FLORENCE—Scholars have long suspected that Caravaggio rendered his amazingly realistic paintings using a primitive camera obscura. Now, according to a report in the Guardian, new research suggests that his use of proto-photographic technique may have gone even further: The 16th-century master is now thought to have spread chemicals onto his canvas in order to create a “film” onto which he would burn images that became the basis for his preliminary sketches.

“We were already sure Caravaggio projected images of his sitters, but we have now found mercury salt in his canvases, which is light-sensitive and used in film,” said Roberta Lapucci, head of conservation at Florence’s SACI institute.

Centuries before the invention of photography, artists, including Caravaggio, knew how to project images using lens and mirrors. “You get the image by turning the whole studio into the camera obscura, but you need darkness, and the problem is you cannot paint in darkness,” said Lapucci. “X-ray fluorescence shows the presence of the mercury salt in his canvases. That is not uncommon because it was used in glue, but we are awaiting proof he was using it on the surface, in his primer.”

Even still, the image burned into the primer would last just 30 minutes and was visible only in the dark. “Therefore he used a white lead paint to sketch, mixed with barium sulphate which was luminous, and which we have found traces of. That way he could see where he was sketching.”