How the Shroud of Turin was made

or: adventures in medieval optical engineering ... and a challenge for the bold

So … remember this epic tweet?

It all started in late August in San Francisco's Lower Haight. I was sitting alone at a restaurant (Otra … I recommend the Mezcal Negroni) when I came across this tweet…

Perfect bait post. Here's the thing though - with my background in camera hardware, this got me thinking: if I were a 14th-century person with contemporary technical skills, how would I actually make the Shroud? Given that humanity's understanding of optical engineering was in its infancy (we’d barely invented eyeglasses), I started thinking about ways to form an image without a lens.

The first hint — the shroud is often depicted in the negative (like in the tweet above) — but it actually looks like this:

In a sense … the shroud itself is a photonegative … but how?

UV Light

Then it hit me - linen naturally bleaches in the sun. Ultraviolet light exposure causes it to lighten. If you masked off the desired shape before leaving it in the sun, you could form an image on the linen just like a photographic negative. And that's when another piece clicked into place: the pinnacle of fine art in 14th century France was stained glass.

Sainte-Chapelle in Paris

Imagine this scenario: a monk or artisan notices that a linen curtain, after extended exposure to sunlight through a stained glass window, has taken on the pattern of the glass. In the post-crusades era, there was a massive market for holy relics, creating real demand for... let's call them "creative reproductions."

This is about the point where I got in touch with Nat — given the history with the Vesuvius Challenge I thought this would be up there in terms of historical quests.

We sent someone to Sainte-Chapelle with a variety of light meter equipment to attempt to measure if the medieval-era glass would actually let through enough UV to do this — and didn’t exactly get a null result beyond “churches are dim inside”. I separately read up on the history of soda-lime glassmaking and became convinced enough UV was let through to allow linen to naturally bleach.

Contemporary depictions in Stained Glass

Online research led me to a blog post that showed how the depiction of Jesus Christ on the Shroud closely resembled contemporary stained glass depictions from the same period. But while the original blogger saw this as evidence supporting the Shroud's authenticity, I drew a different conclusion: the artist behind the Shroud was simply inspired by the French artistic style of the time. Makes perfect sense, right?

Initial Testing

Now it was time to begin some physical testing. I happened to have a UV-C bulb in my basement (as one does), so I ordered some raw linen from Amazon. A couple hours later... results! Clear evidence that UV light could create the effect we see in the Shroud. I ran a less dramatic test in my window using natural sunlight over about two days, confirming that UV-A (in a semi-shaded backyard in San Francisco) could produce similar results.

UV-exposed linen on top

The Glass Size Question

But here's where it gets really interesting. I initially worried about how medieval artisans could produce stained glass windows large enough for a full-body image. The churches of the time had massive windows, but they were typically constructed from smaller pieces joined with lead cames.

Cames connecting pieces of stained glass

Me attempting to work out if light could “get around” lead cames joining individual pieces of glass and avoid them showing on the linen.

The answer? The cylinder glass method. This technique was invented near the modern French/German border prior to the shroud’s creation, and involves blowing massive cylinders of glass and then “unrolling” them to form flat sheets.

There's actually a company in Germany, Lamberts, that still makes glass using this technique - can see how this works below:

This method could absolutely produce human-sized sections of glass in the 1350s. Even if the sheets weren't perfect, the fold patterns we see in the Shroud could have cleverly masked any imperfections.

A Parallel Discovery

Here's where the story takes an unexpected turn. Through a series of connections, I learned about N.D. Wilson, who had independently explored this theory around 2005. His experiments (documented at shadowshroud.com) perfectly aligned with where my investigation was headed. He demonstrated that painting a design on glass and placing linen behind it would create exactly the kind of negative image we see in the Shroud.

Painted window pane used as a shroud replica mask. (shadowshroud.com)

Resulting image

The Final Pieces

The last question is how a “3D-ish” image was practically formed on the shroud. The three-dimensional appearance likely came from a combination of the spacing between the mask pattern (think thickness of a glass sheet) and the sun's movement across the window throughout the day. N.D. Wilson’s work actually replicated this effect by aligning the glass/linen at different angles relative to the sun. A perfect replication likely requires a bit of raytracing work to validate what actually happened in the real shroud.

So... What Now?

Could we make a perfect 1:1 replica using these techniques? Absolutely. Get cylinder-blown glass from Lamberts, paint or glaze a pattern derived from the raytraced inverse of the Shroud image with the "glass shadow mask" over it, and you'd likely end up with something remarkably similar to the original Shroud. While I initially planned to do exactly that, after discovering the prior work, I think it's better to leave this challenge open to the community. I’m very excited to see if someone else can pick up the torch here and make a full-scale recreation of the shroud.