Polonca Ropret,
Institute for the Protection of Cultural Heritage of Slovenia, Restoration Center,
Poljanska 40, 1000 Ljubljana, Slovenia

Peter Bukovec
Faculty of Chemistry and Chemical Technology, University of Ljubljana,
Aškerčeva 5, 1000 Ljubljana, Slovenia

Darko Hanžel
J.Stefan Institute, Jamova 39, 1111 Ljubljana, Slovenia
 
 

Abstract
Brown layers have occurred just below the façade surface of some archaeological buildings. They are a consequence of chemical changes, which can give rise to further destruction.
Brown layers and undamaged plaster underneath the brown layers consist of  CaCO3, SiO2, Ca2SiO4, CaMg(CO3)2, silicate minerals containing Ca, Al, Mg, K, Fe, metal hydroxides and sulphates, with some surface bound water. In both layers coordination of Fe3+ ions is tetrahedral and octahedral. In brown layers there are more Fe3+ ions at octahedral sites and there is an evidence of a change in chemical environment of ferric ions. The difference between brown layers and undamaged plaster underneath the brown layers is in goethite (a-FeOOH) formation. It is microcrystalline and some of the Fe3+ ions are substituted by a non magnetic ion, probably Al3+. Iron in oxidation state 2+ is present only in the brown layer of sample ZMV 7. One of the possible ways of its genesis is reduction of Fe3+ due to SO2 present in the air.