Scientific analysis at the Hamilton Kerr Institute has mainly focused upon artists’ materials and their methods. Artists’ materials can be conveniently divided into organic and inorganic materials. Methods of analysis can be conveniently divided into destructive and non-destructive techniques. Non-destructive techniques (including surface examination of the painting with a stereo-microscope, IR, UV and X-ray examination, etc.) always have preference over invasive techniques that require paintings to be sampled.
Artists’ materials, organic
Organic artists’ materials include supports, such as canvas or panel, and adhesives, surface coatings or paint media. Organic supports are traditionally fibres (mainly flax, but including hemp, jute, cotton, etc.) and various types of wood (oak, poplar, chestnut, pine, lime, etc.). The adhesives, coatings and media include naturally sourced protein glues, egg tempera, oils, resins and waxes. Since the Institute specialises in Old Master paintings, the synthetic analogues are not often encountered in the original works, although they may be present as materials added by conservators. The Institute’s facilities to analyse organic materials are limited.
Gas Chromatography can be used to identify naturally-sourced and modified drying (unsaturated tri-glyceride) oils, including the most commonly used linseed, walnut and poppy oils. Gas Chromatography can also identify standard artists’ pre-treatments of these oils, such as partial pre-polymerisation caused by ‘heat-bodying’ oil. Gas Chromatography can also indicate the presence of naturally-sourced terpenoid resins such as those found in mastic and dammar, etc. However, in the absence of a coupled Mass-Spectrometer, these materials cannot be unambiguously identified.
Some adhesives, surface coatings and paint media may be generically identified using staining techniques applied to microscopic samples or paint cross-sections. Natural fibres can also be readily identified from their microscopic morphology and in cross-section. Students and interns are encouraged to use these methods routinely in conjunction with other methods of material identification.
Artists’ materials, inorganic
Inorganic artists’ materials associated with paintings include supports such as stone (slat slabs, alabaster polychromy, etc.), multi-media decorative elements (glass, paste, gems, etc.), metallic structural components and pigments. The majority of scientific analysis undertaken at the Institute involves the identification of pigments. Artists’ pigments include natural and synthetic compounds and can be identified to varying levels of specificity with a number of methods.
Polarised Light Microscopy
Polarised light microscopy can identify the majority of pigments found in Old Master paintings using very small samples taken from exposed surface layers. All students are trained in the technique and the identification of pigments is routinely undertaken by students, interns and staff. In some instances, pigment identification using PLM is definitive, in other cases a generic identification is possible and other techniques may be used if elaboration is required.
Paint cross sections can be used in conjunction with other methods, such as PLM, to identify artists’ materials and methods. Some pigments and media can be generically identified in microscopic paint cross sections, but they are most useful for determining the artists’ techniques – the ways in which materials are used. Paint cross sections can also provide insight into an object’s conservation history – the ways in which it has interacted with its environment or the ways in which individual components within the paint layer interact with each other.
Scanning Electron Microscopy
Scanning electron microscopy is used routinely to elaborate upon artists’ materials. Pigment dispersions (before mounting for PLM), un-mounted samples or paint cross sections embedded in resin and polished can all be examined to determine surface morphologies and the identity and distribution of elements for most pigments. Where the work of art is sufficiently small (individual folios from medieval manuscripts or fragments of painted Egyptian papyri, for example) sampling for EDX analysis is unnecessary, as the Institute also has access to an Environmental SEM.
The Institute has developed excellent working relationships with a number of renowned experts in various relevant specialist analytical areas. As the need arises, therefore, in addition to the above, we are able to draw upon analytical methods that include, but are not restricted to, the following –
- Gas Chromatography-Mass Spectrometry (GCMS)
- High Pressure Liquid Chromatography (HPLC)
- Fourier-transform Infra-Red Spectroscopy (FTIR)
- Raman Spectroscopy
- Lead isotope analysis
- Trace element identification
- Coccolith identification
Whereas the study of artists’ materials results in material identifications of various levels of specificity, the study of artists’ methods has a less reductive outcome. Artistic processes are, of necessity, less amenable to unambiguous characterisation than artistic products. Artist’s methods are studied using the same scientific techniques that are used in the study of artists’ materials, but with a greater reliance on statistical tools. They are also studied by analysing the products of reconstructions.