About Glass
Modern life just would not be possible without glass.
From the jar that holds the morning marmalade, the mirror in which we brush our
teeth, the windows and car windscreen we look through, the computer screen many
of us use at work every day to the light bulb we switch off last thing at
night; glass is around us everywhere. But what is this amazing substance, where
does it come from and how is it made?
What is glass?
Glass is a combination of sand and other minerals that
are melted together at very high temperatures to form a material that is ideal
for a wide range of uses from packaging and construction to fibre optics. A
form of glass occurs naturally within the mouth of a volcano when the intense
heat of an eruption melts sand to form Obsidian, a hard black glassy type of
stone. Man first used this as tips for spears. Today man has mastered the
glass-making process and can make many different types of glass in infinitely
varied colours formed into a wide range of products. Glass, chemically, is
actually more like a liquid, but at room temperature it is so viscous or
'sticky' it looks and feels like a solid. At higher temperatures glass
gradually becomes softer and more like a liquid. It is this latter property
which allows glass to be poured, blown, pressed and moulded into such a variety
of shapes.
How glass is made?
Glass is made by melting together several minerals at
very high temperatures. Silica in the form of sand is the main ingredient and
this is combined with soda ash and limestone and melted in a furnace at
temperatures of 1700°C. Other materials can be added to produce different
colours or properties. Glass can also be coated, heat-treated, engraved or
decorated. Whilst still molten, glass can be manipulated to form packaging, car
windscreens, glazing or numerous other products. Depending on the end use, the
composition of the glass and the rate at which it is allowed to cool will vary,
as these two factors are crucial in obtaining the properties the glassmaker is
seeking to achieve.
Recycling Glass
The British Glass Recycling Glass website, is a site for
children, students and grown-ups (parents, teachers and other members of the
public) to find out about Recycling Glass and how it benefits the environment
when you recycle it.
Types of Glass
When people speak of glass, they ordinarily
mean a transparent, shiny substance that breaks rather easily. They may think
of the glass in windows and the glass used in eyeglasses as being the same
material. Actually, they are not. There are many kinds of glass. Several
important kinds of glass are discussed in this article.
Flat glass is used chiefly in windows. It is also used in mirrors, room
dividers, and some kinds of furniture. All flat glass is made in the form of
flat sheets. But some of it, such as that used in automobile windshields, is
reheated and sagged (curved)
over molds.
Glass containers are used for packaging food, beverages,
medicines, chemicals, and cosmetics. Glass jars and bottles are made in a wide
variety of shapes, sizes, and colors. Many are for common uses, such as
soft-drink bottles or jars for home canning. Others are made from special glass
formulas to make sure there will be no contamination or deterioration of blood
plasma, serums, and chemicals stored in them see.
Optical glass is used in eyeglasses, microscopes, telescopes,
camera lenses, and many instruments for factories and laboratories. The raw
materials must be pure so that the glass can be made almost flawless. The care
required for producing optical glass makes it expensive compared with other
kinds of glass.
Fiberglass consists of fine but solid rods of glass, each
of which may be less than one-twentieth the width of a human hair. These tiny
glass fibers can be loosely packed together in a woollike mass that can serve
as heat insulation. They also can be used like wool or cotton fibers to make
glass yarn, tape, cloth, and mats. Fiberglass has many other uses. It is used
for electrical insulation, chemical filtration, and firefighters' suits.
Combined with plastics, fiberglass can be used for airplane wings and bodies,
automobile bodies, and boat hulls. Fiberglass is a popular curtain material
because it is fire-resistant and washable.
Laminated safety glass is a “sandwich” made by combining
alternate layers of flat glass and plastics. The outside layer of glass may
break when struck by an object, but the plastic layer is elastic and so it
stretches. The plastic holds the broken pieces of glass together and keeps them
from flying in all directions. Laminated glass is used where broken glass might
cause serious injuries, as in automobile windshields.
Bullet-resisting glass is thick, multilayer laminated glass.
