I'm actually a geophysicist and I understand that the glass doesn't flow. As I read the article and thought of the potential implications I was reminded of the

I'm actually a geophysicist and I understand that the glass doesn't flow. As I read the article and thought of the potential implications I was reminded of the old cartoon where the middle-aged man with a beer gut or paunch sees an attractive woman approaching so he stiffens his back, stands up straight and puffs out his chest as she passes so he can appear younger and more fit and virile. Once she has passed he lets gravity take control again. I set up that scenario (or joke as has been noted by another reply) with the idea that detecting a middle-aged spread in an old window like that would be nearly impossible if the stained glass used to create the image, Jesus in this case, was constructed from a series of small pieces of glass constrained by a framework of lead caming as is used in stained glass construction. In this case, all the flow, if it occurred at all would be constrained at multiple points in the framework so that the image would remain nearly identical over the aeons as long as the framework existed in good condition. However, if glass did indeed flow and if that flow was detectable over the few hundred years that have passed since those cathedrals were constructed and also if each window was made from a single sheet of this glass with an image painted on the glass - then the image itself could be expected to exhibit noticeable changes over significant time periods and it would naturally follow that young Jesus would eventually become middle-aged Jesus with a paunch, like the guy in the cartoon. It was late at the time and my mind was wandering again, punching at the lines while trying to leak some color to the outside.