When Chemistry Meets Canvas: Lead Soaps
- symthasreeskoganti
- May 5
- 3 min read
Fairfax, VA
Paintings of old sit in museums and galleries, simply safe in storage, in the middle of restoration, or perhaps hung up on walls awaiting the eyes of admiring audiences. While certainly beautiful, such art, whether painted by Rembrandt, Guda, or any other, carries historical significance as a product from a certain period--acting as a time capsule containing information about techniques, materials, values, and more. However, a common phenomenon threatens to dampen the splendor of these paintings: lead soaps.
Lead soaps are one of the many types of deterioration that can affect paintings. Many art pieces, specifically oil paintings, are usually formed from pigments with both inorganic and organic components. These pigments are mixed with drying oil(s) that act as a binder, creating a hardened film that ensures the paints dry to a durable finish. The resultant mixtures of the two are oil paints, which are applied in layers on a canvas or wood surface. Deterioration can occur wherever these components interact as a product of the resultant interactions, with lead soaps specifically happening when pigments containing lead react with fatty acids from the drying oils or protective coats.
Though lead paint is now known to be toxic and a frequent source of lead poisoning, paints like lead white and lead tin-yellow were, respectively, found in the 7th century BCE and 13th century CE. Both were commonly used in the 15th to 17th centuries for their opacity and fast drying by many artists. Notable examples include Rembrandt's Belshazzar's Feast (1636/1638) and Portrait of Gerard de Lairesse (1665/1667). With these lead oil paints being a popular choice for 15th-17th century artists, a period falling within Europe’s Renaissance, the cultural and historical significance of art pieces produced within this upwelling of arts and sciences is emphasized. Thus, so is a need for a better understanding of lead soaps to potentially predict and prevent them in the field of conservation.
Lead soap aggregates present a concern in art conservation because of their ability to damage artwork. Spanning up to 100–200 μm in diameter and possibly breaking through the paint’s surface, these soaps can increase transparency in paint films (the solid layer that forms when applied paint dries). This alteration can reveal the initial brushstrokes/sketches or even the material surface underneath to viewers. Though concern continues to increase, little is known about lead soap chemistry and formation. Though it is known that it forms from the lead of heavy-metal oil paints and fatty acids from oils used, not all paintings with such materials have resulted in lead soaps, and the timeline for formation (if it occurs) seems to be variable. However, even if a long time may pass before soap formation, visible damage such as that mentioned above happens within 10-15 years once the chemical process has begun. The many uncertainties regarding soap formation certainly don’t help conservationists’ efforts at trying to preserve these works, but continued research and funding towards this problem seem to be lessening the gap.
While many researchers have contributed towards a new understanding of lead soaps, research conducted by the Metropolitan Museum of Art and the University of Delaware stands out as an example of what collaboration between STEM and the arts can accomplish. Methods such as solid-state nuclear magnetic resonance (ssNMR), which identify atomic-level structure and dynamics in solids with spectroscopy, were used to understand the make-up of the lead soaps and reactions that formed them. Three lead salts from paints, lead palmitate, lead stearate, and lead azelate, were studied and found to likely have two different formations from chemically bonding (coordinating) with the fatty acids. This structural discovery gives conservators more information, calling for further research to pursue unanswered questions.
While there is still no perfect solution to prevent or remove lead soaps, metal chelators and careful storage/cleaning offer the best remedies at the moment. Later research, however, could potentially develop stabilizing agents or other materials to use against lead-soap-induced deterioration. Such research, like that above, only benefits from interdisciplinary collaboration. The combined efforts of art and research institutions allow for quicker progress towards scientific knowledge and the next leap to application in the field of conservation. Not only does it bring forth a better understanding of materials science (a chemistry that is easily applicable to areas outside of art), but it directs the attention of an increasingly STEM-driven world to all that the humanities have to offer.
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