| GLASS KNOWLEDGE
Things you might want to
know about:
How To Cut
Mirror Strips
Need to Know More?
Ask a Glazier
WEIGHTS:
Glass Weight per Square Foot
1/8"=1.64
lbs.sq.ft.
3/16"=2.45
lbs.sq.ft.
1/4"=3.27
lbs.sq.ft.
3/8"=4.91
lbs.sq.ft.
1/2"=6.54
lbs.sq.ft.
3/4"=9.84
lbs.sq.ft.
1"=13.11
lbs.sq.ft.
MEASURING:
Formula
for figuring Square Feet:
From inches
to feet- L"xW" divided by 144= Square Feet
Formula
for figuring Linear Inches
rectangles: (width x 2) + (length x 2) = linear inches
circles: Diameter x 3.2 = linear
inches
octagons: Diameter x 3.3 = linear
inches
hexagons: Diameter x 3.5 = linear inches
ovals (width x 2) + (length x 2) + linear
inches
ARGON GAS:
From the
Greek word argos (inactive)
Atomic Number: 18
Atomic Mass: 39.948
Thermal Conductance: 47.87% LOWER
than air
BTU over hour foot degree F: [(.0139-.0094)/.0094]
Density: 38.01% MORE DENSE
than air
Pounds per cubic foot: [(.1111-.0805)/.0805]
Viscosity: 22.16%MORE VISCOUS
than air
Pound seconds per square foot times ten to the negative fifth:
[(.0441-.0361)/.0361]
Argon is about a 30%
lower thermal conductivity than does air. This translates into about a
16% energy improvement in a standard LowE^2 IGU at better than 90% fill.
If you have an IGU with a
50% fill, for example, the U factor improvement will be 8% and at 75%
fill the improvement will be 12%...you might note the straight line
progression on u value improvement based on the amount of argon fill.
The air we breathe is 1%
argon. If the IGU is filled to anything over that level, then the argon
in the IGU wants to reach equilibrium with the 1% in the atmosphere. The
job of the IGU manufacturer is to ensure that the argon in the IGU stays
in the IGU. A 1% per year loss of argon due to natural dissipation thru
the IGU edge seal (not resulting from a failed seal) is about the best
that is currently available to modern technology. This is readily
achievable and is becoming something of the industry standard.
SAFETY GLASS:
Laminated - A process by which two or more
lites of glass are sandwiched about a polyvinyl layer to give
the glass strength against penetration. It is not shatter proof
or unbreakable. The most common application that everyone
should be familiar with is automobile windshields.
Tempered - The process of heat-treating
glass, to provide much stronger characteristics than annealed,
or un-tempered glass. Once again, tempered glass is not shatter
proof or unbreakable. It is designed to break into very small
pieces to help alleviate severe lacerations. This process is
used on automobile side and rear windows as well as storefronts
and doors that are required by local building codes.
Table:
Uniform load strength - Heat-Strengthened Glass
Note: Data obtained from Fed. Spec. DD-G-1403.
The values have not been verified
| Nominal Glass
Thickness. mm (in) |
Average Breaking
Pressure times Glass Area.
Pa x m2 (lbf x ft2). Minimum value |
| 3 (1/8) |
5895 (1,325) |
| 5 (3/16) |
14,800 (3,325) |
| 6 (1/4) |
24,000 (5,400) |
| 8 (5/16) |
32,700 (7,350) |
| 10 (3/8) |
55,000 (12,000) |
| 12 (1/2) |
70,000 (16,000) |
Here it is -
everything you always wanted to know
about glass -
enjoy!
|
Glass Facts
History of Glass
|
Archaeological
findings indicate that glass was first made in the Middle East,
sometime in the 3000's B.C. In the beginning glass manufacturing
was slow and costly. Glass melting furnaces were very small and
hardly produced enough heat to melt glass properly. In ancient
times, glass was a luxury item and few people could afford it.
An unknown person discovered the blowpipe in the 1st century
B.C. on the Phoenician coast. Glass manufacturing flourished
in the Roman empire and spread from Italy to all countries under
Roman jurisdiction. Due to mass production, glass become an
everyday object and was removed from the list of luxuries.
