# Degradation Products Assessment of the Wooden Painted Surfaces from a XVIIth Heritage Monastery
## 1 Introduction
In the tempera technique, mineral pigments are mixed with water and egg yolk. Common pigments were used, as follows: iron oxide for red, iron oxide-hydroxide for yellow, and lapis lazuli for blue [14; 15; 16; 17]. Taking into account all the identified species, the mechanism of metallic soap formation is the following: the polymerization of the oil and the binding of the carboxyl group to the pigment used, following these steps: the migration of metal ions in the network of polymerized oil; the binding of the carboxylate ion to the metal; the crystallization of the metal soap determined by the presence of saturated fatty acids; the diffusion of metal ions and fatty acids to the core of the metal soap, and its migration to the painted surface [18]. In the case of zinc soap, the zinc-containing pigments and drying varnishes readily react with fatty acids in oil-based paints to form zinc carboxylates. Through the formation of these carboxylates, processes such as severe cleavage, the loss of paint, disfigurement due to some agglomerated formations, increased transparency and surface efflorescence occur, are to list only a few problems associated with zinc soaps. As they crystallize, zinc soaps can cause severe delamination - both between and within layers of paint-resulting in the loss of structural integrity of the painting [19]. Different phases were identified during zinc soap formation: amorphous zinc soaps, considered as the intermediate stage, and crystalline zinc soaps, as the final stage in the degradation process, which causes delamination and increased brittleness of the paint [20; 21]. There are many studies related to the deterioration of paintings associated with the aggregation of metallic soaps. In general, zinc-based aggregates contain soaps formed from palmitic and stearic acids [22; 23]. By oxidation, the paint becomes more polar, and the zinc stearate and palmitate formed become less compatible with the paint matrix and form a separate phase [14; 15; 16; 17]. However, the azelaic acid, a product of oxidative degradation of the predominant C18 unsaturated fatty acids in dried oils, is identified near the perimeter of the aggregates [24]. The white crusts that appear on paintings from the 17 th-18 th century can be caused by humidity gradients and capillarity as factors responsible for migration to the surface. The oleic acid levels could be an indicator of embrittlement in zinc oxide paints, due to the oxidation of oleic acid (C18:1) to azelaic acid during paint curing, by comparison with the modern paints, where the formation of fatty acid soaps immobilizes and prevents migration of stearic and palmitic acids to the surface [25]. Metal soaps are complexes of metal ions from pigments with long-chain fatty acids. The adoption of ZnO by the paint industry in artists' paints in France appeared as early as 1784 [26], and in Great Britain, zinc-white watercolor paints appeared in 1834 [27]. The first occurrence of zinc white in oil tubes was only reported in 1860. There is documentary evidence that zinc oxide was progressively incorporated into prepared paints and pigment materials, often undeclared [28]. The characterization of the materials and degradation products (metal soaps) found in this type of artwork, and how metal soaps influence the appearance of the artwork and the selection of the restoration procedures, have to be investigated. Additionally, the paint layers' deterioration based on lead pigments appears as craters (holes with 100 um diameter and protrusions), as whitish protruding materials [15]. For the identification of the components of the painting layer, among the most used specific analytical techniques are: scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), which allows the elemental analysis of the pigments and additives used, gas chromatography coupled with mass spectrometry (GC-MS), the latter allowing the identification of chemical changes in paints during oil drying [29].