The Reales Alcazares of Sevilla is a building inscribed in the World Heritage List as the oldest royal palace still in use in Europe. The Reales Alcazares of Seville constituted a monumental area during the emiral, califal, taifa-abbadita, almoravide, almohade and castilian-medieval periods [1, 2]. The Alcazar of Isbiliya (the ancient name of Seville) was constructed at the end of the first millennium as a fortress during the omeya control of Seville. The Alcazar expanded during the subsequent centuries due to the city’s increasing political influence within the Iberian Peninsula, and new palaces were built. Abd al Rhaman built the Emiral Palace in 914. The Taifa Palaces of Almuvarak were constructed in the 11th and 13th centuries. In the 13th century, the Golden Tower was built and the Yahward Wall was finished. After the Christian conquest, the Atarazanas and the Gothic Palace were constructed, and several improvements were performed in the 14th century. The Justice Room was constructed, and later the Taifa Palace was rebuilt adjacent to the first building, and the actual Mudejar Palace was constructed. During the subsequent centuries and continuing through the present day, the Reales Alcazares was continuously enriched, especially with regard to Renaissance pavilions and gardens, which vastly enlarged the architectonic area.
The Reales Alcazares of Seville is considered to be one of the greatest surviving examples of the Mudejar style. This style did not involve the creation of new structural shapes, but a reinterpretation of Western cultural styles with Islamic influences. For instance, to enliven the planar surfaces of the walls, the Mudejar style developed complicated tiling patterns that have never been surpassed in sophistication.
High-valued wall paintings are located in the Mudejar Palace. Golden medallions, representing the coat of arms of the Leon and Castile Kingdoms, decorate several chamber walls. These artefacts are extremely degraded and require conservation; therefore, studying the materials used to manufacture them and discovering the causes of deterioration is necessary. These medallions have probably been restored several times through centuries.
The first and more stable material used to create a gilded surface was gold. A review performed by Sandu et al.  took into consideration the gilded surfaces in polychrome art objects (easel and mural paintings, polychrome wooden objects, gilt leather and parchment) and described the materials, gold application techniques and imitations from a technical, analytical and conservation point of view.
There are various ways to make gold leaf adhere to a large surface area, but the use of fine red clays is the most common method [4, 5]. For the application of gold on small mural surfaces, the mordant gilding technique is generally used; pieces of gold leaf are attached to the desired areas with an adhesive [6–8]. Cennino Cennini  notes the ingredients of the mordant that are boiled together: linseed oil, lead white, verdigris and varnish (sandarac or amber). Thompson  describes two types of mordant used in medieval gilding: one is oily and mainly used on panels, wood or wall; another is watery and is used mainly on paper in books sometimes, but never on walls.
X-ray fluorescence spectroscopy is the most widely used technique for elemental chemical analysis due to a number of favourable analytical characteristics [10–12]. Recently, the need to perform in situ non-invasive analysis of art objects, that cannot be easily removed from their location, has led to the development of portable X-ray fluorescence (XRF) equipment. XRF is recognised as being an accurate technique for quantitative analysis. The concentration of one element in a sample is identified as a combination of several factors, such as the measured net peak area within a correcting term corresponding to the effects of the matrix composition and the calibration constant [13–15].
X-ray diffraction is considered to be the best experimental technique for characterising the crystalline phase of artworks. This technique, which is usually performed on powders, is a micro-destructive technique because a small amount of the artwork is collected before performing the experiments. Non-destructive X-Ray diffraction (XRD) equipment was developed to study planar samples using techniques such as grazing incidence X-ray diffraction, which provides very precise information about the surface, atomic interface arrangement in the crystalline structure and composition of the depth profiles [16–18]. In addition, a recent study used Goebel crystal facilities to examine non-planar samples .
Raman spectroscopy has become a very useful tool for the analysis of artwork because it is non-destructive, rapid, reproducible and sensitive and also because of its relative low cost compared with other techniques [20–26]. Nevertheless, some artwork materials fail to give an identifiable Raman spectrum either because they are poor Raman scatters or because of their fluorescence.
Small plain pieces or fragments of artwork can be easily transported to a laboratory and studied directly by non-invasive XRD, micro-Raman, micro-FTIR, and SEM/EDX without altering the samples [27–31], that are then returned to their original location.
This study details the use of non-invasive techniques for the characterisation of materials used to make the gilding on the medallions found in the Mudejar Palace of Reales Alcazares. The main objectives of this paper were to elucidate and reconstruct the processes employed to make the gilding. We also examined the reliability of non-invasive techniques, such as XRF, XRD, FTIR, Raman spectroscopy and SEM-EDX, for studying the golden medallions.