This glass can stop even heavy-caliber bullets at close range. Bullet-resisting
glass is heavy enough to absorb the energy of the bullet, and the several
plastic layers hold the shattered fragments together. Such glass is used in
bank teller windows and in windshields for military tanks, aircraft, and
special automobiles.
Tempered safety glass, unlike laminated glass, is a single piece
that has been given a special heat treatment. It looks, feels, and weighs the
same as ordinary glass. But it can be several times stronger. Tempered glass is
used widely for all-glass doors in stores, side and rear windows of automobiles,
and basketball backboards, and for other special purposes. It is hard to break
even when hit with a hammer. When it does break, the whole piece of glass
collapses into small, dull-edged fragments.
Colored structural glass is a heavy plate glass, available in many
colors. It is used in buildings as an exterior facing, and for interior walls,
partitions, and tabletops.
Opal glass has small particles in the body of the glass that disperse
the light passing through it, making the glass appear milky. The ingredients
necessary to produce opal glass include fluorides (chemical
compounds containing fluorine). This glass is widely used in lighting fixtures
and for tableware.
Foam glass, when it is cut, looks like a black honeycomb. It is filled
with many tiny cells of gas. Each cell is surrounded and sealed off from the
others by thin walls of glass. Foam glass is so light that it floats on water.
It is widely used as a heat insulator in buildings, on steam pipes, and on
chemical equipment. Foam glass can be cut into various shapes with a saw.
Glass building blocks are made from two hollow half-sections
sealed together at a high temperature. Glass building blocks are good
insulators against heat or cold because of the dead-air space inside. The
blocks are laid like bricks to make walls and other structures.
Heat-resistant glass is high in silica and usually contains
boric oxide. It expands little when heated, so it can withstand great
temperature changes without cracking. This quality is necessary in cookware and
other household equipment, and in many types of industrial gear.
Laboratory glassware includes beakers, flasks, test tubes, and
special chemical apparatus. It is made from heat-resistant glass to withstand
severe heat shock (rapid change in temperature). This glass is
also much more resistant to chemical attack than ordinary glass.
Glass for electrical uses. Glass has properties that make it useful
in electrical applications: ability to resist heat, resistance to the flow of
electric current, and ability to seal tightly to metals without cracking.
Because of these properties, glass is used in electric light bulbs and for
picture tubes in television sets.
Glass optical fibers are glass fibers used to transmit information
as pulses of light. Thin, extremely pure optical fibers are used to carry
telephone and television signals and digital (numeric) data
over long distances. Glass optical fibers are also used in control board
displays and in medical instruments.
Glass tubing is used to make fluorescent lights, neon signs, glass
piping, and chemical apparatus. Glass tubing is made from many kinds of glass
and in many sizes.
Glass-ceramics are strong materials made by heating glass to
rearrange some of its atoms into regular patterns. These partially crystalline materials
can withstand high temperatures, sudden changes in temperature, and chemical
attacks better than ordinary glass can. They are used in a variety of products,
including heat-resistant cookware, turbine engines, electronic equipment, and
nose cones of guided missiles. Glass-ceramics have such trade names as
Pyroceram, Cervit, and Hercuvit.
Radiation-absorbing and radiation-transmitting
glass can transmit,
modify, or block heat, light, X rays, and other types of radiant energy. For
example, ultraviolet glass absorbs the ultraviolet rays of the sun but
transmits visible light. Other glass transmits heat rays freely but passes
little visible light. Polarized glass cuts out the glare of brilliant light.
One-way glass is specially coated so that a person can look through a window
without being seen from the other side.
Laser glass is an optical glass containing small amounts of substances
that enable the glass to generate laser beams efficiently. Such glass is used
as the active medium in solid-state lasers, a type of laser
that sends light out through crystals or glass (One substance commonly used in
laser glass is the element neodymium. Researchers are using glass lasers in an
attempt to harness nuclear fusion (the joining of atomic nuclei) as
a source of commercially useful amounts of energy. In their experiments,
powerful glass lasers heat hydrogen atoms until hydrogen nuclei fuse, releasing
large amounts of energy.
