I was told by a condensed matter physicist that glass is a liquid with a very high viscosity (it would be more precise to say that it is a supercooled liquid). The example given was that in cathedral windows the glass flows very, very slowly, so they are thicker at the bottom.
I recently read this article in Scientific American, which contradicts the previous statement.
Are there any serious studies about this aspect of glass? Is it only a problem of definition?