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Introduction
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.
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
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