"Invisible glass" is used principally for coated camera
lenses and eyeglasses. The coating is a chemical film that decreases the normal
loss of light by reflection. This allows more light to pass through the glass.
Photo chromic glass darkens when exposed to ultraviolet rays
and clears up when the rays are removed. Photo chromic glass is used for
windows, sunglasses, and instrument controls.
Photosensitive glass can be exposed to ultraviolet light and
to heat so that any pattern or photograph can be reproduced within the body of
the glass itself. Because the photographic print then becomes an actual part of
the glass, it will last as long as the glass itself.
Photochemical glass is a special composition of
photosensitive glass that can be cut by acid. Any design can be reproduced on
the glass from a photographic film. Then when the glass is dipped in acid, the
exposed areas are eaten away, leaving the design in the glass in three
dimensions. By this means, lacelike glass patterns can be made.
Heavy metal fluoride glass is an extremely transparent glass being
developed for use in optical fibers that transmit infrared rays. Infrared rays
are much like light waves but are invisible to the human eye. In optical
fibers, infrared light transmits better over distance than visible light does.
Chalcogenide glass is made up of elements from the chalcogen group,
including selenium, sulfur, and tellurium. The glass is transparent to infrared
light and is useful as a semiconductor in some electronic devices. Chalcogenide
glass fibers are a component of devices used to perform laser surgery.
Sol-Gel glass can be used as a protective coating on certain solar
collectors or as an insulating material. It is also used to make short, thick
tubes that are drawn into optical fibers. To make Sol-Gel glass, workers
dissolve the ingredients in a liquid. They then heat the liquid. The liquid
evaporates, leaving behind small particles of glass. Heating these
particles fuses (joins) them to form a solid piece of glass.
The temperatures involved in Sol-Gel processes are often lower than those
needed to make ordinary glass.
GLASS FACT
Types of Glass
There
are many different types of glass with different chemical and physical
properties. Each can be made by a suitable adjustment to chemical compositions,
but the main types of glass are:
Glasses
may be devised to meet almost any imaginable requirement. For many specialised
applications in chemistry, pharmacy, the electrical and electronics industries,
optics, the construction and lighting industries, glass, or the comparatively
new family of materials known as glass ceramics, may be the only practical
material for the engineer to use.
Types
of special glass include:
Alkali-barium Silicate Glass
Without
this type of glass, watching TV would be very dangerous. A television produces
X-rays that must be absorbed, otherwise they could in the long run cause health
problems. The X-rays are absorbed by glass with minimum amounts of heavy oxides
(lead, barium or strontium). Lead glass is commonly used for the funnel and
neck of the TV tube, while glass containing barium is used for the screen.
Aluminosilicate Glass
A
small, but important type of glass, aluminosilicate, contains 20% aluminium
oxide (alumina-Al2O3) often including calcium oxide, magnesium oxide and boric
oxide in relatively small amounts, but with only very small amounts of soda or
potash. It is able to withstand high temperatures and thermal shock and is
typically used in combustion tubes, gauge glasses for high-pressure steam
boilers, and in halogen-tungsten lamps capable of operating at temperature as
high as 750°C.
Borosilicate Glass
Most
of us are more familiar with this type of glass in the form of ovenware and
other heat-resisting ware, better known under the trade name Pyrex.Borosilicate
glass, the third major group, is made mainly of silica (70-80%) and boric oxide
(7-13%) with smaller amounts of the alkalis (sodium and potassium oxides) and
aluminium oxide. This type of glass has a relatively low alkali content and
consequently has good chemical durability and thermal shock resistance (it
doesn't break when changing temperature quickly.)As a result it is widely used
in the chemical industry, for laboratory apparatus, for ampoules and other
pharmaceutical containers, for various high intensity lighting applications and
as glass fibres for textile and plastic reinforcement.
Commercial Glass
Most
of the glass we see around us in our everyday lives in the form of bottles and
jars, flat glass for windows or for drinking glasses is known as commercial
glass or soda-lime glass, as soda ash is used in its manufacture.
