In Situ Unilateral ¹H-NMR Studies of the Interaction Between Lead White Pigments and Collagen-Based Binders

The lead white pigment, 2PbCO ₃·Pb(OH) ₂, is thought to play a critical role in the degradation of paint in illuminated manuscripts. Cracking, flaking, and separation of paint films containing lead white on parchment and other works of art have been reported, and have been attributed to the interaction of the pigment with the binding media. A previous study by Fourier transform infrared spectroscopy showed lead white to induce a change in the state of hydration of protein-based binders, though the mechanism of this process is still not well understood. In this work, we apply in situ ¹H unilateral nuclear magnetic resonance (NMR) to explore the nature of the interaction between lead white and collagen-based binders while at the same time we evaluate the feasibility of applying unilateral ¹H-NMR to assess the condition of the paint in medieval-illuminated manuscripts and to follow up the effectiveness of consolidation treatments. Carr, Purcell, Meiboom, and Gill (CPMG) measurements reveal that the addition of lead white to binders derived from collagen, such as bone, rabbit skin or fish glues, increases the T _2eff relaxation constant of these binders. This effect is more pronounced at low relative humidity. The increase in T _2eff suggests that the pigment induces a change in the protein that leads to the formation of more mobile structures, such as peptide fragments, or the partial unfolding of the rigid collagen triple helical structure to a more mobile random coil. The presence of large random coil domains in a collagen-based film has been associated with a lower mechanical strength of the film and therefore with its likeliness to flake.