The technique of the mural paintings of the Tournai Cathedral

Abstract

Studies on mural paintings from the Tournai Cathedral (Belgium) were carried out by micro‐Raman spectroscopy. The analyses were performed on micro‐samples extracted from the remaining paintings and architectonic decorations, and concerned the pigments, the binders and, in some case, the stratigraphy. The palette was established and various pigments were identified, according to the Romanesque and Gothic periods. Red color was largely made of iron oxides. Lapis lazuli was also found in abundance, mainly on the areas that are assigned by historians to the Romanesque period. The green pigments revealed the use of atacamite and posnjakite. It was also found that some of the paint layers were applied using egg yolk as binding medium. These layers cover the stone above a very thin coating made of gypsum and chalk. The presence of oxalates was often noticed. This can be related to aging due to degradation of chalk by aggressive lichen, but also as result of a degradation of organic substances applied during past restorations. The gypsum is also altered by partial or complete dehydration, thereby damaging the artwork on which the painting is based. In humid conditions, iron oxide could be transformed in its hydrolyzed counterpart, whose Raman spectrum was often observed. Finally, a stratigraphic analysis was carried out on a multilayer fragment from the outside whitewash of the Cathedral, and despite a clear visual difference in morphology, each layer showed surprisingly the same chemical composition. Copyright © 2006 John Wiley & Sons, Ltd.

On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments

“…The formed calcite is subjected to degradation processes involving acids, from biological activity or binders degradation (when details are added a secco) [32,53]. Carboxylic acids from binding media are expected [54].…”

“…In fact, coquimbite/paracoquimbite Fe 2 (SO 4 ) 3 ·9(H 2 O) were identified in Pompeii, together with magnetite Fe 3 O 4 and gypsum CaSO 4 ·2H 2 O, as a result of the degradation of the fresco paint layer due to SO 2 pollution [73]. On wall paintings in the Tournai Cathedral, on the other hand, anhydrous haematite was found to be converted to iron oxy-hydroxides due to the humid conditions, which resulted in a visible discolouration of some red areas [53].…”

“…Moreover, ions such as Cl − present in the mortar, sand or in the bricks, can react with the basic carbonate to form copper hydroxychlorides ((Cu 2 Cl(OH) 3 ) atacamite, clinoatacamite, paratacamite botallackite) [119,121,123,[126][127][128] and the copper chloride nantokite [103]. Sulphate ions are also likely to be present in wall paintings, especially from the degradation of calcite to gypsum, from gypsum preparation layers [53], or from SO 2 /SO 3 pollution.…”

“…Copper carboxylates, originating from the binding media interacting with the pigment's cation, were observed for both tempera and oil binders [55,133,134]. Acidic conditions are commonly found in, and surrounding, polychrome objects, as a result of biological activity on the object [53] or of degradation of the organic binders (e.g. oxalic acid [129,135]).…”

“…The release of acetic and formic acid by the organic building materials of the display cases has been ascertained [136][137][138]. The copper oxalate mooloite was studied [32] and identified in various works of art [53,135]. Bluish Cu acetates, corresponding to the well-known pigment verdigris, were identified on pure pigment powders exposed to acetic acid atmospheres [109].…”