By the time of the Crusades, glass manufacture had been revived
in Venice as a result of good contacts with Byzantium. Equipment
was transferred to the Venetian island of Murano, where Soda
Lime glass, better known as cristallo was developed.
Venetian glass-blowers created some of the most delicate and
graceful glass the world has ever seen. Despite their efforts to
keep the technology secret, it soon spread around Europe.
After 1890, glass uses and manufacturing developments increased
so rapidly as to be almost revolutionary. The science and
engineering of glass as a material was much better understood,
and in the late 1950's Sir Alastair Pilkington introduced
a new revolutionary production method (float glass production),
by which 90% of flat glass is still manufactured today.
|
A
brief history of GLASS
| |
The discovery of glass
Natural glass has existed since the beginnings of
time, formed when certain types of rocks melt as a result of
high-temperature phenomena such as volcanic eruptions, lightning
strikes or the impact of meteorites, and then cool and solidify
rapidly. Stone-age man is believed to have used cutting tools
made of obsidian (a natural glass of volcanic origin also known
as hyalopsite, Iceland agate, or mountain mahogany) and tektites
(naturally-formed glasses of extraterrestrial or other origin,
also referred to as obsidianites).
According to the ancient-Roman historian Pliny (AD 23-79),
Phoenician merchants transporting stone actually discovered
glass (or rather became aware of its existence accidentally) in
the region of Syria around 5000 BC. Pliny tells how the
merchants, after landing, rested cooking pots on blocks of
nitrate placed by their fire. With the intense heat of the fire,
the blocks eventually melted and mixed with the sand of the
beach to form an opaque liquid.
This brief history looks, however, at the origins and
evolution of man-made glass.
A craft is born
The earliest man-made glass objects, mainly non-transparent
glass beads, are thought to date back to around 3500 BC, with
finds in Egypt and Eastern Mesopotamia. In the third millennium,
in central Mesopotamia, the basic raw materials of glass were
being used principally to produce glazes on pots and vases. The
discovery may have been coincidental, with calciferous sand
finding its way into an overheated kiln and combining with soda
to form a colored glaze on the ceramics. It was then, above all,
Phoenician merchants and sailors who spread this new art along
the coasts of the Mediterranean.
The oldest fragments of glass vases (evidence of the origins
of the hollow glass industry), however, date back to the 16th
century BC and were found in Mesopotamia. Hollow glass
production was also evolving around this time in Egypt, and
there is evidence of other ancient glassmaking activities
emerging independently in Mycenae (Greece), China and North
Tyrol.
Early hollow glass
production
After 1500 BC, Egyptian craftsmen are known to have begun
developing a method for producing glass pots by dipping a core
mould of compacted sand into molten glass and then turning the
mould so that molten glass adhered to it. While still soft, the
glass-covered mould could then be rolled on a slab of stone in
order to smooth or decorate it. The earliest examples of
Egyptian glassware are three vases bearing the name of the
Pharaoh Thoutmosis III (1504-1450 BC), who brought glassmakers
to Egypt as prisoners following a successful military campaign
in Asia.
There is little evidence of further evolution until the 9th
century BC, when glassmaking revived in Mesopotamia. Over the
following 500 years, glass production centered on Alessandria,
from where it is thought to have spread to Italy.
The first glassmaking "manual" dates back to around 650 BC.
Instructions on how to make glass are contained in tablets from
the library of the Assyrian king Ashurbanipal (669-626 BC).
Starting to blow
A major breakthrough in glassmaking was the discovery of
glassblowing some time between 27 BC and AD 14, attributed to
Syrian craftsmen from the Sidon-Babylon area. The long thin
metal tube used in the blowing process has changed very little
since then. In the last century BC, the ancient Romans then
began blowing glass inside moulds, greatly increasing the
variety of shapes possible for hollow glass items.
