Formation of metal soaps in paint layers exemplify a very complex chemical process, which results from the interaction of fatty components of the binder with certain metal elements in pigments (typically Pb or Zn). The resulting soaps can crystallize, aggregate and protrude to the surface of the painting, where they cause cracking of the paint layer and its falling off. In other cases, the soaps are responsible for colour fading caused by increased transparency of the affected paint layers. Although saponification affects mainly oil paintings, it can occur also in fatty tempera. However, the progression of soap formation in individual cases depends on numerous factors involving the type of reactive pigments, composition of the binder, character of admixtures, concentration of individual components, relative humidity, temperature, type of support, previous cleaning and conservation treatments, etc.

The long-term experimental research focuses on clarification of particular factors affecting saponification in various paint systems. Besides that, analytical methods suitable for detection of secondary changes in paint layers are tested. For proper identification of degradation products, it is often necessary to synthesize and characterize reference compounds due to the paucity of reliable structural data.

Recent publications:
Barannikov R., Vykydalová A., Bezdička P., Hermans J., Plocek J., Švarcová S.: Thermal behavior of mercury carboxylates as paintings’ degradation products. Journal of Thermal Analysis and Calorimetry (2024); https://doi.org/10.1007/s10973-024-13463-3

Garrappa S., Bezdička P., Švarcová S., Hradilová J., Pech M., Hradil D.: Non-invasive evidence of mercury soaps in painted miniatures on ivory, European Physical Journal Plus 138 (2023) 219; https://link.springer.com/article/10.1140/epjp/s13360-023-03847-z

Barannikov R., Kočí E., Bezdička P., Kobera L., Mahun A., Rohlíček J., Plocek J., Švarcová S.: Long-chain mercury carboxylates relevant to saponification in oil and tempera paintings: XRPD and ssNMR complementary study of their crystal structures, Dalton Transactions 51 (2022) 4019-4032; https://pubs.rsc.org/en/content/articlelanding/2022/dt/d1dt04160f

Garrappa S., Hradil D., Hradilová J., Kočí E., Pech M., Bezdička P., Švarcová S.: Non-invasive identification of lead soaps in painted miniatures, Analytical and Bioanalytical Chemistry 413 (2021) 263-278; https://link.springer.com/article/10.1007/s00216-020-02998-7

Švarcová S., Kočí E., Bezdička P., Garrappa S., Kobera L., Plocek J., Brus J., Šťastný M., Hradil D.: Uncovering lead formate crystallization in oil-based paintings, Dalton Transactions 49 (2020), 5044-5054, https://doi.org/10.1039/D0DT00327A

Kočí E., Rohlíček J., Kobera L., Plocek J., Švarcová S., Bezdička P.: Mixed lead carboxylates relevant to soap formation in oil and tempera paintings: the study of the crystal structure by complementary XRPD and ssNMR, Dalton Transactions 48 (2019), 12531-12540.5044-5054, https://doi.org/10.1039/C9DT02040C

Švarcová S., Kočí E., Plocek J., Zhankina A., Hradilová J., Bezdička P.: Saponification in egg yolk-based tempera paintings with lead-tin yellow type I, Journal of Cultural Heritage 38 (2019) 8-19; https://www.sciencedirect.com/science/article/pii/S1296207418306940?via%3Dihub

Top cited:
Garrappa S., Kočí E., Švarcová S., Bezdička P., Hradil D.: Initial stages of metal soaps` formation in model paints: the role of humidity. Microchemical Journal 156 (2020), 104842; https://www.sciencedirect.com/science/article/pii/S0026265X19321848?via%3Dihub (36 citations, 6 per year in average)

Blisters on the surface of the painting (A) as a result of metal soaps’ formation in the red ground layer (by the interaction of oil with minium), growing and protruding to the surface, as shown on microphotos in visible (B) and UV light (C)