Saponification in egg yolk-based tempera paintings with lead-tin yellow type I

0500939 - ÚACH 2020 RIV FR eng J - Článek v odborném periodiku
Švarcová, Silvie - Kočí, Eva - Plocek, Jiří - Zhankina, Assel - Hradilová, J. - Bezdička, Petr
Saponification in egg yolk-based tempera paintings with lead-tin yellow type I.
Journal of Cultural Heritage. Roč. 38, JUN (2019), s. 8-19. ISSN 1296-2074. E-ISSN 1778-3674
Grant CEP: GA ČR(CZ) GA17-15621S
Institucionální podpora: RVO:61388980
Klíčová slova: Saponification * Lead-tin yellow * Egg yolk * Mixed lead carboxylates * Pigment-binder interactions * Gothic painting
Obor OECD: Inorganic and nuclear chemistry
Impakt faktor: 2.553, rok: 2019
Způsob publikování: Open access s časovým embargem

Saponification occurring in paint layers of artworks represents a serious degradation process affecting the appearance and stability of paintings. Formation of metal soaps is mostly reported as a consequence of the interaction of lead white or zinc white with oil binder. We found the distinct symptoms of saponification in paint layers consisting of lead-tin yellow type I and egg yolk binder in a panel painting from the 15th century. This scarce finding induced the study of interactions between lead-tin yellow type I with three different binders: (i) egg yolk tempera, (ii) emulsion of egg yolk and oil, and (iii) oil. The interactions were studied in long-term model experiments using infrared spectroscopy (FTIR) and powder X-ray diffraction (XRD). Based on the results, a set of mixed lead carboxylates of the series Pb(C16)2-x(C18)x, where x = 0, 0.25, 0.5, 0,75, 1, 1.5 and 2, was synthesised as reference materials suitable as a basis for proper identification of neo-formed lead soaps by FTIR and XRD. The formation of lead soaps was detected already within 1 month of reaction in egg yolk containing binders, whereas no soap was formed during 6 months of the experiment in the oil mixture. The neo-formed soaps are various mixed lead carboxylates containing both palmitate (C16) and stearate (C18) in one crystal structure and the proportion of carboxylate anions is affected by the type of the binder. We found the C16:C18 ratio in mixed lead carboxylates is higher in egg yolk tempera samples in accordance with the more abundant palmitic acid in egg yolk fat. On the other hand, the lower C16:C18 ratio in emulsion binder correlates with increased content of stearic acid, suggesting contribution of both binders, i.e. egg yolk and oil, to the soap formation.
Trvalý link: http://hdl.handle.net/11104/0297117