[tt] Wikipedia: Glass

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Glass - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Glass

Glass in the common sense refers to a hard, brittle, transparent,
solid, such as that used for windows, many bottles, or eyewear,
including, but not limited to, soda-lime glass, borosilicate glass,
acrylic glass, sugar glass, isinglass (Muscovy-glass), or aluminium
oxynitride.

In the technical sense, glass is an inorganic product of fusion
which has been cooled to a rigid condition without
crystallizing.^[1]^[2]^[3]^[4]^[5] Many glasses contain silica as
their main component and glass former.^[6]

In the scientific sense the term glass is often extended to all
amorphous solids (and melts that easily form amorphous solids),
including plastics, resins, or other silica-free amorphous solids.
In addition, besides traditional melting techniques, any other means
of preparation are considered, such as ion implantation, and the
sol-gel method.^[6] However, glass science commonly includes only
inorganic amorphous solids, while plastics and similar organics are
covered by polymer science, biology and further scientific
disciplines.

The optical and physical properties of glass make it suitable for
applications such as flat glass, container glass, optics and
optoelectronics material, laboratory equipment, thermal insulator
(glass wool), reinforcement fiber (glass-reinforced plastic, glass
fiber reinforced concrete), and art.

Contents

* 1 General properties, uses, occurrence
  + 1.1 Glass in buildings
  + 1.2 Technological applications
* 2 Glass production
  + 2.1 Glass production history
  + 2.2 Glass ingredients
  + 2.3 Contemporary glass production
  + 2.4 Glassmaking in the laboratory
* 3 Silica-free glasses
* 4 The physics of glass
  + 4.1 Glass versus a supercooled liquid
  + 4.2 Behavior of antique glass
  + 4.3 Physical properties
   o 4.3.1 Color
* 5 History
  + 5.1 South Asia
  + 5.2 Romans
  + 5.3 Anglo-Saxon world
  + 5.4 Islamic world
  + 5.5 Medieval Europe
  + 5.6 Late medieval Northern Europe
  + 5.7 Murano glassmaking
* 6 Glass art
* 7 See also
* 8 References
* 9 Bibliography
* 10 External links

General properties, uses, occurrence

Laboratory glassware made from borosilicate glass (Erlenmeyer flask)
Laboratory glassware made from borosilicate glass (Erlenmeyer flask)
Flat panel display, using thin sheets of special alkali-free glass
Flat panel display, using thin sheets of special alkali-free^[7]
glass

Ordinary glass is prevalent due to its transparency to visible
light. This transparency is due to an absence of electronic
transition states in the range of visible light. The homogeneity of
the glass on length scales greater than the wavelength of visible
light also contributes to its transparency as heterogeneities would
cause light to be scattered, breaking up any coherent image
transmission. Many household objects are made of glass. Drinking
glasses, bowls and bottles are often made of glass, as are light
bulbs, mirrors, aquaria, cathode ray tubes, computer flat panel
displays, and windows.

In research laboratories, flasks, test tubes, and other laboratory
equipment are often made of borosilicate glass for its low
coefficient of thermal expansion, giving greater resistance to
thermal shock and greater accuracy in measurements. For
high-temperature applications, quartz glass is used, although it is
very difficult to work. Most laboratory glassware is mass-produced,
but large laboratories also keep a glassblower on staff for
preparing custom made glass equipment.

Sometimes, glass is created naturally from volcanic lava, lightning
strikes, or meteorite impacts (e.g., Lechatelierite, Fulgurite,
Darwin Glass, Volcanic Glass, Tektites). If the lava is felsic this
glass is called obsidian, and is usually black with impurities.
Obsidian is a raw material for flintknappers, who have used it to
make extremely sharp glass knives since the stone age.

Glass sometimes occurs in nature resulting from human activity, for
example trinitite (from nuclear testing) and beach glass.

Glass in buildings

Main articles: Architectural Glass, Glazing in architecture,
and Window

Glass is commonly used in buildings as transparent windows, internal
glazed partitions, and as architectural features. It is also
possible to use glass as a structural material, for example, in
beams and columns, as well as in the form of "fins" for wind
reinforcement, which are visible in many glass frontages like large
shop windows. Safe load capacity is, however, limited; although
glass has a high theoretical yield stress, it is very susceptible to
brittle (sudden) failure, and has a tendency to shatter upon
localized impact. This particularly limits its use in columns, as
there is a risk of vehicles or other heavy objects colliding with
and shattering the structural element. One well-known example of a
structure made entirely from glass is the northern entrance to
Buchanan Street subway station in Glasgow.

