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VIDEO links: view educational videos showing how glass is made and how
mirrors are made! This link will take you to the Glass Association
of North America website's
video page.
The following information is available here at mammen.com:
I. Glass - a solid or a liquid?
Glass is a liquid! Like all liquids, glass has viscosity,
which means it resists flow. Unlike most liquids that you
are familiar with, however, glass is so viscous at room
temperature that its flow can only be measured with very
delicate instruments. In fact, at room temperature, it would
take the glass in your house 30 thousand million years to
deform the same extent that it would in one minute at 1300
degrees Celsius. Given long enough, glass behaves the same
as any ordinary liquid.
The liquids that constitute glass may freeze, which means to
crystallize into an ordinary solid. The material that
results is said to be "devitrified" and is no longer
considered to be glass. This process must take place at
approximately 1000 degrees Celsius. Therefore, though it
sounds like a paradox, glass is a liquid that is too cold to
freeze!
[1/26/2000 Read
a related article here; but please come back!]
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Education.
II. A brief history of glass.
Glass is rarely found in nature. There are places, such
as near volcanoes, where natural glass occurs, but it is
usually too impure to be clear. Ancient people realized the
value of this natural glass and formed it into weapons and
tools. Even in fairly recent times, tribesmen in Africa and
Australia used broken bottles and glass insulators from
telegraph poles to make arrowheads and tools.
The ancient Egyptians were the first people to make glass.
The experts are divided about when the Egyptians first made
their glass; it was either around 2500 BC or around 3000 BC
. By the sixth century BC, glassmaking techniques were
well-known throughout the eastern Mediterranean. These
people made glass beads and vessels, and they used
glass-mosaic panels to decorate their homes and furniture.
The craftsmen in Alexandria perfected many early glassmaking
techniques. Around 200 BC, the blowing iron was developed.
This tool, which is used still today, allows a glassworker
to blow through one end of the tube to shape a glob of
molten glass at the other end. The Egyptians and Romans were
also very knowledgeable about coloring glass and produced
beautiful pieces, many of which survive today.
Flat glass was first made by the Romans, though it was of
such poor quality that it provided very little illumination
and even less visibility. During the middle ages,
stained-glass windows emerged as an art form. This art
developed between AD 600 - 900 and was perfected by the 13th
and 14th centuries. Around 1200, Venice became the leader in
glassmaking. They guarded their knowledge and skills
carefully, even going so far as to establish a glassworks on
an island (Murano) to prevent the escape of their craftsmen!
By the 15th and 16th centuries, almost all houses had at
least some glass windows. The ingredients and tools used at
this time differed very little from what we use today.
Up until the 1800s, flat glass was made by spinning a disk
of molten glass at the end of a pontil until it was a meter
or more in diameter. This was called crown glass. It was not
very flat and had many imperfections, which is why windows
of that time were made up of small panes - the discs were
cut so that panes were made from the best parts. Another,
much more costly, method was developed in France in 1688:
the cast-plate process. Molten glass was poured onto a
casting table and then rolled flat by iron rollers. The
resulting plate glass was ground and polished by hand. The
first decent mirrors were made from this type of glass.
Around 1900, another alternative to crown glass was
developed. This was also the first attempt at mechanizing
the manufacture of flat glass. A circular metal bait was
lowered into the molten glass and drew up a cylinder up to
12 meters long and 75 centimeters in diameter. The cylinder
was then split, reheated, and unrolled flat.
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Education.
III. Today's flat glass - how it's
made
In order to produce perfectly flat, distortion-free glass
which is fire-finished on both sides, the molten glass has
to be kept from contacting anything solid. In the 1950s, the
Pilkingtons developed the revolutionary "float" process,
which is how most glass is produced today. As you will see,
the name comes from the fact that the molten glass floats on
molten tin to produce a perfectly flat surface. A float
plant, once operational, produces a continuous ribbon of
glass, 24 hours a day, 365 days a year, for up to ten
years.
Raw materials:
Sand, soda ash, dolomite, limestone, salt cake, carbon,
rouge, feldspar, and recycled glass are precisely blended
and fed by a conveyor into the furnace.
Melting:
The raw materials are melted in a huge, brick-lined,
gas-fired furnace that is about 160 feet long by 35 feet
wide. The temperature inside the furnace reaches 2950
degrees Fahrenheit. The molten glass flows out of the
furnace into the next stage of the process.
The tin bath:
The molten glass from the furnace pours onto a molten pool
of tin forming a large pool of glass. Because the viscosity
of the glass is greater than that of the tin, the glass
floats on the surface of the tin. The atmosphere in the tin
bath is carefully controlled to prevent the metal from
oxidizing. Heat is applied from above to melt out any
irregularities; this allows the glass to conform to the flat
surface of the tin and results in very parallel surfaces on
the glass. The glass is of uniform thickness and excellent
optical clarity. The glass enters the tin bath at 2000
degrees Fahrenheit and leaves at 1100 degrees, which is
sufficiently cool so that the rollers in the next process do
not spoil the underside finish. The continuous sheet of
glass that is drawn from this pool of glass is called a
ribbon. The faster it is drawn, the thinner the glass. The
tin bath section is about 200 feet long.
