How big was medieval glass? Could they make big pieces, or were they all little like those in stained-glass windows?
Wilson’s answer:
When I first presented the Shadow Theory, tentatively, to the student body of New St. Andrews College, I did not know the answer to this question. A member of my audience, however, did. As she had worked a great deal within an artisan glass shop that still used medieval technique she was able to describe the method for blowing a sheet and told me that sheets measuring 6’x8’ were fairly common. When I looked into her explanation, before experimenting, I found that she was absolutely right. Throughout the thirteenth century glass blowers were capable of creating large sheets. They would blow cylinders, up to nine feet or more in length and then, while the glass was still molten enough, they would cut off the bowls that formed both the top and the bottom and slice the cylinder lengthwise, unfolding it into a sheet. The sheets would be cut into small pieces for use in leaded stained-glass windows, or painted while large.
As the Shroud is roughly fourteen feet in length, two pieces of glass would be necessary, both at least six feet long. The image of the front of the man would be produced beneath one and the back of the man beneath the other.
When Dr. Antonio Lombatti, Fellow Researcher in Medieval Church History at the Deputazione di Storia Patria in Parma, Italy was recently asked about the availability of glass large enough to produce the Shroud, he responded, “Of course a medieval artist could have enough glass to produce that relic.” He pointed out that six foot painted glass windows were not uncommon, and also mentioned that the length discrepancy between the front and back images of the man in the Shroud (1-2 inches) suggests two different phases of production.
**So, medievals could blow glass sheets large enough for this process, but were they clear enough? Wouldn’t the glass be flawed? How about other materials? Would an image made in sand directly on the cloth, or a thin painted fabric laid out on the linen have the same effect? **
Wilson’s answer:
:Medieval glass, particularly Venetian, would have been plenty clear enough to allow for light penetration (it wouldn’t need to be clear enough for us to watch the hummingbirds in the front garden). That is the first necessary step. As far as flaws go (wavy surface, air bubbles, etc.) at this point I can only extrapolate based on what I’ve seen thus far in my experiments. As I first began work painting images on glass, I soon realized that the sun’s path was important for more than just the three-dimensionality of the image. My paintings, and those of the people helping me, were all positively cluttered with brush-strokes, and brush-strokes are not visible on the Turin Shroud. The theory went this way: as the sun travels it will expose every brush-stroke from a mathematically infinite number of points. The distinct shadow created by a brush-stroke would be dispersed along a line in reverse of the sun’s path. In addition, the sun’s route would shift a degree or two (or three, depending on the amount of time the linen was exposed), and the smoothing out of the brush stroke (and the linear path of that dispersal) would spread out in width as well. The cloth that was exposed beneath a sun lamp was used as a control for just this reason. When exposed beneath a stationary light source, the crudeness of the image remains, or worsens (with brush strokes visible). When the same image is placed over cloth beneath a traveling sun, the strokes disappear and the image becomes fluid. This same process would occur for flaws or waves in the glass. Some of the chunky spots on the painting I used are far more drastic than a glass flaw, and yet they are invisible in my image. Also, due to the proximity of the cloth to the glass (flush beneath it), potential refraction of light due to waving, would become negligible. Even more so because of a traveling sun.
As far as sand on the cloth is concerned, an image would certainly be created, but it would not be three-dimensional as there would be no gap between the sand and the cloth to allow for the sun’s path to shift and sculpt the shading. A thin cloth would work, and is something that I have discussed with several people at times. It would only work if it were stretched tight over a frame (to create the necessary distance between the painting on its surface and the linen beneath it). The frame itself could not be too tall, probably no more than one half, to three-quarters of an inch. However, I find glass more natural and believable to a medieval forger. For one, glass accounts better for those never mentioned wrinkles on the cloth. There are regular constellations of wrinkles on the Shroud that predate the image-formation (there is blood in at least one of them). These wrinkles are generally the same color as the image on the Shroud, but occasionally appear darker. In the shadow theory, if a wrinkle were trapped beneath the glass, the linen within would be as protected, or more, as that beneath the painted portions. The result is a dark image, but also many dark wrinkles. I have yet to encounter any other explanation for these wrinkles, though there may be one.
In the end, like my initial testing of this image-making process, I can’t know exactly how linen will react beneath medieval glass until I actually test it, something I hope to do this summer. However, I believe what I’ve learned from the disappearance of my brush-strokes, will apply equally well to glass flaws.