The Roman connection
The Romans also did much to spread glassmaking technology. With
its conquests, trade relations, road building, and effective
political and economical administration, the Roman Empire
created the conditions for the flourishing of glassworks across
western Europe and the Mediterranean. During the reign of the
emperor Augustus, glass objects began to appear throughout
Italy, in France, Germany and Switzerland. Roman glass has even
been found as far a field as China, shipped there along the silk
routes.
It was the Romans who began to use glass for architectural
purposes, with the discovery of clear glass (through the
introduction of manganese oxide) in Alexandria around AD 100.
Cast glass windows, albeit with poor optical qualities, thus
began to appear in the most important buildings in Rome and the
most luxurious villas of Herculaneum and Pompeii.
With the geographical division of the empires, glass
craftsmen began to migrate less, and eastern and western
glassware gradually acquired more distinct characteristics.
Alexandria remained the most important glassmaking area in the
East, producing luxury glass items mainly for export. The world
famous Portland Vase is perhaps the finest known example of
Alexandrian skills. In Rome's Western empire, the city of Köln
in the Rhineland developed as the hub of the glassmaking
industry, adopting, however, mainly eastern techniques. Then,
the decline of the Roman Empire and culture slowed progress in
the field of glassmaking techniques, particularly through the
5th century. Germanic glassware became less ornate, with
craftsmen abandoning or not developing the decorating skills
they had acquired.
The early Middle Ages
Archaeological excavations on the island of Torcello near
Venice, Italy, have unearthed objects from the late 7th and
early 8th centuries which bear witness to the transition from
ancient to early Middle Ages production of glass.
Towards the year 1000, a significant change in European
glassmaking techniques took place. Given the difficulties in
importing raw materials, soda glass was gradually replaced by
glass made using the potash obtained from the burning of trees.
At this point, glass made north of the Alps began to differ from
glass made in the Mediterranean area, with Italy, for example,
sticking to soda ash as its dominant raw material.
Sheet glass skills
The 11th century also saw the development by German glass
craftsmen of a technique - then further developed by Venetian
craftsmen in the 13th century - for the production of glass
sheets. By blowing a hollow glass sphere and swinging it
vertically, gravity would pull the glass into a cylindrical
"pod" measuring as much as 3 meters long, with a width of up to
45 cm. While still hot, the ends of the pod were cut off and the
resulting cylinder cut lengthways and laid flat. Other types of
sheet glass included crown glass (also known as "bullions"),
relatively common across western Europe. With this technique, a
glass ball was blown and then opened outwards on the opposite
side to the pipe. Spinning the semi-molten ball then caused it
to flatten and increase in size, but only up to a limited
diameter. The panes thus created would then be joined with lead
strips and pieced together to create windows. Glazing remained,
however, a great luxury up to the late Middle Ages, with royal
palaces and churches the most likely buildings to have glass
windows. Stained glass windows reached their peak as the Middle
Ages drew to a close, with an increasing number of public
buildings, inns and the homes of the wealthy fitted with clear
or colored glass decorated with historical scenes and coats of
arms.
Venice
In the Middle Ages, the Italian city of Venice assumed its role
as the glassmaking centre of the western world. The Venetian
merchant fleet ruled the Mediterranean waves and helped supply
Venice's glass craftsmen with the technical know-how of their
counterparts in Syria, and with the artistic influence of Islam.
The importance of the glass industry in Venice can be seen not
only in the number of craftsmen at work there (more than 8,000
at one point). A 1271 ordinance, a type of glass sector statute,
laid down certain protectionist measures such as a ban on
imports of foreign glass and a ban on foreign glassmakers who
wished to work in Venice: non-Venetian craftsmen were themselves
clearly sufficiently skilled to pose a threat.
Until the end of the 13th century, most glassmaking in Venice
took place in the city itself. However, the frequent fires
caused by the furnaces led the city authorities, in 1291, to
order the transfer of glassmaking to the island of Murano. The
measure also made it easier for the city to keep an eye on what
was one of its main assets, ensuring that no glassmaking skills
or secrets were exported.
In the 14th century, another important Italian glassmaking
industry developed at Altare, near Genoa. Its importance lies
largely in the fact that it was not subject to the strict
statutes of Venice as regards the exporting of glass working
skills. Thus, during the 16th century, craftsmen from Altare
helped extend the new styles and techniques of Italian glass to
other parts of Europe, particularly France.
