Spongy hydrogels clean rough paintings

Conservators at museums work hard to keep the artwork in their care clean and free from damage so it can be enjoyed for centuries to come. Regrettably, artists don’t always make that task easy. They often use delicate materials such as paper in their artwork, or industrial paints and pigments that are easily damaged by cleaning solvents. In a technique that’s been especially popular in recent decades, some artists apply paint to canvas in thick brush strokes to create three-dimensional, textured paintings with lots of nooks and crannies for grime to enter and occupy.

For several years, Piero Baglioni and his colleagues at the University of Florence in Italy have been using chemistry and materials science to solve pressing problems in art conservation. Among their recent creations are polymer hydrogels—sheets of soft, rubbery material—that gently and controllably release small amounts of cleaning fluid onto an artwork surface: just enough to remove dirt and residues, but not so much as to damage the original piece.

Although the gels work well for cleaning flat surfaces, they’re too stiff to conform to the irregularities of rough contemporary paintings. Now Baglioni and colleagues have concocted a new hydrogel to solve that problem. They start with a viscoelastic solution of polyvinyl alcohol (PVA) chains in water, which they freeze and thaw to create a porous gel. As the water crystallizes and expands, it presses the chains together into a spongy solid.

In a homogeneous PVA solution, ice crystals grow long, thin, and straight. The resulting freeze–thaw gel is soft and conformable, but its narrow parallel pores aren’t quite effective enough at removing dirt from a mock painting, as shown by the rectangle on the left in the figure. In the final key refinement, the researchers started instead with a mix of long- and short-chain PVA. The short-chain molecules clump together into micron-sized blobs, which disrupt the growth of the ice needles. The new freeze–thaw gel, with its irregular network of round, interconnected pores, is far better at cleaning, as shown by the rectangle on the right.

Beyond just testing on mock-ups, the researchers have already used their gels to restore two valuable Jackson Pollock paintings to their original 1940s glory. The works—Two and Eyes in the Heat , both housed at the Peggy Guggenheim collection in Venice—predate Pollock’s best-known poured paintings. But their surfaces are similarly rough and, until now, hard to clean. (R. Mastrangelo et al., Proc. Natl. Acad. Sci. USA, 2020, doi:10.1073/pnas.1911811117 .)