Glass in buildings can be of a safety type, including wired, heat
strengthened (tempered) and laminated glass. Glass fibre insulation
is common in roofs and walls. Foamed glass, made from waste glass,
can be used as lightweight, closed-cell insulation. As insulation,
glass (e.g., fiberglass) is also used. In the form of long,
fluffy-looking sheets, it is commonly found in homes. Fiberglass
insulation is used particularly in attics, and is given an R-rating,
denoting the insulating ability.

Technological applications

Pure SiO[2] glass (the same chemical compound as quartz, or, in its
polycrystalline form, sand) does not absorb UV light and is used for
applications that require transparency in this region. Large natural
single crystals of quartz are pure silicon dioxide, and upon
crushing are used for high quality specialty glasses. Synthetic
amorphous silica, an almost 100 % pure form of quartz, is the raw
material for the most expensive specialty glasses, such as optical
fiber core. Undersea cables have sections doped with erbium, which
amplify transmitted signals by laser emission from within the glass
itself. Amorphous SiO[2] is also used as a dielectric material in
integrated circuits due to the smooth and electrically neutral
interface it forms with silicon.

Optical instruments such as glasses, cameras, microscopes,
telescopes, and planetaria are based on glass lenses, mirrors, and
prisms. The glasses used for making these instruments are
categorized using a six-digit glass code, or alternatively a
letter-number code from the Schott Glass catalogue. For example, BK7
is a low-dispersion borosilicate crown glass, and SF10 is a
high-dispersion dense flint glass. The glasses are arranged by
composition, refractive index, and Abbe number.

Glass polymerization is a technique that can be used to incorporate
additives that modify the properties of glass that would otherwise
be destroyed during high temperature preparation. Sol gel is an
example of glass polymerization and enables embedding of organic and
bioactive molecules, to add a new level of functionality to
glass.^[8]

Glass production

Main articles: Glass production and Float glass

Oldest mouth-blown window-glass from 1742 from Kosta Glasbruk,
Småland, Sweden. In the middle the mark from the glass blowers pipe
Oldest mouth-blown window-glass from 1742 from Kosta Glasbruk,
Småland, Sweden. In the middle the mark from the glass blowers pipe

Glass production history

Glass melting technology has passed through several stages:^[9]
* Glass was manufactured in open pits, ca. 3000 B.C. until the
invention of the blowpipe in ca. 250 B.C.

* The mobile wood-fired melting pot furnace was used until around
the 17th century by traveling glass manufacturers.

* Around 1688, a process for casting glass was developed, which
led to glass becoming a much more commonly used
material.^[citation needed]

* The local pot furnace, fired by wood and coal was used between
1600 and 1850.

* The cylinder method of creating flat glass was used in the
United States of America for the first time in the 1820s. It was
used to commercially produce windows.^[citation needed]

* The invention of the glass pressing machine in 1827 allowed the
mass production of inexpensive glass products^[10].

* The gas-heated melting pot and tank furnaces dating from 1860,
followed by the electric furnace of 1910.

* Hand-blown sheet glass was replaced in the 20th century by
rolled plate glass.^[citation needed]

* The float glass process was invented in the 1950s.

Glass ingredients

Pure silica (SiO[2]) has a "glass melting point"-- at a viscosity of
10 Pa·s (100 P)-- of over 2300 °C (4200 °F). While pure silica can
be made into glass for special applications (see fused quartz),
other substances are added to common glass to simplify processing.
One is sodium carbonate (Na[2]CO[3]), which lowers the melting point
to about 1500 °C (2700 °F) in soda-lime glass; "soda" refers to the
original source of sodium carbonate in the soda ash obtained from
certain plants. However, the soda makes the glass water soluble,
which is usually undesirable, so lime (calcium oxide (CaO),
generally obtained from limestone), some magnesium oxide (MgO) and
aluminium oxide are added to provide for a better chemical
durability. The resulting glass contains about 70 to 74 percent
silica by weight and is called a soda-lime glass.^[9] Soda-lime
glasses account for about 90 percent of manufactured glass.

As well as soda and lime, most common glass has other ingredients
added to change its properties. Lead glass, such as lead crystal or
flint glass, is more 'brilliant' because the increased refractive
index causes noticeably more "sparkles", while boron may be added to
change the thermal and electrical properties, as in Pyrex. Adding
barium also increases the refractive index. Thorium oxide gives
glass a high refractive index and low dispersion, and was formerly
used in producing high-quality lenses, but due to its radioactivity
has been replaced by lanthanum oxide in modern glasses. Large
amounts of iron are used in glass that absorbs infrared energy, such
as heat absorbing filters for movie projectors, while cerium(IV)
oxide can be used for glass that absorbs UV wavelengths
(biologically damaging ionizing radiation).

