Johannes Vermeer left behind few remnants of his life and even fewer of his creative process. No one knows for certain how the 17th-century Dutch artist made his paintings, what he thought about the world around him, or even what he looked like.

That aura of mystery has sent Jane Jelley, author of the new book Traces of Vermeer, searching for obscure materials, testing and re-testing paints, and even gathering pigs’ bladders from butcher shops—all in the name of uncovering the inscrutable master’s process. “The thing is a big puzzle and people do love puzzles,” Jelley, who is also an artist, said. “How did someone do something? We all want to know, and Vermeer won’t tell us.”

American lithographer and etcher Joseph Pennell was the first to voice the idea, in 1891, that Vermeer employed some sort of optical aid in his paintings. But it wasn’t until the turn of this century, with the help of David Hockney, that the issue went mainstream. In conjunction with physicist and optics expert Charles M. Falco, the British artist published a book detailing the “Hockney-Falco thesis”—that the rise of realism in Western art in the 15th century could be explained by the development of concave mirrors, used by artists including Caravaggio, Velázquez, and, of course, Vermeer.

Some art historians have pushed back against these claims. “These qualities in Vermeer’s work may have been inspired by an interest in the camera obscura,” wrote Metropolitan Museum of Art curator Walter Liedtke in 2003, referencing another technology Vermeer may have utilized, “but its importance to the artist has been greatly exaggerated.”

In this unknown lies questions of legitimacy and brilliance. If Vermeer did use these tools, does that undermine his talent? “There are people on one side of the fence who say, ‘No, no, Vermeer can’t possibly have used a lens, he’s too great for that, he never needed to,’” said Jelley. “And on the other side of the fence are people who say, ‘Well, actually, if we look at his pictures there’s a lot of evidence of optical effects which are slightly strange, which suggest he must have used a lens.’”

Jelley’s experiment began after she attended a talk by architect Philip Steadman, who in 2002 published the book Vermeer’s Camera: Uncovering the Truth Behind the Masterpieces. Steadman argued that Vermeer used a camera obscura, a device in which an image is projected through a small hole onto the opposite wall of a darkened space (such as a box or an enclosed room). The resulting picture appears both upside-down and backwards. What Steadman couldn’t explain was how Vermeer converted the projection into a rightside-up painting.

Jelley had an idea. Inside a camera obscura, she painted over the projected image onto a piece of translucent, oiled paper. Since colors can’t be distinguished in the darkness, she used black paint to establish the tones of the painting, marking the darkest places with the thickest paint and leaving the whites untouched. She then pressed the paper face down onto a primed canvas and rubbed to create a transfer—a simple printing technique that corrects the orientation of the projection. Once outside the camera obscura, she added in the color while looking directly at the subject. Jelley believes the dark underpainting helps create the vibrancy of the images because “some of what we interpret as light could come from the white ground of the canvas itself.”

Her process also addresses another quirk of Vermeer’s works. Scientific analysis reveals that his paintings consist of very few layers of paint; Girl With a Pearl Earring (1665), for example, is made of only four. Perhaps the most interesting layer is the first, which “is not a sketchy outline; but a tonal map of his entire composition, which most unusually, is composed of shapes rather than lines,” writes Jelley. “His dramatic underpaintings appear to have arrived all at once on his canvas, without guide marks, and with little correction.”

Jelley doesn’t believe that her argument—that Vermeer employed a camera obscura as part of his artistic process—makes him any less of a genius, however. “I think painters have always used the latest technologies to help them in their work,” she said, “and we’re not any different now.”

The book Traces of Vermeer comes four years after the release of Tim’s Vermeer, a much-discussed documentary chronicling tinkerer and non-artist Tim Jenison as he painstakingly builds a life-sized set of Vermeer painting The Music Lesson (1662–65) in order to test his own hypothesis about the artist’s techniques.

With a mirrored contraption that he has dubbed a “comparator,” he systematically color-matches, mechanically recreating the scene in paint. “A comparator is basically a mirror,” he explained to Hyperallergic in 2013, “and when you look at the mirror you see your subject reflected across the room. Just off the edge of the mirror, you see your canvas where you’re painting.” The idea is that when you’ve matched the color of the image in the mirror with the color of the paint on your canvas, the edge of the mirror will seem to disappear.

Jenison’s results in the film are compelling, but Jelley sees technical issues in his process—one of the most persuasive being that Jenison painted in sections rather than layers, as analysis of Vermeer’s paintings suggests the painter did.

Still, Jelley doesn’t present her theory as a full explanation of Vermeer’s work either. “The thing is,” she said, “what you can’t ever explain is the nature of genius.”