In the second half of the 15th century, the craftsmen of
Murano started using quartz sand and potash made from sea plants
to produce particularly pure crystal. By the end of the 16th
century, 3,000 of the island's 7,000 inhabitants were involved
in some way in the glassmaking industry.
Lead crystal
The development of lead crystal has been attributed to the
English glassmaker George Ravenscroft (1618-1681), who patented
his new glass in 1674. He had been commissioned to find a
substitute for the Venetian crystal produced in Murano and based
on pure quartz sand and potash. By using higher proportions of
lead oxide instead of potash, he succeeded in producing a
brilliant glass with a high refractive index which was very well
suited for deep cutting and engraving.
Advances from France
In 1688, in France, a new process was developed for the
production of plate glass, principally for use in mirrors, whose
optical qualities had, until then, left much to be desired. The
molten glass was poured onto a special table and rolled out
flat. After cooling, the plate glass was ground on large round
tables by means of rotating cast iron discs and increasingly
fine abrasive sands, and then polished using felt disks. The
result of this "plate pouring" process was flat glass with good
optical transmission qualities. When coated on one side with a
reflective, low melting metal, high-quality mirrors could be
produced.
France also took steps to promote its own glass industry and
attract glass experts from Venice; not an easy move for
Venetians keen on exporting their abilities and know-how, given
the history of discouragement of such behavior (at one point,
Venetian glass craftsmen faced death threats if they disclosed
glassmaking secrets or took their skills abroad). The French
court, for its part, placed heavy duties on glass imports and
offered Venetian glassmakers a number of incentives: French
nationality after eight years and total exemption from taxes, to
name just two.
From craft to industry
It was not until the latter stages of the Industrial Revolution,
however, that mechanical technology for mass production and
in-depth scientific research into the relationship between the
composition of glass and its physical qualities began to appear
in the industry.
A key figure and one of the forefathers of modern glass
research was the German scientist Otto Schott (1851-1935), who
used scientific methods to study the effects of numerous
chemical elements on the optical and thermal properties of
glass. In the field of optical glass, Schott teamed up with
Ernst Abbe (1840-1905), a professor at the University of Jena
and joint owner of the Carl Zeiss firm, to make significant
technological advances.
Another major contributor in the evolution towards mass
production was Friedrich Siemens, who invented the tank furnace.
This rapidly replaced the old pot furnace and allowed the
continuous production of far greater quantities of molten glass.
Increasing automation
Towards the end of the 19th century, the American engineer
Michael Owens (1859-1923) invented an automatic bottle blowing
machine which only arrived in Europe after the turn of the
century. Owens was backed financially by E.D.L. Libbey, owner of
the Libbey Glass Co. of Toledo, Ohio. By the year 1920, in the
United States, there were around 200 automatic Owens Libbey
Suction Blow machines operating. In Europe, smaller, more
versatile machines from companies like O'Neill, Miller and Lynch
were also popular.
Added impetus was given to automatic production processes in
1923 with the development of the gob feeder, which ensured the
rapid supply of more consistently sized gobs in bottle
production. Soon afterwards, in 1925, IS (individual section)
machines were developed. Used in conjunction with the gob
feeders, IS machines allowed the simultaneous production of a
number of bottles from one piece of equipment. The gob feeder-IS
machine combination remains the basis of most automatic glass
container production today.
Modern flat glass
technology
In the production of flat glass (where, as explained earlier,
molten glass had previously been poured onto large tables then
rolled flat into "plates", cooled, ground and polished before
being turned over and given the same treatment on the other
surface), the first real innovation came in 1905 when a Belgian
named Fourcault managed to vertically draw a continuous sheet of
glass of a consistent width from the tank. Commercial production
of sheet glass using the Fourcault process eventually got under
way in 1914.
Around the end of the First World War, another Belgian
engineer Emil Bicheroux developed a process whereby the molten
glass was poured from a pot directly through two rollers. Like
the Fourcault method, this resulted in glass with a more even
thickness, and made grinding and polishing easier and more
economical.