The
main constituent of practically all commercial glass is sand. Sand by itself
can be fused to produce glass but the temperature at which this can be achieved
is about 1700°C. Adding other minerals and chemicals to sand can considerably
reduce the melting temperature.
The
addition of sodium carbonate (Na2CO3), known as soda ash, to produce a mixture
of 75% silica (SiO2) and 25% of sodium oxide (Na2O), will reduce the
temperature of fusion to about 800°C. However, a glass of this composition is
water-soluble and is known as water glass. In order to give the glass
stability, other chemicals like calcium oxide (CaO) and magnesium oxide (MgO) are
needed. These are obtained by adding limestone which results in a pure inert
glass.
Commercial
glass is normally colourless, allowing it to freely transmit light, which is
what makes glass ideal for windows and many other uses. Additional chemicals
have to be added to produce different colors of glass such as green, blue or
brown glass.
Most
commercial glasses have roughly similar chemical compositions of:
70%
- 74% SiO2 (silica) 12% - 16% Na2O (sodium oxide) 5% - 11% CaO (calcium oxide)
1% - 3% MgO (magnesium oxide) 1% - 3% Al2O3 (aluminum oxide)
Flat
glass is similar in composition to container glass except that it contains a
higher proportion of magnesium oxide.
Within
these limits the composition is varied to suit a particular product and
production method. The raw materials are carefully weighed and thoroughly
mixed, as consistency of composition is of utmost importance in making glass.
Nowadays
recycled glass from bottle banks or kerbside collections, known as cullet, is
used to make new glass. Using cullet has many environmental benefits, it
preserves the countryside by reducing quarrying, and because cullet melts more
easily, it saves energy and reduces emissions.
Almost
any proportion of cullet can be added to the mix (known as batch), provided it
is in the right condition, and green glass made from batch containing 85% to
90% of cullet is now common.
Although
the recycled glass may come from manufacturers around the world, it can be used
by any glassmaker, as container glass compositions are very similar. It is,
however, important that glass colours are not mixed and that the cullet is free
from impurities, especially metals and ceramics.
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Glass Ceramics
Some
of these "Glass ceramics", formed typically from lithium
aluminosilicate glass, are extremely resistant to thermal shock and have found
several applications where this property is important, including cooker hobs,
cooking ware, windows for gas or coal fires, mirror substrates for astronomical
telescopes and missile nose cones. An essential feature of glass is that it
does not contain crystals. However, by deliberately stimulating crystal growth
in glass it is possible to produce a type of glass with a controlled amount of crystallization
that can combine many of the best features of ceramics and glass.
Glass Fiber
Glass
fiber has many uses from roof insulation to medical equipment and its
composition varies depending on its application. For building insulation and
glass wool the type of glass used is normally soda lime. For textiles, an
alumino-borosilicate glass with very low sodium oxide content is preferred
because of its good chemical durability and high softening point. This is also
the type of glass fiber used in the reinforced plastics to make protective
helmets, boats, piping, car chassis, ropes, car exhausts and many other items.
In recent years, great progress has been made in making optical fibers which
can guide light and thus transmit images round corners. These fibers are used
in endoscopes for examination of internal human organs, changeable traffic
message signs now on motorways for speed restriction warnings and
communications technology without which telephones and the internet would not
be possible.
Lead Glass
Commonly
known as lead crystal, lead glass is used to make a wide variety of decorative
glass objects. It is made by using lead oxide instead of calcium oxide, and
potassium oxide instead of all or most of the sodium oxide. The traditional
English full lead crystal contains at least 30% lead oxide (PbO) but any glass
containing at least 24% PbO can be described as lead crystal. Glass containing
less than 24% PbO, is known simply as crystal glass. The lead is locked into
the chemical structure of the glass so there is no risk to human health. Lead
glass has a high refractive index making it sparkle brightly and a relatively
soft surface so that it is easy to decorate by grinding, cutting and engraving
which highlights the crystal's brilliance making it popular for glasses,
decanters and other decorative objects. Glass with even higher lead oxide
contents (typically 65%) may be used as radiation shielding because of the
well-known ability of lead to absorb gamma rays and other forms of harmful
radiation.
