Please cite this article in press as: V. Matovi ´ c, et al., The origin of syngenite in black crusts on the limestone monument King’s Gate (Belgrade Fortress, Serbia) – the role of agriculture fertiliser, Journal of Cultural Heritage (2011), doi:10.1016/j.culher.2011.09.003 ARTICLE IN PRESS G Model CULHER-2552; No. of Pages 12 Journal of Cultural Heritage xxx (2011) xxx–xxx Available online at www.sciencedirect.com Original article The origin of syngenite in black crusts on the limestone monument King’s Gate (Belgrade Fortress, Serbia) – the role of agriculture fertiliser Vesna Matovi ´ c a,∗ , Suzana Eri ´ c a , Aleksandar Kremenovi ´ c a , Philippe Colomban b , Danica Sre ´ ckovi ´ c-Bato ´ canin a , Nenad Matovi ´ c c a University of Belgrade, Faculty of Mining and Geology, Department of Mineralogy, Petrology, Crystallography and Geochemistry, Djusina 7, Belgrade, Serbia b LADIR, UMR 7075 CNRS, and Université-Pierre-and-Marie-Curie, 94230 Thiais, France c The Highway Institute, Kumodraska 257, Belgrade, Serbia a r t i c l e i n f o Article history: Received 25 February 2011 Accepted 1 st September 2011 Available online xxx Keywords: Limestone Black crust Syngenite Gypsum Fertiliser a b s t r a c t Black crusts formed on limestone built into the King’s Gate represent the most important process in stone deterioration that is occurring in this part of the monumental complex of the Belgrade Fortress (Serbia). Of special importance is the association of salts (namely gypsum and syngenite). Syngenite is a common secondary deposit on granite monuments and on medieval stained glass (i.e., on K-containing materials). However, its appearance over calcareous substrates is not apparent, particularly in cases where cement mortar was not used for bounding. The origins of the potassium and sulphate ions required for syngenite formation are related to meteoritic water, which penetrates the soil above the arch of King’s Gate. Water dissolves some soil components and becomes enriched with various ions before coming into contact with the limestone blocks. Enriched water contains two times more K + and SO 4 2- ions than pristine meteoritic water does. The source of the required ions is potassium-sulphate that is present in agricultural fertilisers that are used above the monument. The proposed mechanism for syngenite formation was additionally supported with laboratory experiment. The results of X-ray diffractometry and SEM-EDS analyses of limestone treated with potassium-sulphate solution and sulphuric acid suggest the possibility that the syngenite was formed over calcite: CaCO 3 + K 2 SO 4 + H 2 SO 4 → K 2 Ca(SO 4 ) 2 H 2 O + CO 2 ↑ However, the complex mechanisms of gypsum and syngenite formation under natural conditions (vari- able concentration of potassium and sulphate ions, intermediates phases, temperature changes, humidity, the amount of disposable water etc.) do not exclude the possibility of syngenite formation over gypsum. © 2011 Elsevier Masson SAS. All rights reserved. 1. Research aims Syngenite is the common mineral in black crusts derived on buildings constructed of potassium rich building materials (gran- ite, cement mortar, etc.). However its presence on potassium poor materials like limestone is extremely strange. The black crusts that have developed on the limestone blocks built into the King’s Gate (Belgrade Fortress, Serbia) are the domi- nant and most destructive decay forms. The presence of syngenite beside common gypsum and calcite in black crusts on limestone blocks along with the variable amounts of syngenite motivated the ∗ Corresponding author. Tel.: +38 11 13 33 67 02. E-mail addresses: vesna.matovic@rgf.bg.ac.rs, matovicnenad@yahoo.com (V. Matovi ´ c), suzanaeric@yahoo.com (S. Eri ´ c), akremen@eunet.rs (A. Kremenovi ´ c), colomban@glvt-cnrs.fr (P. Colomban), danicabat@yahoo.com (D. Sre ´ ckovi ´ c-Bato ´ canin). aims of this study. Therefore, in this study were taken into consid- eration: • variable composition of the black crusts depending on the posi- tion of the limestone blocks; • determination of the source of the ions responsible for the pres- ence of different phases in the black crusts; • the explanation of the mechanism of black-crust formation, par- ticularly with regard to syngenite-bearing crusts. 2. Introduction Cultural monuments all over the world have been traditionally made of different stone types. Monuments in Serbia, especially in Belgrade were built most frequently of limestone and sandstone due to their pronounced abundances, decorative appearance and properties conducive to ease of manipulation. 1296-2074/$ – see front matter © 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2011.09.003