An off-shoot of evolution in flat glass production was the
strengthening of glass by means of lamination (inserting a
celluloid material layer between two sheets of glass). The
process was invented and developed by the French scientist
Edouard Benedictus, who patented his new safety glass under the
name "Triplex" in 1910.
In America, Colburn developed another method for drawing
sheet glass. The process was further improved with the support
of the US firm Libbey-Owens and was first used for commercial
production in 1917.
The Pittsburgh process, developed by the American Pennvernon
and the Pittsburgh Plate Glass Company (PPG), combined and
enhanced the main features of the Fourcault and Libbey-Owens
processes, and has been in use since 1928.
The float process developed after the Second World War by
Britain's Pilkington Brothers Ltd., and introduced in 1959,
combined the brilliant finish of sheet glass with the optical
qualities of plate glass. Molten glass, when poured across the
surface of a bath of molten tin, spreads and flattens before
being drawn horizontally in a continuous ribbon into the
annealing leer.
Conclusion
Although this brief history comes to a close nearly 40 years
ago, technological evolution naturally continues. Not yet ready
to be "relegated" to a history of glass are areas such as
computerized control systems, coating techniques, solar control
technology and "smart matter", the integration of
micro-electronic and mechanical know-how to create glass which
is able to "react" to external forces.
|
|
5000 BC
3500 BC
16th century BC
1500 BC
9th century BC
650 BC
27 BC-AD 14
AD 100
7th-8th
centuries
1000
11th century
1271
1291
14th century
15th-16th
centuries
1674
1688
Industrial Revolution
late 19th century
1900-1925
1905-1914
1910
1917
1928
1959
|
History of Glass
The mysterious physical, optical and aesthetic properties of glass have
always intrigued man. Even the most sophisticated 20th century man is
amazed and bemused by this solid, which he has been told is really a
rigid un-crystallized liquid. The product and the process used to
manufacture it seem to smack of alchemy, for glass is nothing but coarse
sand and soda ash transformed into smooth transparent forms.
According to the Roman historian Pliny, who wrote in Naturalis Historica
in 77 A.D., man first produced glass by accident about the year 5000
B.C. Phoenician sailors feasting on a beach near Belus in Asia Minor,
could find no stones on which to place their cooking pots; therefore,
they set them on blocks of soda carried by their ship as cargo. As the
fire's heat increased, the sand and soda turned to molten glass.
Pliny's anecdote now is considered apocryphal, but it contains an
accurate recipe for producing glass: heat plus silica and soda ash.
Ornamental glass beads dating from 2500 B.C. have been found in Egypt,
and glass rods from even earlier have been uncovered in Babylon. The
first useful glass objects date to Egypt's 18th dynasty, about 1500 B.C.
Egyptians attached metal rods to silica paste cores, which they dipped
repeatedly into molten glass to produce small bottles. The cores later
were removed. The goblet of Thutmose III, made about 1490 B.C. and now
at New York's Metropolitan Museum of Art, was produced in this manner.
Glassblowing, a Babylonian discovery, probably came about when
glassmakers using the core-dipped method switched to hollow metal rods
to hold silica paste cores and then discovered that molten glass could
be blown into shapes. After this discovery, which dates to about 250
B.C., glass vessels suddenly became easy and inexpensive to produce.
Romans imported Syrian and Babylonian glassmakers, and small bowls and
bottles were selling for only a Roman penny in 200 B.C. Pliny the Elder
noted in 79 B.C. that fine glass cups were replacing cups of precious
metals as a status symbol among the Roman rich.
Glass, however, did not replace shutters at the windows of Roman homes.
The Romans tried but failed to cast transparent flat glass to enclose or
ornament their homes. Slabs 1/2" thick have been excavated - including a
32 by 44-inch piece at Pompeii - but Romans did not discover the art of
grinding and polishing cast glass to make it transparent. Instead of
glass, the rich used thin, translucent sheets of alabaster to enclose
wall openings.
