Old Window Glass - Owners Manual

All glass is an amorphous solid or a super-cooled liquid. The glass in old windows is not wavy and distorted because it has buckled in the sun or sagged from the pull of gravity. It is wavy because the outside diameter of a cylinder is greater than its inside diameter. Before the 1960’s manufacturers made two types of clear window glass. Plate glass was poured out in large slabs and ground flat, a very expensive and labor intensive process.

Photo 1: Window glass was made the old fashioned way - by blowing air into a bubble of molten glass. To get beyond the limitations of the human form, huge machines blew compressed air into cylindrical bubbles forty feet high and two feet wide.
Photo 2: First the mechanical blow pipe, the “bait” is dipped into a pot of molten glass.
Photo 3: The “baits” with their “gathers” of molten glass are pulled skywards as compressed air is blown into the cylinders.
Photo 4: The giant cylinders are gently lowered onto wooden racks.
Photo 5: A skilled workman uses a red-hot rod to crack the cylinders into various lengths.
Photo 6: Another craftsman slits each cylinder lengthwise so it can be opened.
Photo 7: The slit cylinder is placed in a slumping furnace. As the glass softens, it is “ironed” into flat sheets.
Photo 8: The flattened sheets are annealed - cooled carefully to release any inner stress.
Photo 9: The final product is graded from AA – sheets with the least distortion, to B – the kind with waves and bubles that we admire today. Double strength cylinder glass (24 oz. per square foot) could be made into 60” x 70”sheets. Sheets of single strength glass (18 oz. per square foot) could be as large as 36” x 50”. In the early 1960’s Pilkington Glass in Great Britain developed and licensed a way to float molten glass on a bed of molten tin. Both the polished plate glass and cylinder glass processes were phased out. To see how float glass is made go to: How Float Glass is Manufactured Modern float glass is perfectly flat, flawless and inexpensive. Makers of modern “reproduction glass” artificially add in the imperfections that the original process sought to eliminate. The old glass has an organic charm, a depth and fire-polish, that mechanical float glass can never achieve.

Excerpted from: Glass, Paints, Varnishes and Brushes, Their History and Manufacture, Pittsburgh Plate Glass Company, published by The Lakeside Press, a division of R.R. Donnelley & Sons, 1923.