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Old Plate Glass - Owners Manual

The phrase “polished plate” is still used to denote that very finest in sheet glass. When we see how plate glass was made in the first half of the 20th Century, we understand why it was and still is so valuable – these factory workers were truly skilled craftsmen.

Plate 1:   Glass making begins in a siliceous sand mine like this one in Crystal City, Missouri. Plate 1
Plate 2:   The glass ingredients are melted at 2,500 to 3,000° Fahrenheit for nearly 24 hours before they fuse into glass. Each clay pot holds 3,000 lbs of material. To be strong enough to withstand the furnace heat, it takes three years, start to finish, to make each pot. Each pot has a useful life of twenty days. Plate 2
Plate 3:   The molten glass is poured onto a water cooled steel table 32 feet long and 20 feet wide. The roller, also water cooled, weighs 25 tons. The glass is rolled to ½ inch thick. Plate 3
Plate 4:   The rolled sheet is moved to a cooling oven, or lehr, 8oo feet long to make a 5 hour trip from red hot to room temperature. At the end of the trip inspectors crawl over it marking all imperfections which will be cut away before the glass is polished. The cooled glass, called rough plate, is translucent, not transparent. Plate 4
Plate 5:   Cut sheets of rough plate are embedded in wet Plaster of Paris on circular polishing tables. Many sheets fitted carefully onto one table. Each table weighs 70 tons. The polishing wheels are driven by 500 horsepower motors. Plate 5
Plate 6:   Iron polishing plates weighing 124,000 lbs are lowered onto the glass and fed with water and increasing finer grades of sand. The final polish is done with emery powder which gives the glass a satin finish.

The glass is moved to a second set of felt wheels and polished to perfect transparency with rouge, also know as iron oxide or super fine rust.

Plate 6
Plate 7:   One the first side is polished, the plates are turned over and the second side is polished in the same way.

Finally the glass is washed in muriatic acid to remove the plaster. Each ½ inch plate is now ¼ inch thick.

Plate 7
Plate 8:   The finished product is inspected, defective areas cut off and the sheets cut to size for shipping.

Making plate glass is expensive. 30% of the original material is lost as gas during melting, half of the remaining material is lost in polishing and 20% of the final product is lost through breakage or rejected as defective.

Plate 8


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.



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Old Plate Glass - Owners Manual