With the breakdown of the Roman Empire, glassmaking technology stagnated
in Europe; in fact, it almost disappeared. True, Gothic cathedrals of
the late 12th century and later featured brilliant bits of colored
glasses, complex designs and rate and were prohibitively expensive. Even
the rich still shuttered their windows, and the Middle English word for
windows - "wind eyes" - underlined the fact that wall openings enclosed
in glass were, for all practical purposes, nonexistent.
During the 13th and 14th centuries, glassmaking was revived in Venice as
a result of that Italian state's trade contacts with Byzantium.
Soda-Lime was developed by glassmakers of the island or Murano in about
1450, and Venetians termed this clear, thin glass cristallo. Despite
attempts to keep their technology secret, it soon spread north over the
Alps to Germany, France, Belgium and England.
In England, where deforestation was a problem as early as the 15th
century, glassmakers were required after 1615 to use coal instead of
wood in the glassmaking process. About 1675, the English learned to add
lead oxide to the basic glass formula, and the resulting solid, heavy
and durable vessels progressively replaced the fragile glasses of
Venice.
Flat glass for windows was still rare during much of the 17th and 18th
centuries. Small panes were made by blowing a large glob of glass,
removing it from the blowing iron and then rotating the glass quickly so
it would spread and flatten. Such glass had a dimple in its center, many
air bubbles and a pattern of concentric circles, but it was transparent
and effective in keeping out the weather. At the end of the 17th
century, the French learned how to grind and polish cast glass to
produce plate glass, but only the rich could afford it.
During the 1800s, glass technology improved rapidly. A hand-operated
split mold developed in 1821 that ended the age of blowing individual
bottles, glasses and flasks. A semi-automatic bottle machine perfected
50 years later mass-produced bottles and turned them into the everyday
miracle they are today.
Great strides were made in the manufacture of flat glass during the 19th
century. Compressed air technology led to flatter, better glass panes.
Controlled amounts of air were used to blow a large glass cylinder,
which was slit lengthwise, reheated and allowed to flatten under its own
weight. Large, relatively inexpensive lites of glass were produced in
this manner. As a result of such technological advances, window areas
that required 18 to 24 panes to enclose in 1730 could be increased
dramatically and glass prices dropped by the 1860s, glass-enclosed "wind
eyes" were commonplace in the humblest homes.
Plate glass, that wickedly expensive French product, also became
commonplace by the end of the 19th century. Water power, then steam and
then electricity made the grinding and polishing of heavy glass plates
faster and easier. By the 1860s, smart stores and office buildings in
Europe and North America glistened with plate glass. France, Belgium and
Germany monopolized the production of the product until 1883, when the
Pittsburgh Plate Glass Company became the first successful manufacturer
of the product in the United States. By 1895, the company could produce
20 million square feet of plate glass a year, and imports from Europe
fell sharply.
With the 20th century came an era of revolutionary technology. Machines
were developed, improved and perfected to produce endless ribbons of
sheet (window) glass, to produce plate glass polished and ground
simultaneously on both sides and to produce float glass on a bed of
molten tin. Also developed were processes to strengthen glass through
thermal and chemical tempering, to add tints to glass for reduced heat
transmission and glare and to coat glass with transparent metal and
metal oxide films that reflected heat or conducted electricity. And
products marrying these processes and developments were created to help
make life more convenient, more comfortable, safer and more beautiful.
In retrospect, the romance of glass is not an Egyptian producing a
bottle for a Pharaoh or window glass being made from a cylinder, a pane
at a time, in a one-man glass house. The true romance of glass is the
story of the reasonable cost for use in architecture, transportation,
industry, science and the home. Billions of people now benefit because
technology has made glass a versatile, easy-to-use miracle.
The Nature of Glass
Glass is not easily described.
Its physical structure does not conform to liquid, solid or gas. Glass
actually is more of a liquid than the solid it appears to be. Its
complex nature has intrigued man from ancient times.
