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Many paintings from the early 1500s, including the "Mona Lisa," were painted on wooden panels that required a thick, "ground layer" of paint to be laid down before artwork was added.

“In this case, it’s interesting to see that indeed there is a specific technique for the ground layer of ‘Mona Lisa,’” he said in an interview with The Associated Press.

“If it were really hot it would cause the canvas to shrink locally and cause the paint to bubble up somewhat. Hot could seep in faster and disrupt the ground layer, the layer the paint is attached to.”

On land, thawing permafrost - a ground layer that remains frozen - threatens to damage infrastructure such as roads, building foundations and runways, a June 2022 report by the Canadian Climate Institute said.

Like a watercolorist, she sometimes composed around these empty areas: For the bright sheen in a sitter’s hair, for instance, or the light-catching folds of a coat sleeve, she would leave quite large areas of the prepared canvas’s exposed white ground layer.