From prehistoric times onwards, both artists and craftsmen have sought to manipulate natural and/or manmade materials to produce artefacts of either utilitarian or aesthetic value. Initially, such artefacts, for example obsidian fist axes or ivory sewing needles, were relatively simple and (thus) closely related to the original, natural materials from which they were made. These could be wood, bone, clay, various types of stone, different vegetable fibres (cotton, flax …) and animal tissues (leather, hair …), indigenous metals (copper, gold, silver …), naturally formed glass (obsidian), (powdered) minerals with an intense colour etc. As material culture expanded and ancient technologies evolved, artefacts of increasing sophistication were made, for example by combining several materials in one artefact (metal-inlaid wooden tools, painted earthen­ware bowls …). Another important evolution was that, next to natural materials, more manmade materials such as silicate glass, refined metals/metal alloys, ceramics and other products of proto-chemical workshops gradually began to be used. While the materials that could be employed for the manufacture of artefacts were originally limited to those that were locally available, an increase in trade between early human communities also expanded the variety of available materials, allowing new or more expensive equivalents of local starting materials to be used. In turn this stimulated the imagination of artists and artisans, leading to cultural innovations both on the material level as well as the artistic level. The technological advances realized during past historical periods such as the Ancient Kingdom in Egypt, Mediaeval Europe, 18–19th century China and Japan and post-colonial America can be reconstructed by careful material study of the artefacts that have survived from these periods. Such artefacts are usually (relatively) rare and require appropriate conditions to guarantee their long-term preservation and the possibility for scholars and the general public alike to appreciate them, either for their techno-historical or their culturo-aesthetic value. These valuable, often irreplaceable, objects `out of the past' (such as paintings/drawings, tapestries, garments, photographs, objects in metal, wood, clay, stone, glass …) are generally denoted cultural heritage (CH) artefacts.

Depending on the (complexity of the combination of) materials that were employed to manufacture these CH artefacts, the degree to which the individual materials may have survived the passage of time since their creation may vary widely. This broadly depends on the one hand on the characteristics of the material itself (for example, artefacts made of gold will corrode far less than those made of bronze) and on the other hand on the environmental circumstances in which the artefacts have passed important parts of their lifetime. For example, stained window glass may resist significant corrosion when kept intact inside the window frames of a well ventilated cathedral, while the same glass, buried for 400 years in moist soil, may become severely weakened in mechanical strength and develop brown staining. While there is a general belief that oil paintings are complex but essentially static assemblages of widely different (in)organic materials, at or just below the seemingly placid surface of these works of art chemical reactions are taking place that slowly alter the chemical make-up of the paint layers. While some of these reactions are the result of intimate contact between the different materials, most are driven forward by external physicochemical factors. A prime stimulus for reduction–oxidation (redox) reactions is light absorption by coloured substances (molecules) in the ultraviolet (UV) and visible (Vis) range. Such reactions can lead to the spontaneous in situ formation of secondary chemicals that will often differ from the original materials in their macroscopic properties (such as colour, volume and porosity). Both the organic components of paint (the protein-, saccharide- or lipid/oil-based binding media, organic dyes etc.) and/or the inorganic components (mostly pigments based on metal ions) may be affected. In a similar manner, statues and related CH artefacts composed of different metals may develop significant corrosion at locations where different materials meet or where other sources of mechanical stress were applied to the material. In the degradation of glass, metal and paint alike, another important factor is the often cyclic variation in relative humidity in museums, warehouses or archaeological depots, which causes the condensation and re-evaporation of minute moisture droplets within the microporous material that may have formed on their surface. The latter can act as miniature galvanic cells in which redox reactions can occur at the interface between the original material, the material that is covering or touching it and water. In addition, phenomena such as the crystallization of salts, the leaching of metal cations from the original materials and their in situ recombination with short-chain organic acids such as formates, acetates, oxalates or longer carboxylic acids may lead to insoluble metal soaps that can either protect or undermine the mechanical integrity of a CH artefact. Cycles of condensation/evaporation may also transport secondary alteration compounds towards the surface, leading to the formation of weathering crusts. These crusts can be partly crystalline; almost all have a colour and texture that is quite different from those of the original material.

