Please cite this article in press as: E. Franceschi, F. Locardi, Strontium, a new marker of the origin of gypsum in cultural heritage? Journal of Cultural Heritage (2013), http://dx.doi.org/10.1016/j.culher.2013.10.010 ARTICLE IN PRESS G Model CULHER-2808; No. of Pages 6 Journal of Cultural Heritage xxx (2013) xxx–xxx Available online at ScienceDirect www.sciencedirect.com Original article Strontium, a new marker of the origin of gypsum in cultural heritage? Enrico Franceschi ∗ , Federico Locardi Department of Chemistry and Industrial Chemistry, University of Genoa, Via Dodecaneso 31, 16146 Genova, Italy a r t i c l e i n f o Article history: Received 13 November 2012 Accepted 28 October 2013 Available online xxx Keywords: Strontium X-ray fluorescence Gypsum Cultural heritage Paintings a b s t r a c t A new possible methodology for recognizing the nature of gypsum in artworks, in particular for wall paintings, was developed. Calcium sulfate can be an alteration product of the calcium carbonate, or a component used by the Artist himself; the problem of identification of the presence and the nature of gypsum could be found detecting the presence of strontium. This element could be used as a marker since, differently from the alteration form, in mineral gypsum simultaneous presence of calcium and strontium occurs. The correlation between these two elements may be recognized using non-invasive in situ X-ray fluorescence measurements. In the present work, we tested this occurrence in various mineral samples of gypsum and alabaster as well in tempera, fresco and Egyptian paintings. Considering the new possible role of strontium indicating the presence of natural gypsum, we expect to provide a valuable tool for conservation scientists, restorers and art historians. © 2013 Published by Elsevier Masson SAS. 1. Research aim The object of this study is focused on strontium, an element that could be used as a new marker in order to identify the origin of gypsum in cultural heritage: natural or an alteration product? Several mineral samples, which led to the identification of a possible association of calcium with strontium, were analysed. This correlation was subsequently tested on some real case studies: wall paintings, stucco, icons, and painting on wood. 2. Introduction The technique of wall painting, known under the Italian term of a fresco, is widespread in all age and in every country. Many authors in the past described the good fresco technique, emphasizing the right combination of ingredients and the painting procedure on wet plaster [1–5]. Due to the alkalinity of the medium, a limited number of pigments (chiefly earths of various colours) mixed simply with water or lime water can be used. In order to have a greater colour palette, painters have often used variants of the technique by paint- ing the dried plaster. In this case, different a secco techniques and binders have been used, as egg white or egg yolk, various kinds of natural substances as wax, honey, casein, animal glue, resins or oils [6]. In our recent studies, we encountered also the use of gyp- sum as inorganic medium for the pigments and, in the same time, ∗ Corresponding author. Tel.: +39 01 03 53 87 56; fax: +39 01 03 53 61 02. E-mail addresses: franceschi@unige.it (E. Franceschi), federicolocardi@gmail.com (F. Locardi). forming a fine plaster. It is well known that a widespread alteration of mural paintings is due to the phenomenon of sulphating of the lime, caused by the environmental pollution or water penetration, especially in the surface. This phenomenon is very dangerous for the artworks as it may cause the detachment of the paint film. In the same time, if calcium sulfate di-hydrate (CaSO 4 .2H 2 O) in the form of gypsum has been used by the painter, analyses are not able to differentiate between these two possibilities. So, one of the main diagnostic problem remains the correct individuation of the gyp- sum in the wall paintings if it is really due to alteration processes or not. We encountered a similar problem in studying the preparation layers and painted layers of some Albanian medieval icons and an ancient Egyptian sarcophagus. In fact, for the paintings made on wood, one of the common preparation layer was usually obtained by mixing animal glue with gypsum [7] or, in fewer cases, with lime [8]. In a recent work conducted on mural paintings, we have encoun- tered a difficulty in determining the source of gypsum presence. In all cases where the study is conducted using micro-sampling and cross-section methods, it is possible to examine the different layers separately, so generally the preparation method used by the painter is clear. On the contrary, in the case of in situ study, using X-ray flu- orescence, in presence of lead pigments, as lead white or red lead, it is practically impossible to distinguish between a gypsum or a lime preparation. In fact, a peak overlap occurs when sulphur (SK 2,31 keV) and lead (PbM 2,35 keV) are present in the same sam- ple and the precise identification of sulphur becomes very difficult. However, if the gypsum presence is confirmed, also with another analytical methods, remains the problem of its formation: natural or alteration gypsum. 1296-2074/$ – see front matter © 2013 Published by Elsevier Masson SAS. http://dx.doi.org/10.1016/j.culher.2013.10.010