Cooling the ribbon:
The ribbon enters a cooling tunnel, called an annealing lehr
(pronounced leer) that is 400 feet long. The glass gradually
cools to 120 degrees Fahrenheit as it passes through this
section. This relieves the glass of thermal stresses and
makes it cool enough to handle. Stainless steel conveyors
convey the glass through the lehr.
Inspection:
The ribbon passes through an inspection booth where defects
in the glass are marked so they can be cut out and removed
during the cutting operation.
Cutting and packaging:
The ribbon passes through a series of cutting heads that cut
the glass as it passes by. After cutting, powdered Lucite is
sprayed on the glass as a separator to prevent scratching
during packaging and shipping. The glass is then packed into
a variety of containers for shipment to Mammen Glass and
Mirror, Inc.
Depending on the thickness of the glass being made, it takes
from 15 to 70 minutes from the time the glass enters the tin
bath until it reaches the end of the line.
[Special thanks to Guardian Industries for their input
on this page]
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Education.
IV. Glass fabrication
Cutting:
Glass is cut by "scoring" one side of the glass with a glass
cutter, and then snapping the glass apart along the score.
Most of the glass cutters used today use a precision-ground
steel cutting wheel. While hand-held cutters are still used
quite a bit, most of our cutting is done on our
state-of-the-art computer-controlled glass cutting line. The
system consists of 3 tables that are each 9 feet by 12 feet;
one for loading the glass, one for scoring the glass, and
one for "breaking out" the scores. In this continuous
operation, a PC controls the cutting head, which can cut
rectangles, circles, ovals, and many intricate shapes with
accuracy to within several thousandths of an inch. Glass
orders are optimized before cutting on another PC inside the
office to utilize time and materials as efficiently as
possible.
Polishing:
One process that the cut glass can undergo is polishing. In
this process, a machine-finished high-lustre edge is put on
the glass. The large machines that perform this operation
use from 2 to 5 "diamond wheels," that is, steel wheels with
minute industrial diamond bits impregnated in the cutting
surface. Next, polishing wheels begin to shine the freshly
ground surface. Finally, wheels made of felt that are soaked
with a special cerium-oxide solution give the edge its
high-lustre finish. We highly recommend this type of edge
for desk tops, furniture tops, shelves, or anywhere else
where the edge of your glass will show.
Beveling:
Beveling is very similar to polishing, except that the
grinding and polishing wheels work more on the surface of
the glass than the edge of the glass (or mirror!). It takes
skilled craftsmen to operate beveling machinery, especially
the labor-intensive machines that can bevel circles, ovals,
and other shapes. We are very proud of our employees and the
high-quality beveled products that they produce.
Drilling:
Our precision-drilling equipment drills holes from both
sides of the glass that meet in the center. The special
drill bits used are very similar to the diamond wheels used
in polishing and beveling. Holes can be drilled for many
purposes: hinges, electrical outlets or switches in mirrors,
or the hole you buy your movie tickets through!
Special fabrication:
Mammen Glass and Mirror, Inc. employs skilled glassworkers
who can cut, grind, polish, drill, and notch glass manually
to your exacting specifications. We like to say, "We sell
glass that hasn't been invented!" because if you can think
it up, we can make it.
Tempering:
Through this process of thermal toughening, a piece of glass
is heated to a very high temperature, and then the surface
is rapidly chilled. Once the glass is entirely cool, the
interior is in a permanent state of tension, and the surface
is in a permanent state of compression. Glass that is
toughened in this way can be 4 to 10 times stronger on the
surface than regular glass. Once glass is tempered, it
cannot be cut; all cutting and other fabrication must be
done before the glass is tempered. When the tempered glass
breaks, it instantly shatters into tiny pieces, about the
size of rock salt. Federal law mandates that we use tempered
glass (or another form of safety glass) in and near all
doors, and in other specified applications. We recommend it
anywhere that an added level of safety is desired. For more
information, see our page about safety
glass.
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Education.
V. How to buy glass
We are happy to meet all your glass needs; and we know
that the first one could well be helping you figure out what
those needs are! Glass is something that most people don't
buy every day, and there are thousands and thousands of
products available. We are happy to consult with you about
your needs (see our feedback section and our pricing
section). To get started, though, consider the
following:
What thickness will you need?
For single-pane residential windows, single-strength glass
is generally used, and we can cut it for you while you wait.
Commercial windows are generally 1/4" thick. Desktops and
similar applications also usually use 1/4" or thicker glass.
For shelving, you must consider many variables; we suggest
you consult with us. Tables with pedestal bases usually use
3/4" thick glass.
Clear or tinted?
Glass is available in a variety of colors besides clear, the
most common by far being bronze, gray, and green.
Size?
Be sure to measure before you are ready to buy! If you are
unsure, we can provide this service in most cases. Glass is
measured to sixteenths of an inch in most cases.
Installation?
Will you want to pick up your glass, have it delivered, or
have it installed?
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