Optical Glass
Optical
glasses will be found in scientific instruments, microscopes, fighter aircraft
and most commonly in spectacles.
The
most important properties are the refractive index and the dispersion. The
index is a measure of how much the glass bends light. The dispersion is a
measure of the way the glass splits white light into the colours of the
rainbow. Glass makers use the variations in these characteristics to develop
optical glasses.
Sealing Glass
A
wide variety of glass compositions are used to seal metals for electrical and
electronic components. Here the available glasses may be grouped according to
their thermal expansion which must be matched with the thermal expansions of
the respective metals so that sealing is possible without excessive strain
being induced by differing levels of expansion. For sealing to tungsten, in
making incandescent and discharge lamps, borosilicate alkaline earths-aluminous
silicate glasses are suitable. Sodium borosilicate glasses may be used for
sealing to molybdenum and the iron-nickel-cobalt (Fernico) alloys are frequently
employed as a substitute, the amount of sodium oxide permissible depending on
the degree of electrical resistance required. With glasses designed to seal to
Kovar alloy, relatively high contents of boric oxide (approximately 20%) are
needed to keep the transformation temperature low and usually the preferred
alkali is potassium oxide so as to ensure high electrical insulation.Where the
requirement for electrical insulation is paramount, as in many types of vacuum
tube and for the encapsulation of diodes, a variety of lead glasses (typical
containing between 30% and 60% lead oxide) can be used.
Technical Glass
Technical
is the term given to a range of glasses used in the electronics industry.
Without
borate glass the computer revolution would not have been possible as it's
vitally important in producing electrical components. This type of glass,
contains little or no silica and is used for soldering glass, metals or
ceramics as it melts at the relatively low temperature of 450-550°C, well below
that of normal glass, ceramics and many metals.
Glass
of a slightly different composition is used for protecting silicon
semi-conductor components against chemical attack and mechanical damage. Known
as passivation glass it is vital in microelectronics technology and the
production of the silicon chips inside computers.
Another
type of glass - Phosphate Glass - which is a semi conductor, is used in the
construction of secondary electron multipliers.
Chalcogenide
glass - Similar semi conductor effects are also characteristic of a type of
glass that can be made without the presence of oxygen. Some of them have
potential use as infrared transmitting materials and as switching devices in
computer memories because their conductivity changes abruptly when particular
threshold voltage values are exceeded.
Vitreous Silica
Silica
glass or vitreous silica is of considerable technical importance as it has a
very low thermal expansion. This difficult to make glass contains tiny holes
created using acids and is used for filtration. Porous glasses of this kind are
commonly known as Vycor.
All Types Of Glass
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Float Glass (5700)
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Acid Etched Glass (107)
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Alarm Glass (3)
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Antique Mirror (7)
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Bent Glass (61)
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Body Tinted Glass (50)
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Fire Resistent Glass (77)
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Insulating Glass (227)
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Laminated glass (184)
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Low-e glass (69)
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Mirror (402)
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Pattern glass (105)
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Photovoltaic glass (24)
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Reflective glass (13)
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Sand-blasted glass (82)
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Self-Cleaning glass (15)
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Tempered glass (323)
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Wired glass (21)
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Borosilicate glass (28)
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Solar control glass (94)
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Screen-printed glass (86)
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Ultra Thin Glass (7)
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Figured Glass (58)
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Wired Glass (7)
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Bevelled Glass (82)
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Bent laminated glass (39)
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Bent tempered glass (54)
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Led Glass (2)
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Sheet Glass (166)
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Bullet Proof Glass (16)
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History of Glass
A Brief History of glass
From our earliest origins, man has been making use of
glass. Historians have discovered that a form of natural glass - obsidian -
formed for instance, within the mouth of a volcano as a result of the intense
heat of an eruption melting sand - was first used by man as tips for spears.
Archaeologists have found evidence of man-made glass
which dates back to 4000BC; this took the form of glazes used for coating stone
beads. It was not until 1500 BC that the first hollow glass container was made
by covering a sand core with a layer of molten glass.
