The application of NMR spectroscopy to characterization in cultural heritage

Date
2017
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University of Delaware
Abstract
Soap formation in traditional oil paintings occurs when heavy-metal containing pigments, such as lead white, 2PbCO3·Pb(OH)2, lead tin yellow type I, Pb2SnO4, and zinc oxide, ZnO, react with fatty acids in the binding medium. These soaps lead to transparency of paint layers and may form aggregates which protrude through the paint surface damaging the integrity of the artwork. The factors that trigger soap formation and the mechanism(s) of the process are not yet well understood. To elucidate these issues, chemical and structural information is necessary, which can be obtained using solid-state nuclear magnetic resonance (ssNMR) spectroscopy. ☐ Despite their versatility, only a few single-crystal X-ray structures of lead carboxylates exist, due to difficulties with solubility. In particular, the structures of long-chain metal carboxylates have not been reported. There are two types of coordination geometry around lead centers, based on whether the lone electron pair in Pb(II) is stereochemically active or inactive. These geometries are commonly referred to as hemidirected and holodirected structures. By studying a series of lead carboxylates, ranging from lead hexanoate (C6) to lead hexadecanoate (C18), with 13C and 207Pb solid-state NMR and infrared spectra, and single-crystal X-ray diffraction of two soaps, the series can be divided in two groups based on chain length. ☐ In more modern paintings, due to concerns about toxicity, zinc white eventually replaced lead white as the common base pigment and filler. It was later discovered that zinc white also reacts with oil binders, forming soaps which often leads to brittle paint which tends to flake. Initial measurements of FTIR and 13C NMR spectra were performed on several zinc soaps to gain insight into their structure. ☐ The basic lead white pigment has proven challenging to observe by NMR spectroscopy using traditional techniques. Dynamic nuclear polarization (DNP) is used here to enhance the ultra-wideline 207Pb solid-state NMR spectra, enabling, for the first time, detection of the basic lead carbonate phase of lead white pigment. Variable-temperature experiments revealed that the short relaxation time of the basic lead carbonate phase hinders the acquisition of the NMR signal at room temperature. The DNP enhancement is twice as large for lead palmitate as it is for the basic lead carbonate, permitting detection of the formation of a lead soap in an aged paint film by 207Pb ssNMR spectroscopy; which may aid in the detection of deterioration products in smaller samples removed from works of art. ☐ Lead−tin yellow type I (LTY-I) has been used by artists from at least the early 15th century until the early 18th century, which has also been linked to soap formation. 119Sn and 207Pb ssNMR spectroscopy were used to characterize the pigment. A combination of NMR techniques and DFT molecular cluster calculations, allowed identification of the individual species in LTY-I and determine their 119Sn and 207Pb chemical-shift tensors. The presence of starting materials from the synthesis, minium and tin(IV) oxide, was also verified. Knowledge of the chemical-shift tensor components and the impurities in LTY-I is important for examining the chemistry of degradation processes and soap formation. ssNMR was used to detect reaction between Pb2SnO4 and added palmitic acid in a model paint sample containing LTY-I. ☐ Finally, 13C NMR was used to track the formation of lead soaps in a basic lead white paint film under different humidity conditions. Initial fits suggest this process is diffusion-driven and the reactive event is a first-order reaction, based on the reaction profile tracked over the course of several days. ☐ Modern techniques in solid-state NMR spectroscopy has provided new insights into the local lead geometries in lead soap formations and the reaction process.
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