Besides the chemicals mentioned, in some furnaces recycled glass
("cullet") is added, originating from the same factory or other
sources. Cullet leads to savings not only in the raw materials, but
also in the energy consumption of the glass furnace. However,
impurities in the cullet may lead to product and equipment failure.
Fining agents such as sodium sulfate, sodium chloride, or antimony
oxide are added to reduce the bubble content in the glass.^[9]

A further raw material used in the production of soda-lime and fiber
glass is calumite, which is a glassy granular by-product of the iron
making industry, containing mainly silica, calcium oxide, alumina,
magnesium oxide (and traces of iron oxide).^[11]

For obtaining the desired glass composition, the correct raw
material mixture (batch) must be determined by glass batch
calculation.

Contemporary glass production

Following the glass batch preparation and mixing the raw materials
are transported to the furnace. Soda-lime glass for mass production
is melted in gas fired units. Smaller scale furnaces for specialty
glasses include electric melters, pot furnaces and day tanks.^[9]

After melting, homogenization and refining (removal of bubbles) the
glass is formed. Flat glass for windows and similar applications is
formed by the float glass process, developed between 1953 and 1957
by Sir Alastair Pilkington and Kenneth Bickerstaff of the UK's
Pilkington Brothers, which created a continuous ribbon of glass
using a molten tin bath on which the molten glass flows unhindered
under the influence of gravity. The top surface of the glass is
subjected to nitrogen under pressure to obtain a polished finish.
^[12] Container glass for common bottles and jars is formed by
blowing and pressing methods. Further glass forming techniques are
summarized in the table Glass forming techniques.

Once the desired form is obtained, glass is usually annealed for the
removal of stresses.

Various surface treatment techniques, coatings, or lamination may
follow to improve the chemical durability (glass container coatings,
glass container internal treatment), strength (toughened glass,
bulletproof glass, windshields), or optical properties (insulated
glazing, anti-reflective coating).

Glassmaking in the laboratory

New chemical glass compositions or new treatment techniques can be
initially investigated in small-scale laboratory experiments. The
raw materials for laboratory-scale glass melts are often different
from those used in mass production because the cost factor has a low
priority. In the laboratory mostly pure chemicals are used. Care
must be taken that the raw materials have not reacted with moisture
or other chemicals in the environment (such as alkali oxides and
hydroxides, alkaline earth oxides and hydroxides, or boron oxide),
or that the impurities are quantified (loss on ignition).^[13]
Evaporation losses during glass melting should be considered during
the selection of the raw materials, e.g., sodium selenite may be
preferred over easily evaporating SeO[2]. Also, more readily
reacting raw materials may be preferred over relatively inert ones,
such as Al(OH)[3] over Al[2]O[3]. Usually, the melts are carried out
in platinum crucibles to reduce contamination from the crucible
material. Glass homogeneity is achieved by homogenizing the raw
materials mixture (glass batch), by stirring the melt, and by
crushing and re-melting the first melt. The obtained glass is
usually annealed to prevent breakage during processing.^[13]^[14]

See also: Optical lens design, Fabrication and testing of optical
components

Silica-free glasses

Besides common silica-based glasses, many other inorganic and
organic materials may also form glasses, including plastics (e.g.,
acrylic glass), carbon, metals, carbon dioxide (see below),
phosphates, borates, chalcogenides, fluorides, germanates (glasses
based on GeO[2]), tellurites (glasses based on TeO[2]), antimonates
(glasses based on Sb[2]O[3]), arsenates (glasses based on
As[2]O[3]), titanates (glasses based on TiO[2]), tantalates (glasses
based on Ta[2]O[5]), nitrates, carbonates and many other
substances.^[6]

Some glasses that do not include silica as a major constituent may
have physico-chemical properties useful for their application in
fibre optics and other specialized technical applications. These
include fluorozirconate, fluoroaluminate, aluminosilicate, phosphate
and chalcogenide glasses.