Glass blowing became the most common way to make glass
containers from the First Century BC. However, the glass made during this time
was highly colored due to the impurities of the raw material. It was not until
the First Century AD when colorless glass was produced and then colored by the
addition of coloring materials.
The secret of glass making came to Britain with the
Romans. However, the skills and technology required to make glass were closely
guarded by the Romans and it was not until the Roman Empire disintegrated that
skills for glass making spread throughout Europe and the Middle East.
The Venetians, in particular, gained a reputation for
technical skill and artistic ability in the making of glass bottles and a fair
number of the city's craftsmen left Italy to set up glassworks throughout
Europe.
In Britain, there is evidence of a glass industry around
Jarrow and Wearmouth dating back to 680AD, while from the 13th Century, there
is evidence of there having been a glass industry in the Weald and the
afforested area of Surrey and Sussex around Chiddingford.
A major milestone in the history of glass occurred with
the invention of lead crystal glass by George Ravenscroft. He attempted to
counter the effect of clouding that sometimes occurred in blown glass by
introducing lead to the raw materials used in the process.
The new glass he created was softer and easier to
decorate and had a higher refractive index, adding to its brilliance and
beauty, and it proved invaluable to the optical industry. It's thanks to
Ravenscroft's invention that optical lenses, astronomical telescopes,
microscopes and the like became possible.
The modern glass industry only really started to develop
in Britain after the repeal of the Excise Act in 1845 relieved the heavy
taxation that had been enforced. Before that time, excise duties were placed on
the amount of glass melted in a glasshouse and levied continuously from 1745 to
1845.
Joseph Paxton's Crystal Palace at the Great Exhibition of
1851 marked the beginning of the discovery of glass as a building material. The
revolutionary new building encouraged the use of glass in public, domestic and
horticultural architecture. Glass manufacturing techniques also improved with
the advancement of science and better technology.
By 1887 glass making developed from traditional mouth
blowing to a semi-automatic process when Ashley introduced a machine capable of
producing 200 bottles per hour in Castleford, Yorkshire - more than three times
quicker than the previous production methods.
Twenty years later, in 1907, the first fully automated
machine was developed in America by Michael Owens from major glass
manufacturers Owens of Illinois, and used at its factory in Manchester,
Illinois making 2,500 bottles per hour.
Other developments followed rapidly, but it was not until
the First World War, when Britain became cut off from essential glass suppliers
that glass became part of the scientific sector. Up until then glass was seen
as a craft rather than a precise science.
Today, glass making is a modern, hi-tech industry
operating in a fiercely competitive global market where quality, design and
service levels are critical to maintaining market share.
Modern glass plants are capable of making millions of
glass containers a day in many different colours, but green, brown and clear
remain the most popular.
Few of us can imagine modern life without glass. It
features in almost every aspect of our lives - in our homes, our cars and
whenever we sit down to eat or drink. Glass packaging is used for many
products, wines, spirits and beers all come in glass as do medicines and
cosmetics not to mention numerous foodstuffs.
With increasing consumer concern for the environment,
glass has again come into its own proving to be an ideal material for
recycling. Glass recycling is good news for the environment. It saves used
glass containers being sent to landfill and less energy is needed to melt
recycled glass than to melt down raw materials, thus saving energy. Recycling
also reduces the need for raw materials to be quarried thus saving precious
resources.
The Future of glass
Glass as a material in its own right will always exist.
But many new applications and manufacturing processes will involve glass in
combination with other materials. Optical fibres, for example, are currently
manufactured with one or more different coatings, which are often plastics.
With the increasing sophistication of opto-electronic devices, there is an
increasing need to combine optical and electronic devices for many applications
such as transmission of audio, video and data information. Glasses and
ceramics, either alone or composite with other materials, will find increasing
application in biological and medical areas. Materials such as photo chromic, electro
chromic and thermochrominc glasses, which respond to external stimuli, are
being developed with various, sometimes unusual, applications.
By :- M Z HAQUE
Asst. Marketing Manager
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