crystallisation and chemical attack from pollutants. Biofilms can engender and mediate these processes, having a significant impact on the surface of carved stone, including staining, alteration of the movement of moisture, and physical stresses on microstructure. Longer-term climatic variability also brings about change to the physical conditions affecting the historic environment, for instance increased rainfall exacerbates problems caused by water ingress and increases both macro- and microscopic biological growth. In conservation contexts, responses to these forms of material degradation often result in steps to measure, record, protect, and/or repair carved stones (e.g. Sheltering monuments: Case Study 17). There is a long and continuing tradition of regular repair and maintenance using traditional craft techniques and materials). However, the development of heritage science during the 20th century has led to the introduction of new techniques for measuring change, analysing materials, protecting them from decay, and consolidating vulnerable components. For instance, petrographic analysis is used for characterisation and the determination of provenance and biocides for the management of biofilms. As a result of these techniques, the nature of carved stone, and their dynamic relations with their physical environments is altered to some degree, whether directly or indirectly. For instance, rates of weathering can be modified and signs of wear and age removed.
Science-based materials analysis techniques provide information on the physical and compositional condition of stone when carved, as it changes through its lifespan and what its condition is now, both on the surface and internally. Some methods of analysis which use specially developed statistical-based algorithms to isolate patterns in the worked surface have the capacity to reveal information about the stone-carver (Kitzler Åhfeldt 2013), possibly their place of work, their support workers and clients, the nature of tools used in the carving process and the procuring and movement of stone.
Figure 79: Historic Environment Scotland Conservation Directorate uses X-ray fluorescence to try to identify a white coating on the St Andrews Sarcophagus. © Sally Foster
As historic and prehistoric carved stone is highly valued, direct sampling for laboratory analysis is limited or impossible. This leads to the application of visual characterisation, classification and condition assessment methods, using descriptive 'field' methods and terminologies derived from the geological and physical geography disciplines (see Giesen et al. 2014). Increasingly, a range of instrumental analytical techniques are applied on the rare occasions that materials can be transferred to the laboratory. There are also variants of many methods available for non-destructive testing in situ, removing the need for destructive sampling of materials (or on whole objects in the laboratory, but without destructive sub-sampling). Provenance discrimination and material condition assessment requires compositional, mostly mineralogical analysis using combinations of techniques such as thin section petrography, X-Ray Diffraction, X-Ray Fluorescence, magnetic susceptibility, nuclear magnetic resonance, FTIR spectroscopy, Raman spectroscopy and quantitative colour measurement. These investigative techniques are being brought together into schemes to assess the risks to and vulnerability of stone to changing environments, including changing conservation practice and management regimes.
There is an increasingly strong working relationship between archaeologists and geologists and other scientists in the heritage sector and as a result an excellent understanding of Scottish rock micro fragments is developing. This leads to a continuing refinement in the techniques which allow carved stones to have their original source in the landscape pinpointed (Magnetic susceptibility: Case Study 9).