In order to obtain insights into the chemical (and in particular redox) transformations (i) that were used to manufacture the objects/materials, (ii) that slowly (or more rapidly) can change the outlook and/or threaten the long-term integrity of CH artefacts and (iii) that were introduced during conservation treatments, X-ray absorption spectroscopy (and methods related to it) has proven to be particularly valuable in the last few decades.

Since XAS experiments are almost exclusively conducted at synchrotron facilities, most XAS studies are performed on small samples that are taken from CH artefacts. Only in a few rare cases have XAS or related measurements been performed directly and in a non-invasive manner on the artefacts as a whole. For this purpose, the latter were transported to the appropriate synchrotron beamline. The displacement of unique, irreplaceable or very valuable artefacts to synchrotron-radiation (SR) facilities is hampered by the fact that they are difficult to organize and/or fund (insurance costs). Moreover, most XAS beamlines are designed to accommodate relatively small samples and have difficulty in accepting large-scale CH artefacts such as entire statues or paintings.

The experience of the last 20 years has shown that XAS is most often employed to characterize (the degradation of) inorganic materials such as glass, ceramics, minerals, pigments and paints (see Fig. 1). On the other hand, in the case of wood, paper, textiles etc., where an inorganic (minor) component may be responsible for the degradation of the organic matrix, XAS can provide very valuable clues about the chemical transformations that have caused the unwanted damage.

Figure 1 (a) Number of papers published annually describing the use of XAS and related methods for the characterization of various types of CH materials. (b) Distribution of published papers over the chemical elements.

The use of XAS and related methods in cultural heritage has been reviewed by several authors (Cotte et al., 2010; Bertrand et al., 2012; Janssens et al., 2010; Farges & Cotte, 2016). Fig. 1(a) shows the number of published studies in the period 1995–2016. The first applications of XAS generally start appearing from 1995 onwards, with a regular number of papers appearing annually since 2004. Among these, K-edge XAS of the elements sulfur, manganese, iron and copper is frequently employed to study predominantly ceramic, pigmented and glassy materials. Iron- and copper-rich metallic artefacts and related problems are also well studied by means of XAS.

