[mtbopfuyn]

They wouldn't need a man-sized piece of glass. They'd only need to place several end to end.

Using the sun is a simple process which I've done many times to safely bleach out antique fabric. A solution of salt or lemon juice accelerates the process so the 10 period isn't needed.

[Swordmaker]

Using the sun is a simple process which I've done many times to safely bleach out antique fabric.

This technique is apparently such a bleaching process... however, bleaching from light exposure does not stop. Wilson shadowed certain portions of his linen and exposed the rest through the glass... remove the glass and the rest of the linen should "catch-up" and bleach out as well. The shroud image is now at least 700 years old and shows no signs of having the image bleach out.

[A.J.Armitage]

They wouldn't need a man-sized piece of glass. They'd only need to place several end to end.

Wouldn't the place where the pieces met leave some kind of imprint on the pattern?

Or would the waves or lumps in the glass?

This seems like something to check.