The Mona Lisa is the famous Leonardo da Vinci painting of a woman with a mysterious smile. This week, the painting gave up a secret. 

Scientists using X-rays to examine the chemical structure of a small part of the painting discovered a technique Leonardo used in the work. An oil paint used for it was a special, new chemical mixture, which suggests that the Italian artist was in an experimental mood when he worked on the painting in the 16th century. 

"He loved to experiment, and each of his paintings is completely different technically,"said Victor Gonzalez, a chemist who has studied the chemical element (成分) of several works by Leonardo and other artists. The researchers found a rare lead compound (铅化合物) — plumbonacrite, in Leonardo's first layer of paint. The discovery proved that da Vinci most likely used lead oxide to thicken and help dry his paint. The paint in the study is about the thickness of a human hair, lying in the top right area of the painting.

The scientists looked into its atomic structure using X-rays, moving particles at the speed of light, permitting researchers to look deeper into the paint structure. " Plumbonacrite is really a fingerprint of his recipe, as it's the first time we can chemically confirm it," Gonzalez said.

Dutch artist Rembrandt may have used a similar mixture when he was painting in the 17th century. Gonzalez and other researchers have found plumbonacrite in his work, too. Leonardo is thought to have put lead oxide powder, which has an orange color, in the oil to make it thicker and dry faster. "What you will get is an oil that has a very nice golden color," Gonzalez said. "It flows more like honey. "

But the Mona Lisa — said by the Louvre to be a portrait (肖像) of Lisa Gherardini, the wife of a Florentine silk businessman — and additional works by Leonardo still have other secrets to tell. "What we are saying is just a little brick in the knowledge," Gonzalez said.