This months newsletter is the first of a two part series on glass. The articles are designed to better educate you on glass itself; i.e. how it is made, types of glass, and glass safety.

Glass: It’s Everywhere! 

There isn’t a place you can go where glass doesn’t affect our lives in some capacity.  Even on that remote deserted island where castaways are awaiting rescue from their plight a glass bottle is used to carry their rescue note; glass is even a portion of the castaways life.  Skyscrapers, encased in glass, provide the architect with an avenue for his artistic expression by providing a building that looks sleek, finished, and cutting edge.  We drink from glasses, and look through glasses. We use cameras with glass lenses to capture the picture of our dreams.  The applications are endless. 

So what does glass consist of, and how is it made?  What should we be concerned about when we inspect homes with glass shower doors, glass sliding deck doors, and insulated glass windows and skylights?

The origins of glass are lost in prehistory.  Glass appears naturally as obsidian and was used as a material for small bottles and glass beads that date from around the year 3500 B.C.  The first recorded use for glass was for windows that appeared in Roman times. The largest known piece of Roman glass was a crudely cast window from a bath at Pompeii. By the tenth century A.D., the Venetian island of Murano had become the glass making capital of the world.   

The first methods of glass production, at the various Murano glass factories, used either the “crown” or the “cylinder” method. In the late 1700’s and early 1800’s window glass was called “crown” glass. Both the crown and cylinder methods blew a large sphere or bubble then by spinning the bubble the glass was made flat.

In the crown process the cylinder was adhered to a large iron rod called a “punty,” which was opposite the blow pipe.  The blow pipe was removed.  The glassworker would spin the punty rapidly, which would create a large cylinder of glass, or crown, from which the individual panes were eventually flattened and from which panes would be cut. With this method one pane always had the “bull’s eye” where the punty was originally attached and had been cracked off.

In the cylinder process the glass is swung back and forth in a pendulum fashion and then the cylinder was cut off and slit lengthwise. The end portions of the processed glass were cut off and the end product was made into a long rectangular sheet of glass. By the end of the nineteenth century, flat glass was mass produced and was a common material. Later, the cylinder method used compressed air to produce glass that could be split lengthwise, like before, but then the glass was reheated and allowed to flatten on an iron table.   

  Using blowpipe to shape glass    
Source: Wikimedia Foundation

But neither of these methods provided the optical quality for the desired fine mirrors of the day. For this reason the first “plate glass” methods of production were introduced in the late seventeenth century, in France.  Molten glass was cast into frames, spread into sheets by rollers, cooled, and then ground flat and polished. The results were near perfect in their quality, but very expensive to complete.  Mechanization of some portions of the grinding and finishing portions of the process would drop the prices to where glass would find its way to store fronts all across the continent.   By 1851 the “cylinder” method had evolved enough to where the Crystal Palace, in London, was outfitted with over 900,000 square feet of glass.

By the early 1900’s the cylinder glass method of production was replaced by the “drawn glass” method where molten glass was pulled, or drawn, in flat sheets directly from the molten glass. These “plates” of glass would proceed down the production lines for grinding and polishing of the plate glass thus enabling the manufacture of glass to proceed in one continuous production line. 

The mass production technology of modern glass manufacturing is relatively new. It did not appear until after WWII, with a significant change occurring in 1959 when a British company developed a new production method where glass was “floated” across a bath of liquid tin. In this process the glass hardens before it hits a “hard” surface.  Float glass came to the US in the early 1960’s and accounts for over 90 percent of the glass production in the United States today.  Float glass offers nearly the same optical quality that plate glass offers.

The main components used in the manufacture of glass are sand, soda ash, lime and smaller amounts of other chemicals, depending on what it will be used for.  The raw materials are delivered to the glass plant in railroad cars where the powders are then stored in large silos. The various components are then proportioned by weight and mechanically mixed. This mixture is called a “batch.”  Recycled glass or waste glass from previous batches, called cullet, may also be added to the mix. After mixing, the batch goes to the furnaces in batch cars, in hoppers, or on conveyor belts.

The mixture is melted at temperatures that range from 2600 to 2900 degrees Fahrenheit, in furnaces that are called continuous tanks. The largest continuous tanks can melt 400 to 600 tons of mixture per day for the production of flat glass.  The process is continuous with raw materials being fed into the loading end as rapidly as the molten glass is removed from the working end.  

In part two of this newsletter we provide information about the types of glass and saftey legislation. View part II

To learn more about home construction, visit our website's Anatomy of a Home section

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