Protective coatings of wax and oil on mediaeval wall paintings in Croatia

U tekstu se razmatraju rezultati laboratorijskih istraživanja uzoraka uzetih s površine srednjovjekovnih zidnih slika iz šest hrvatskih crkava, sagrađenih u razdoblju od posljednje četvrtine 11. stoljeća do druge četvrtine 14. stoljeća. Uzorci su uzimani iz crkava u Šilovu Selu na otoku Šipanu (crkva Sv. Ivana Krstitelja), u Stonu (crkva Sv. Mihovila), u Zadru (zvonik crkve Sv. Marije i katedrala Sv. Stošije), Dubrovniku (franjevački samostan Male braće) i u Zagrebu (kapela Sv. Stjepana). Uzorci su analizirani metodama tankoslojne kromatografje i infracrvene spektroskopije, u kojima je u zaštitnome sloju zidnih slika utvrđena prisutnost voskova i ulja. Temeljno pitanje o kojemu se u tekstu raspravlja jest je li riječ o izvornim zaštitnim slojevima ili se radi o slojevima koji su na zidne slike naneseni u prijašnjim restauratorskim zahvatima. U pokušaju da se odgovori na to pitanje, razmatraju se antički i srednjovjekovni izvori, te poznati povijesni podaci o zidnim slikama i oni vezani uz prijašnje konzervatorsko-restauratorske zahvate.The paper discusses the results of laboratory analyses of samples taken from the surfaces of mediaeval wall paintings in six Croatian churches, erected in the period between the last quarter of the 11th century and the second quarter of the 14th century. The samples were taken from churches in Šilovo Selo on the island of Šipan (St. John the Baptist’s), in Ston (St. Michael’s), Zadar (belfry of St. Mary’s and Cathedral of St. Anastasia), Dubrovnik (Franciscan monastery of Friars Minor) and Zagreb (Chapel of - St. Stephen). The analytical methods applied were thin-layer chromatography and infrared spectroscopy, and they revealed the presence of waxes and oils in the protective layers of the wall paintings. The main issue raised by the paper is whether these are the original protective coatings or coatings that have been applied to the wall paintings within the scope of previous restoration interventions. In an attempt to answer that question, antique and mediaeval sources have been consulted, and known historical data on wall paintings, and information concerning previous conservation and restoration works, have been studied. The conclusion has been drawn that further analysis of protective coatings from the surfaces of the wall paintings is needed and that new samples should be taken for the purpose of confirming the previous results. Such additional investigation should primarily determine whether the wall paintings in the Zadar belfry were really protected with microcrystalline waxes. In addition, it should establish what kind of wax was used in Ston, and what kind of oil in Zagreb. Nonetheless, regardless of any additional analysis, the laboratory testing carried out thus far indicates that the analysed mediaeval wall paintings were indeed protected by wax or oil coatings. The research of archival material has not yielded any confirmation of the thesis that these coatings are the results of previous conservation and restoration interventions, so the conclusion may be drawn that they are, in fact, the original protection. This technological detail, though seemingly minor, is very significant both from the point of view of the history of mediaeval wall painting technology in Croatia, and from the point of view of future conservation and restoration research of wall paintings, especially those created before the middle of the 14th century. The fact that very little is known about the waxing of wall paintings in the Middle Ages suggests that this was an antique tradition, which was either preserved on the Croatian coast and islands in towns that existed already in classical antiquity, or was brought there from countries of strong antique traditions, primarily from the south of the Apennine peninsula. Although the southern part of Croatia was exposed to strong Byzantine influences during the centuries in discussion, and although these influences brought with them stylistic and iconographic solutions in painting, no trace of waxing as a method of protecting wall paintings has been discovered in any of the neighbouring Balkan countries. One of the reasons could be very practical, and very pertinent for the whole concept of such protection. It might be assumed that the wall paintings were coated with wax in the coastal region and on islands in order to protect them from frequent changes in microclimatic conditions, including the sudden and large oscillations of relative air humidity, and from aerosols, particularly chlorides (NaCl), that they were exposed to. Furthermore, it is worth noting that the smooth surface of paintings – or marble – is more resistant to the causes of fast degradation, and this could be a reason for the preference for smooth surfaces which continued in countries with more demanding atmospheric conditions. Another reason might lie in the fact that foreign influences were arriving faster and easier across the sea, from the southern Apennine peninsula, than from the Balkan hinterlands. Additional laboratory analyses of the protective coating of the frescos in St. John’s church at Šilovo ceresin wax, Selo have not confirmed the presence of but rather they have established that the coating was made of a mixture of natural turpentine oils and waxes. The staining of wall paintings with oil was probably modelled on the oil-varnishing of paintings executed in tempera on board. Due to the less demanding techno- logical process, oil easily replaced wax. Nonetheless, it is very likely that the primary purpose of the staining of wall paintings, especially those in the Zagreb Chapel of St. Stephen, was a desire to achieve a similar – if not the same – aesthetic and optical impression to that given by varnished tempera paintings on board