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Uncovering lead formate crystallization in oil-based paintings
Silvie Švarcová1, Eva Kočí1, Petr Bezdička1
1Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-ŘeŽ 1001, 250 68 Husinec-ŘeŽ, Czech Republic. svarcova@iic.cas.cz koci@iic.cas.cz bezdicka@iic.cas.cz garrappa@iic.cas.cz plocek@iic.cas.cz stastny@iic.cas.cz hradil@iic.cas.cz.
Dalton Transactions (Cambridge, England : 2003)
|March 19, 2020
Summary
Newly formed lead formate (Pb(HCOO)2) crystals are detected during oil paint drying. This study reveals lead pigments degrade via formic acid, impacting artwork over time.
Area of Science:
- Art conservation science
- Materials science
- Chemistry
Background:
- Lead carboxylates form in oil paintings, causing deterioration.
- Their formation is often assumed to be a long-term process.
- Early detection of lead carboxylates during paint drying is possible.
Purpose of the Study:
- To investigate chemical changes in lead pigments during oil paint drying.
- To uncover the degradation mechanism of minium (Pb3O4) in drying oil films.
Main Methods:
- X-ray powder diffraction (XRPD)
- Solid-state NMR (ssNMR) spectroscopy (13C and 207Pb)
- Fourier-transformed infrared spectroscopy (FTIR)
- Gas chromatography-mass spectrometry (GC-MS)
Main Results:
- A degradation mechanism of Pb3O4 via lead formate (Pb(HCOO)2) crystallization was identified.
- Formic acid formation during oil autoxidation was confirmed.
- Pb3O4 reacts with volatile formic acid, forming Pb(HCOO)2.
- Co-existence of crystalline Pb(HCOO)2, amorphous phases, and remaining Pb3O4 was observed.
Conclusions:
- Lead formate (Pb(HCOO)2) is an intermediate in the degradation of Pb3O4 to lead soaps or carbonates.
- Understanding these early-stage chemical changes is crucial for art conservation.
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