The black image of the mural is striking, as nothing similar has been found anywhere else in Slovenia.^[1] The relevant samples also indicate that the layer, whose colour ranges from dark brown to black, was applied directly to the plaster (Fig. 2–3). In some parts of the scene with the female saints and the one with a saintly legend, the original blue and green colours have also been preserved. This suggests the use of azurite and malachite, as confirmed by the Raman spectroscopy and XRF analysis. In al fresco murals, azurite often transforms to green-coloured basic copper chloride (atacamite, paratacamite, etc.) due to moisture, alkaline lime, and chloride salts. Therefore, for many parts of the mural, it is impossible to ascertain whether the original colour was blue or green. Both pigments – azurite, in particular – can further degrade to dark tenorite, which Raman analyses have confirmed for most of the black colour samples. This can occur in alkaline environments (lime) or due to high temperatures. Historical sources document several fires in Ormož, for example in 1487, 1647, 1704, and 1778,^[2] but it is unknown whether the church was also affected each time. Moreover, azurite and malachite are less stable when finely ground. The cross-sections confirmedextremely fine grains of the pigment (Figs. 4–5). On the other hand, XRF analyses have also revealed the presence of lead pigments, which are highly unstable and also tend to darken, degrading into platternite, mainly due to sulphur in the atmosphere.^[3] The lead pigments could not be identified either with the XRF technique or Raman spectroscopy, nor was it possible to establish whether only one or several pigments were used (lead white, massicot, or minium). The third potential cause of the darkening could be the degradation of the organic binder. FTIR has identified the presence of a certain protein binder in the darkened colour layers, which are also very dense and compact (Figs. 2–3), suggesting the use of some organic substance as well. The pigment was probably impregnated with egg tempera, which lowers the thermal stability of azurite and malachite. This could also have led to the darkening of both the pigments and the binder. In the case of the Ormož mural, the phenomenon may be a combination of all three factors and the transformation of all three materials (copper and lead pigments, protein binder) – or just one of them.^[4] Apart from the copper and lead pigments, the painter’s palette also included white lime, yellow and red earth (haematite), an unidentified organic black pigment, and probably green earth (XRF: aluminium, potassium, manganese, iron). In some places, the latter was obviously mixed with malachite (Fig. 5), making it more stable and preserving the green colour, as seen in the depiction of the saintly legend on the northern wall of the main nave. Lime from the plaster served as the main binder, though some organic binder, probably egg yolk, was also added to many of the pigments, as indicated by the FTIR analyses.

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[1] KRIŽNAR, KAVKLER, DOLENEC 2024.
[2] VNUK 2005.
[3] Cf. Alessia COCCIATO, Luc MOENS, Peter VANDENABEELE, On the stability of mediaeval inorganic pigments. A literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments, Heritage Science, 5/12, 2017, https://heritagesciencejournal.springeropen.com/articles/10.1186/s40494-017-0125-6#citeas; Artist’s Pigments. A Handbook of Their History and Characteristics, 1–2 (ed. Robert L. Feller), London 2012².
[4] KRIŽNAR, KAVKLER, DOLENEC 2024.