Yes, tapping on frescoes can reveal defects

The US Capitol building in Washington, DC, is adorned with multiple lavish murals created in the 19th century by Italian artist Constantino Brumidi. These include panels in the Senate first-floor corridors, Room H-144, and the rotunda. The crowning glory is The Apotheosis of Washington on the dome of the rotunda, 180 feet above the floor. Brumidi worked in various mediums, including frescoes. Among the issues facing conservators charged with maintaining the Capitol building frescoes is delamination. Artists apply dry pigments to wet plaster to create a fresco, and a good fresco can last for centuries. Over time, though, the decorative plaster layers can separate from the underlying masonry, introducing air gaps. Knowing precisely where such delaminated areas are, and their exact shape, is crucial to conservation efforts, yet the damage might not be obvious to the naked eye. Acoustician Nicholas Gangemi is part of a research group led by Joseph Vignola at the Catholic University of America that has been using laser Doppler vibrometry to pinpoint delaminated areas of the Capitol building frescoes. It's a non-invasive method that zaps the frescoes with sound waves and measures the vibrational signatures that reflect back to learn about the structural conditions. This in turn enables conservators to make very precise repairs to preserve the frescoes for future generations. It's an alternative to the traditional technique of gently knocking on the plaster with knuckles or small mallets, listening to the resulting sounds to determine where delamination has occurred. Once separation occurs, the delaminated part of the fresco acts a bit like the head of a drum; tapping on it produces a distinctive acoustic signature. But the method is highly subjective. It takes years of experience to become proficient at this method, and there are only a small number of people who can truly be deemed experts. "We really wanted to put that experience and knowledge into an inexperienced person's hands," Gangemi said during a press briefing at a virtual meeting of the Acoustical Society of America. So he and his colleagues decided to put the traditional knocking method to the test.

Tap, tap, tap...

Since they didn't want to perform knocking experiments on the actual frescoes—"we're not professionals at this sort of technique"—the researchers created their own surrogate frescoes in the lab and embedded their own defects. In this case, the defects were delaminated areas in the shape of a ginkgo leaf. They embedded that leaf-shaped layer of air on a brick backing structure and then applied two layers of plaster. "We know the shape of the defect, whereas on the real artwork, we might not necessarily know the shape," said Gangemi. "This allowed us to test if we're actually resolving the shape of the defect correctly." They used a little hammer to tap on the fresh surrogates in a 19x19 grid pattern, recording the radiated sound. Next, they took advantage of advances in signal processing to analyze those recordings and reconstruct the measured resonances back into a color-coordinated grid. For instance, when one is not tapping on the defect area, the resonance shows up as blue, whereas delaminated areas show up as orange. Among the findings: A resonance between 200 to 500 hertz reveals a substantial difference between delaminated and non-delaminated plaster. Gangemi et al. were able to exploit this feature to examine different frequency bands in hopes of achieving better resolution of the leaf-shaped defect. Higher frequencies, they found, brought the shape into better focus but also added more noise. The researchers were able to identify a "sweet spot" frequency range where the shape is accurately resolved with minimal noise. In short, "We've proven that these tap tests are viable and that different resolutions can give us different benefits," said Gangemi. This work is largely a proof of principle, but it should be possible to use it as the basis for simple, accessible tools for smartphone or computer applications, allowing even untrained users to spot delamination points in frescoes.