Why is Old Glass Wavy?

Old glass has wavy imperfections because of the way it was made, not because it continues to flow. That's a commonly repeated myth.

Plein air painting from Montgomery Place, New York

Very old glass was made by spinning a glass crown disk from a central rod, flattening it, and cutting rectangular pieces out of it. Later, in the early 1900s, glass was made in tall cylinders which were reheated and flattened. After the mid 20th century it has been made by floating molten glass on molten tin, which results in perfectly smooth glass. Whichever way it's made, it doesn't change shape appreciably after being installed in a window.

Antique glass, with its wavy imperfections, distorts both the view through the window, and the reflection of anything behind the viewer. Those distortions are minimal if objects (like the bust) are close to the glass, but the ripple effect grows as objects are farther from the glass. 
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Read more:
The Craftsman Blog All About Wavy Glass
Corning Museum of Glass Does Glass Flow?