Under extremes of pressure and temperature solids may exhibit large
structural and physical changes which can lead to polyamorphic phase
transitions.^[15] In 2006 Italian scientists created an amorphous
phase of carbon dioxide using extreme pressure. The substance was
named amorphous carbonia(a-CO[2]) and exhibits an atomic structure
resembling that of Silica.^[16]

The physics of glass

The standard definition of a glass (or vitreous solid) requires the
solid phase to be formed by rapid melt quenching.^[2]^[3]^[4] Glass
is therefore formed via a supercooled liquid and cooled sufficiently
rapidly (relative to the characteristic crystallisation time) from
its molten state through its glass transition temperature, T[g],
that the supercooled disordered atomic configuration at T[g], is
frozen into the solid state. Generally, the structure of a glass
exists in a metastable state with respect to its crystalline form,
although in certain circumstances, for example in atactic polymers,
there is no crystalline analogue of the amorphous phase ^[17]. By
definition as an amorphous solid, the atomic structure of a glass
lacks any long range translational periodicity. However, by virtue
of the local chemical bonding constraints glasses do possess a high
degree of short-range order with respect to local atomic
polyhedra^[18]. It is deemed that the bonding structure of glasses,
although disordered, has the same symmetry signature
(Hausdorff-Besicovitch dimensionality) as for crystalline
materials^[19].

Glass versus a supercooled liquid

Glass is generally treated as an amorphous solid rather than a
liquid, though both views can be justified.^[20] However, the notion
that glass flows to an appreciable extent over extended periods of
time is not supported by empirical research or theoretical analysis
(see viscosity of amorphous materials). From a more commonsense
point of view, glass should be considered a solid since it is rigid
according to everyday experience.^[21]

Some people consider glass to be a liquid due to its lack of a
first-order phase transition ^[20]^[22] where certain thermodynamic
variables such as volume, entropy and enthalpy are continuous
through the glass transition temperature. However, the glass
transition temperature may be described as analogous to a
second-order phase transition where the intensive thermodynamic
variables such as the thermal expansivity and heat capacity are
discontinuous. Despite this, thermodynamic phase transition theory
does not entirely hold for glass, and hence the glass transition
cannot be classed as a genuine thermodynamic phase transition.^[4]

Although the atomic structure of glass shares characteristics of the
structure in a supercooled liquid, glass is generally classed as
solid below its glass transition temperature.^[23] There is also the
problem that a supercooled liquid is still a liquid and not a solid
but it is below the freezing point of the material and will
crystallize almost instantly if a crystal is added as a core. The
change in heat capacity at a glass transition and a melting
transition of comparable materials are typically of the same order
of magnitude indicating that the change in active degrees of freedom
is comparable as well. Both in a glass and in a crystal it is mostly
only the vibrational degrees of freedom that remain active, whereas
rotational and translational motion becomes impossible explaining
why glasses and crystalline materials are hard.

Behavior of antique glass

The observation that old windows are often thicker at the bottom
than at the top is often offered as supporting evidence for the view
that glass flows over a matter of centuries. It is then assumed that
the glass was once uniform, but has flowed to its new shape, which
is a property of liquid. ^[24] The likely source of this unfounded
belief is that when panes of glass were commonly made by
glassblowers, the technique used was to spin molten glass so as to
create a round, mostly flat and even plate (the Crown glass process,
described above). This plate was then cut to fit a window. The
pieces were not, however, absolutely flat; the edges of the disk
would be thicker because of centripetal force relaxation. When
actually installed in a window frame, the glass would be placed
thicker side down for the sake of stability and visual sparkle.^[25]
Occasionally such glass has been found thinner side down or on
either side of the window's edge, as would be caused by carelessness
at the time of installation.

Mass production of glass window panes in the early twentieth century
caused a similar effect. In glass factories, molten glass was poured
onto a large cooling table and allowed to spread. The resulting
glass is thicker at the location of the pour, located at the center
of the large sheet. These sheets were cut into smaller window panes
with nonuniform thickness. Modern glass intended for windows is
produced as float glass and is very uniform in thickness.

Several other points exemplify the misconception of the 'cathedral
glass' theory:

* Writing in the American Journal of Physics,^[26] physicist Edgar
D. Zanotto states "...the predicted relaxation time for GeO[2]
at room temperature is 10^32 years. Hence, the relaxation period
(characteristic flow time) of cathedral glasses would be even
longer".
* If medieval glass has flowed perceptibly, then ancient Roman and
Egyptian objects should have flowed proportionately more -- but
this is not observed. Similarly, prehistoric obsidian blades
should have lost their edge; this is not observed either
(although obsidian may have a different viscosity from window
glass).^[20]
* If glass flows at a rate that allows changes to be seen with the
naked eye after centuries, then the effect should be noticeable
in antique telescopes. Any slight deformation in the antique
telescopic lenses would lead to a dramatic decrease in optical
performance, a phenomenon that is not observed.^[20]
* There are many examples of centuries-old glass shelving which
has not bent, even though it is under much higher stress from
gravitational loads than vertical window glass.

