Wall paintings typically contain low concentrations of organic materials within a largely inorganic matrix and are characterised by their high porosity and long-term exposure to severe environmental conditions. The identification of organic materials within specific paint or plaster layers is challenging and the inherent characteristics of wall painting samples present further complications. Embedding materials (such as epoxy, polyester and acrylic-based resins) used to produce cross-sections often infiltrate porous and leanly bound samples, and compromise the interpretation of Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectra and the qualitative identification of natural organic materials. An alternative method for the preparation of cross-sections of wall painting samples was developed using cyclododecane (C(12)H(24)) as a temporary consolidant and barrier coating to encapsulate the sample, and to provide necessary support to produce a cross-section through microtoming. Impacts of traditional and novel sample preparation techniques on the identification of organic materials with micro-FTIR-ATR were examined for both replica and real wall painting samples.

The widespread occurrence of Indian yellow on an early 17 th-century wall painting in Rajasthan (India) was initially indicated by photo-induced luminescence imaging of the painted scheme in the Badal Mahal within the Garh Palace (Bundi). The presence of the organic pigment was subsequently confirmed by HPLC-ESI-Q-ToF. The results of a multianalytical study focusing on two samples from the wall painting and two reference pigments from the British Museum and National Gallery (London, UK) are presented here. The research focused on the possible causes for the different yellow/orange hues observed in the painting samples. Analysis of cross-sections with SEM-EDS revealed similar elemental composition for the Indian yellow paint layers, but different underlying layers, indicating a variation in painting technique. The composition of the Indian yellow samples was investigated by HPLC-ESI-Q-ToF with both positive and negative ionisation. In addition to euxanthic acid and euxanthone, a sulphonate derivative of euxanthone was found to be present in all samples, while relative amounts of the three components varied. Flavonoid molecules-morin, kaempferol, quercetin and luteolin-were also detected in one wall painting sample (characterised by a brighter yellow colour) and not in the sample that was more orange. 2 The optical properties of the samples were characterised by photoluminescence spectroscopy in both solid state and aqueous solution. The contribution of each organic compound to the emission spectrum of Indian yellow in solution was also investigated by time-dependent density functional theory (TDDFT) calculations. Small differences in terms of spectral shift were observed in solid state experiments, but not in solution, suggesting that the spectral differences in the emission spectrum were mostly due to different contributions of solid-state arrangements, most likely driven by π-π stacking and/or hydrogen bonds. However, a slight difference at high energies was observed in the spectra acquired in solution and TDDFT calculations permitted this to be ascribed to the different chemical composition of the samples. Time-resolved measurements highlighted di-exponential lifetime decays, confirming the presence of at least two molecular arrangements. Py(HMDS)-GC-MS was also used for the first time to characterise Indian yellow and the trimethylsilyl derivative of euxanthone was identified in the pyrograms, demonstrating it to be a suitable marker for the identification of the pigment in complex historic samples.

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The design and evaluation of site-specific injection grouts for the stabilisation of delaminated wall paintings is often challenging to perform in situ, due to constraints such as time, availability of materials and reliable testing procedures. In this research, a rigorous design and testing methodology, which includes the development of a new adhesion test, was adopted on site for the development of injection grouts to be used in water-sensitive situations. Water reduced mixtures were obtained by partly substituting it with ethanol. Previous research (Pasian et al. 2018) demonstrated in the laboratory the potential suitability of water-ethanol grouts. In the present paper, water-reduced grouts were designed, tested and applied on site for the first time.

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