The change in publication pace in 2004 is related to the fact that SR-based X-ray microprobes (hard X-ray region, E > 5 keV) and X-ray microscopes (soft X-ray region, E < 5 keV) started routine operation at second- and third-generation synchrotron facilities around this period. Since CH materials are usually multi-component and often also strongly heterogeneous in nature, while degradation phenomena tend to change the material properties predominantly at the very surface, i.e. in the topmost few micrometres, only those forms of X-ray spectroscopy that can extract localized information are relevant in this context. This can take essentially two forms: either as a form of full-field microscopy in which spectroscopic information is obtained from all pixels of the sample area under study in a parallel manner or via a scanning X-ray microspectroscopy approach in which only a very tiny spot on the sample surface is irradiated, yielding spectroscopic information. Full-field forms of XAS-based microanalysis include scanning transmission X-ray microscopy (STXM), a method relaying on the absorption of monochromatic X-rays that traditionally operates in the 0.1–1 keV range and that `interrogates' the K absorption edges of elements such as carbon, nitrogen and oxygen (Chen, 1997; de Smit et al., 2008; Solomon et al., 2005; Urquhart & Ade, 2002). It is therefore suitable for extracting information on the organic fraction of CH materials. In recent years, the possibility of performing spectroscopic measurements in full-field imaging mode in transmission geometry has also been realized (see Section 4; Pouyet et al., 2015; Meirer et al., 2013). (Hard) X-ray microprobes, generally operating in the energy range from 1–2 to 23 keV, can address a much larger series of K, L (and occasionally M) edges of relevant elements. Since these devices frequently employ rhodium-coated mirrors, the absorption-edge energy of this element (23.229 keV) in many cases defines the upper working energy of X-ray microprobes. Next to silicon, accessible elements include alkaline and alkaline-earth elements such as sodium, magnesium, potassium and calcium (the major constituents of glass, ceramics …) and all first-row transition elements (Ti–Zn, the major constituents of most metallic CH artefacts and of most inorganic pigments, inks …; Cabaret et al., 2010), which may all be studied via their K edges. XAS of heavier elements (with K edges above 23 keV, i.e. Pd–Ba), which are occasionally present in metal alloys and glass/ceramics, can be performed using their L edges (situated in the 3–6 keV energy range). The same applies to the heavy elements mercury, lead and bismuth, elements that are frequently encountered in paints, with L-edge energies from 12 to 16 keV; in rare cases the M edges of these elements are also used (2–3 keV range). The X-ray microprobes very often work in X-ray fluorescence (XRF) mode and only occasionally employ transmission mode for recording XAS data. XRF-mode XAS data acquisition allows information to be obtained over a wider concentration range and is also more compatible with strongly absorbing sample matrices, the thickness of which is less critical/limiting. Since reproducible energy scanning without measureable changes of the position, orientation or size of the primary X-ray microbeam is not possible in practice at many experimental stations, in many CH studies data acquisition is limited to micro-XANES (X-ray absorption near-edge spectroscopy) measurements. XANES measurements involve recording X-ray absorption in a region of around 100 eV around the edge of interest. Only when the X-ray microprobe endstation is specifically optimized for it or when fairly homogeneous materials/samples (such as glass and some types of metal alloys) are examined does it become possible to record XAS spectra over a larger energy range (>500 eV) in order to record laterally resolved extended X-ray absorption fine-structure (EXAFS) data. Such studies are much rarer than those relying on micro-XANES data. Another limiting factor in this respect is that due to the complex and heterogeneous character of the samples analyzed, leading to complex speciation, the X-ray absorption signal is averaged and almost no information can usually be deduced beyond the first coordination sphere. In these situations, XANES analysis may/must be considered as sufficient and has often been preferred to full EXAFS data collection, interpretation and modelling.

Since the extraction of micro-XAS data from CH materials is frequently realized by means of X-ray microprobes, the same (CH) material is often examined by means of a combination of X-ray-based microprobe modalities: next to information on valence state (micro-XANES), nearest-neighbour types, number and orientation (micro-EXAFS), microscopic X-ray powder diffraction (micro-XRPD) is very frequently employed to allow a more specific identification of the crystalline compounds that are present and to visualize their distribution throughout the examined area. A mode of operation that is frequently combined with the recording of micro-XANES spectra from individual sample locations is the recording of micro-XRF maps (yielding distribution maps of one or more chemical elements) at a few relevant energies close to an absorption edge of an element of interest (for example iron) to obtain semi-quantitative distribution maps of this element in various valence states (for example Fe²⁺ and Fe³⁺). This modus operandi is gradually being replaced by full-field (FF) or full-scale (FS) XANES imaging.

Since in a number of cases X-ray emission spectroscopy (XES) and resonant inelastic X-ray spectroscopy (RIXS) can offer a higher spectral resolution than `regular' XAS and thus may allow more species-specific information to be obtained, in the near future they might become very useful for CH investigations, provided that it will become possible to employ them in microprobe mode (van Bokhoven & Lamberti, 2016).

While most current CH studies make use of `hard' (E > 3 keV) X-rays, in the soft X-ray range a number of analytical possibilities could be better exploited. Scanning transmission X-ray microscopy (STXM) is a method that provides speciation information on carbon, nitrogen and oxygen that is often complementary to that obtained at transition-metal K edges (Rouchon & Bernard, 2015; Ferrand et al., 2014). The equivalent of STXM in the hard-energy regime (sometimes called full-field XANES) is also proving itself to be useful for CH investigations (Cotte et al., 2017).

Note: This chapter was originally written in 2016.