The ‘Mona Lisa’ has given up a little secret, after scientists used X-rays to peek into the chemical structure of a tiny speck of the celebrated work of art.

The scientists came up with a new perspective into the techniques used by Leonardo da Vinci when he crafted his portrait of the woman with the enigmatic smile.

The research, published Wednesday in the Journal of the American Chemical Society, says the oil-paint recipe he used as his base layer had a unique chemical signature.

A distinctive compound in Leonardo’s first layer of paint suggests the famously experimental Italian Renaissance master may have been in an inventive mood when he started the ‘Mona Lisa’ early in the 16th century.