Soap Formation in National Gallery Oil Painting

Can you recognize this painting? We can’t either from a one micron scan, but we do know that these images expose different chemical components found in a tiny piece of an oil painting. The formation of soap aggregates is of growing concern to art conservation and restoration, especially in traditional 15th to 20th century oil paintings where soap forms when heavy metal-containing pigments react with fatty acids.

Lead and other heavy metal soap formation appears to be the leading cause of deterioration in these masterpieces. In some cases, the formation of metal soaps causes transparency where paint layers become so deformed that the underlying color or woodgrain of the artwork’s panel supports are revealed. Formerly believed to be the result of gas bubbles or even insect eggs, Photo-induced Force Microscopy (PiFM) shows clear evidence that these deformations are caused by the formation of metal soaps. Nanoscale microscopy techniques like PiFM have become viable tools in art conservation.

National Gallery Oil Painting – PiFM used in Art Conservation

Photo-induced Force Microscopy (PiFM) was used to map a soap cross-section of a tiny piece of an oil painting. The PiFM images were taken at fixed wavenumbers and cropped to a common area due to thermal drift in the images. (Sample courtesy of the National Gallery of Art, Washington, D.C.)

“Since the initial revelation, lead and other heavy metal soaps have been detected and reported to be the cause of deterioration in hundreds of oil paintings dating from the fifteenth to the twentieth centuries.” Silvia A. Centeno, Jaclyn Catalano, Cecil Dybowski, Professor, Nicholas Zumbulyadis, Yao Yao, Anna Murphy “Investigating the Formation and Structure of Lead Soaps in Traditional Oil Paintings”

Soaps are long chain fatty acids that can bind to metal to form metal carboxylates. The metal soap found in these masterpieces is a product of the painting itself, formed over centuries by the marriage of free fatty acids and metal ions within the layers of paint beneath the artwork’s surface. Sometimes, the metal soap protrudes through the paint layer, appearing like a microscopic bubble. It results in the eruption of a metal soap globule through a painting’s surface, in a process called delamination in which layers of paint deform, lift up, and flake off.

To explain the complex chemistry taking place within a masterpiece, Dr. Joen Hermans, a physical chemist who studies metal soap formation at the University of Amsterdam, says, “We are literally watching paint dry” [1].

All images were acquired during our live demo on the floor of the ACS Fall 2017 Exhibition (sample courtesy of the National Gallery of Art, Washington, D.C.) With PiFM, we can deconvolute mixtures of disparate components through IR signals. Note that PiFM can highlight chemical components that are difficult to see in AFM topography and phase images with exceptional spatial resolution.