The state of the art in technical art history currently offers a range of analytical techniques to support interpretation and conservation. Three key forms are invasive analysis, non-invasive investigation, and non-invasive hyperpectral imaging. Using these techniques involves certain trade-offs. Is it acceptable to remove part of the artwork for sampling? Does the artwork need to be investigated in situ? Is a ‘spot analysis’ sufficient or does the whole of the artwork need to be considered?
Invasive investigation involves removing small samples from an artwork for subsequent analysis by instrument by laboratory instrument.
Multiple techniques are available to support instrumental analysis including sampling for cross-sections, light microscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and X-ray fluorescence (XRF) analysis. These enable the stratigraphy of a sample to be analysed, and the materials (pigments, media, coatings) to be identified and their degradation studied.
However, using an invasive approach comes with certain disadvantages. Most importantly, it is invasive and so requires a small part of the object to be removed. The techniques are also labour intensive, both in terms of sample preparation and analysis. In addition, only a ‘snapshot’ of the whole object is investigated, usually a sample of less than 1mm in extent.
Non-invasive investigation, by contrast, involves the use of portable instruments to take spot measurements and instruments that can scan and create multispectral images. This includes the use of macro XRF, portable p-Raman, p-FTIR and p-XRF.
The key advantage of this approach is that the instrument can be taken to the object. However, there are also disadvantages. Spot analysis only delivers a ‘snapshot’ of the whole artwork, and there are limitations about the depth of analysis of vibrational spectroscopies.
Macro-XRF has been established as a non-destructive analytical technique for investigating the distribution of element across a wide field of view on objects, mainly paintings. Reflectance hyperspectral imaging in the visible (400-700 nm) and in the near-infrared (750-2500 nm) spectral ranges for the investigation of works of art has been used for a number of years.
The potential of mid-IR imaging (2700-10,000nm) for the study of painted cultural heritage with a hyperspectral imaging system in the IR fingerprint region would be transformative in the field of non-invasive analysis of artworks. This would be particularly useful for the identification of oils, resins and coatings.