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What Causes the Waves in Old Glass?

Gwen Bruno
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Have you ever wondered why the window panes in many old houses have a wavy, distorted look to them? The charm of old glass is due to the process by which it was manufactured.

Old window glass is wavy because of its manufacturing process.

Crown Glass

In the early 1800s, window glass was made by blowing a bubble of glass, which was then spun until it was flat. This process left a bump, or crown, in the center.

"Bull's-eye" Windows

Despite the imperfection of crown glass, early Americans did not let any part of it go to waste; such “bull’s-eye” glass is commonly found in old buildings.

Cylinder Glass

By the 1850s, crown glass was superseded by cylinder glass, in which molten glass was blown into a cylindrical shape, then opened up into a clear sheet of window glass.

How To Tell The Difference

When you look at crown glass from the side, you will see subtle swirls or ripples caused by the spinning process. Cylinder glass has faint parallel ripples, caused by the differences in the circumferences of the cylinder that made it.

Cleaning Old Glass

Aging and weathering leaves minute pits in the surface of old glass. The antique window specialists at Fairview Glass Co. recommend cleaning old glass with a paste glass cleaner that can be buffed.