The American Society for Testing and Materials defines glass as "an
inorganic product of fusion which has cooled to a rigid condition
without crystallizing". Glass can be considered, then, an unusual
material which has the random atomic arrangement of liquid but which
somehow has been "frozen" in place so that it is a solid and permanent
substance. Glass can be transparent, translucent or opaque. It is
non-porous, non-absorptive and impervious to the common elements and
many harsh chemicals and liquids. It is exceptionally resistant to
abrasion and surface scratches. It is one of the best electrical
insulation materials, yet can be treated to conduct electricity. Glass
has lower head conductivity than most metals and can possess a very low,
zero or even negative coefficient of expansion. Because it contains a
large proportion of silica and is produced by the action of heat upon
that silica, it is generally categorized as a ceramic. Glass, however,
stands in a class by itself, quite distinct from other ceramics. Most
ceramic materials are shaped cold and then fired to produce the desired
result; glass is shaped at extremely high temperatures and then allowed
to cool. It again may be made semi-plastic, plastic or even molten by
the further application of heat. For this reason, glass also is
considered a thermoplastic material, which softens when heated and
hardens when cooled.
The Flat Glass Recipe
Glass goes back millenniums formed by nature as obsidian, or black
glass, a hard non-corrosive, semi opaque substance fused by volcanic
eruptions and enduring centuries of erosion.
This natural glass is composed of three elements of the earth-sand, soda
and lime. These same elements in varying forms also make up the basic
composition of manufactured glass products ranging from containers and
glassware to windshields and windows for high-rise commercial buildings.
About 50 other chemical elements are used in modern glassmaking, in
major and minor ways, to affect color, viscosity or durability, or to
impart some desired physical property. But nature's original ingredients
are still basic elements in the formulation of glass.
Glass largely is an open chain of silicon atoms with atoms of various
oxides occupying the spaces between. It is this loose structure that
permits transparency. Silica, or sand, is the most important ingredient
in glassmaking since it is the source of, and provides the structure
for, transparency. But sand requires soda and lime for practical
glassmaking.
Today, an average batch mix used to manufacture flat glass products
contains about 70 percent silica sand, 13 percent lime, 12 percent soda
and small amounts of other materials. About one-quarter of the batch is
in the form of cullet, or cleaned and crushed glass recovered from
previous glassmaking operations.
Silica or silicon dioxide, which is converted into glass by the action
of heat is very difficult to fuse, requiring extremely high
temperatures. Ancient scientists discovered that other materials such as
soda, when melted in close contact with sand, would permit the melting
of silica at much lower temperatures. Such materials are known as
fluxes, and soda was probably the first flux.
The primary forms of soda used in glassmaking are soda ash (sodium
carbonate) or caustic soda (sodium hydroxide). When a mixture of sand
and soda dissolves in the molten soda, forming sodium silicate.
Depending on the proportions of sand and soda, this sodium silicate is
more or less soluble in water and is known as water glass. To overcome
water solubility of glass, another element, lime, is required.
Lime (calcium oxide) usually is introduced into the glass batch mix in
the form of limestone. Its use in correct proportion causes formulation
of a soda-lime-silicate composition that is virtually unaffected by
moisture or acids. Lime also renders the glass more viscous at the
working temperature, shortens the setting time and improves weathering
properties.
Because of its low melting range, the soda-lime-silicate composition
undoubtedly was the type used by ancients to produce the earliest known
vessels, vases, semi-precious glass stones and beads, and, much later,
the earliest form of window glass. Today, soda-lime-silicate is the
basis for float glass, and of course, products fabricated from it.
Other materials are added to produce different properties in the basic
flat glass product or to replace one of the basic elements to produce
different types of commercial glasses. Lead, for example, in the form of
lead oxide, may be used to replace lime, and is introduced to increase
brilliance, density and index of refraction. Lead glasses included
optical and ophthalmic glasses and the finest stemware and art objects.
Boron, substituted in whole or in part for the silica, increases the
refractive index, deepens the color produced by various other coloring
materials, and greatly reduces the coefficient of thermal expansion.
Borosilicate glasses are used for such high heat resistant products as
ovenware, laboratory glasses and range surfaces. Metallic oxides are
added to produce tinted or colored glasses.
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