Old Window Glass Did Not Slowly Sag
The ripples in historic windows are not evidence that glass has been flowing downward for centuries. At ordinary temperatures, glass is so rigid that its shape does not noticeably change over a human lifetime. The familiar explanation—that old panes sagged under gravity, leaving their bottoms thicker—is a myth.
The unevenness was already there when the panes were made. Before modern methods could produce consistently flat sheets, glassmakers shaped molten glass by hand. In one common process, a blown cylinder was split and flattened; in another, a spinning blob spread into a circular sheet. Neither method reliably produced perfectly uniform thickness, so the finished pane could bend and distort the view.
Why the Waviness Looks Like Sagging
A pane with uneven thickness can appear to have a thicker edge at the bottom simply because of how it was cut, positioned, or selected for installation. Some panes were installed thick-edge down, but that does not show that gravity moved the glass there. Others were mounted in different orientations.
The distortion is therefore a record of manufacturing, not slow motion. Light passing through regions of different thickness changes direction slightly, making straight lines outside look wavy. In old houses, that small optical quirk is a visible trace of how glass was shaped before uniformly flat window panes became practical.
