Why glass is not a slow liquid

Glass is not a slow liquid at room temperature, because its atoms are locked in place even though they are arranged without the repeating order of a crystal. That disordered but rigid state is an amorphous solid, not a liquid waiting to slump.

Old windows are not thicker at the bottom because the glass has crept downward over centuries. The panes were uneven when they were made, and the glazier usually set the thick edge at the bottom for a practical fit.

The mechanism is more particular than the slogan. In a liquid, atoms continually rearrange and the material takes the shape of its container on a human timescale. In window glass those rearrangements are frozen out, the structure is stuck in whatever disorder it had when it cooled through the glass transition, and the pane holds its shape.

Follow it one step further. Crown and cylinder methods of making sheet glass produced pieces that were thicker at one edge than the other. Installing the thick part downward is a choice a fitter makes, and finding it there centuries later is evidence of that choice, not of flow.

The case worth keeping in mind is specific. Medieval and early modern windows often show the thick edge down, and people have taken the pattern as a clock of liquid motion. Panes installed the other way, thick edge up or to the side, sit in some old buildings too, which is an awkward fact for a gravity-driven liquid.

That case has a boundary. The claim is about ordinary temperatures, far below the softening range. Heat glass toward the temperatures where it is worked and it does flow, which is how blowing and pressing are possible, and that hot flow is not a cold window sagging.