A quick note on historic glass

posted Jan 11, 2016, 1:14 PM by Phillip Barlow   [ updated Jan 11, 2016, 1:30 PM ]

Have you ever heard that old glass is wavy because it has sagged over time?  Common myth, but not true.  The waviness, ripples, bubbles, and other imperfections are due to the manufacturing process of their time.  In fact, it wasn’t until c1960 that the “float glass” technique produced the very clear and consistent glass that we know today.    Glassmakers up until that point were aware of the imperfections and even provided guidance for proper glazing that took them into account.

“All window glass, no matter by what process it is made, has a “grain” that extends in the direction the glass was drawn.  Ordinarily, in glazing, this “grain” should be vertical in the sash or frame, as the glass presents a much better appearance when viewed in a reflected light if so glazed.  But when glazing sash to be used in automobiles, railroad coaches, or other vehicles, the “grain” should be horizontal in the sash.  When so glazed the view from the vehicle is clearer and freer from distortion than when the “grain” is vertical in the sash.”

Monro, W. L., & American window glass company, Pittsburgh. (1926). Window glass in the making: An art, a craft, a business. Pittsburgh: American window glass Co. Page 99.


Check out the light passing through the windows.  Can you tell which panes are original and which have been replaced?

Most of the historic glass that we have in Colorado would have been "Cylinder Glass," which is glass that has been produced by blowing air into molten glass, extruding a long cylinder and then cutting the cylinder.  The cylinder is then reheated, laid flat, and smoothed.  This process of drawing the glass produced the ripples that we associate with old windows.  Check out the London Crown Glass Company page for quick descriptions of glass manufacturing techniques.  http://londoncrownglass.com/Manufacturing.html

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