Some glasses have a glass transition temperature close to or below
room temperature. The behaviour of a material that has a glass
transition close to room temperature depends upon the timescale
during which the material is manipulated. If the material is hit it
may break like a solid glass, however if the material is left on a
table for a week it may flow like a liquid. This simply means that
for the fast timescale its transition temperature is above room
temperature, but for the slow one it is below. The shift in
temperature with timescale is not very large however as indicated by
the transition of polypropylene glycol of -72 °C and -71 °C over
different timescales. ^[17] To observe window glass flowing as
liquid at room temperature we would have to wait a much longer time
than any human can exist. Therefore it is safe to consider a glass a
solid far enough below its transition temperature: Cathedral glass
does not flow because its glass transition temperature is many
hundreds of degrees above room temperature. Close to this
temperature there are interesting time-dependent properties. One of
these is known as aging. Many polymers that we use in daily life
such as rubber, polystyrene and polypropylene are in a glassy state
but they are not too far below their glass transition temperature.
Their mechanical properties may well change over time and this is
serious concern when applying these materials in construction.

Physical properties

The following table lists some physical properties of common
glasses. Unless otherwise stated, the technical glass compositions
and many experimentally determined properties are taken from one
large study.^[27] Unless stated otherwise, the properties of fused
silica (quartz glass) and germania glass are derived from the
SciGlass glass database by forming the arithmetic mean of all the
experimental values from different authors (in general more than 10
independent sources for quartz glass and Tg of germanium oxide
glass). Those values marked in italic font have been interpolated
from similar glass compositions (see Calculation of glass
properties) due to the lack of experimental data.
Properties Soda-lime glass (for containers)^[28] Borosilicate (low
expansion, similar to Pyrex, Duran) Glass wool (for thermal
insulation) Special optical glass (similar to
Lead crystal) Fused silica Germania glass Germanium selenide glass
Chemical
composition,
wt% 74 SiO[2], 13 Na[2]O, 10.5 CaO, 1.3 Al[2]O[3], 0.3 K[2]O, 0.2
SO[3], 0.2 MgO, 0.01 TiO[2], 0.04 Fe[2]O[3] 81 SiO[2], 12.5
B[2]O[3], 4 Na[2]O, 2.2 Al[2]O[3], 0.02 CaO, 0.06 K[2]O 63 SiO[2],
16 Na[2]O, 8 CaO, 3.3 B[2]O[3], 5 Al[2]O[3], 3.5 MgO, 0.8 K[2]O, 0.3
Fe[2]O[3], 0.2 SO[3] 41.2 SiO[2], 34.1 PbO, 12.4 BaO, 6.3 ZnO, 3.0
K[2]O, 2.5 CaO, 0.35 Sb[2]O[3], 0.2 As[2]O[3] SiO[2] GeO[2] GeSe[2]
Viscosity
log(y, Pa·s) = A +
B / (T in °C - T[o]) 550-1450°C:
A = -2.309
B = 3922
T[o] = 291 550-1450°C:
A = -2.834
B = 6668
T[o] = 108 550-1400°C:
A = -2.323
B = 3232
T[o] = 318 500-690°C:
A = -35.59
B = 60930
T[o] = -741 1140-2320°C:
A = -7.766
B = 27913
T[o] = -271.7 515-1540°C:
A = -11.044
B = 30979
T[o] = -837
Glass transition
temperature, T[g], °C 573 536 551 ~540 1140 526 ± 27^[29]^[30]^[31]
395 ^[32]
Coefficient of
thermal expansion,
ppm/K, ~100-300°C 9 3.5 10 7 0.55 7.3
Density
at 20°C, g/cm^3 2.52 2.235 2.550 3.86 2.203 3.65 ^[33] 4.16 ^[32]
Refractive index n[D]^[34] at 20°C 1.518 1.473 1.531 1.650 1.459
1.608 1.7
Dispersion at 20°C,
10^4×(n[F]-n[C])^[34] 86.7 72.3 89.5 169 67.8 146
Young's modulus
at 20°C, GPa 72 65 75 67 72 43.3 ^[35]
Shear modulus
at 20°C, GPa 29.8 28.2 26.8 31.3
Liquidus
temperature, °C 1040 1070^[36] 1715 1115
Heat
capacity at 20°C,
J/(mol·K) 49 50 50 51 44 52
Surface tension,
at ~1300°C, mJ/m^2 315 370 290
Chemical durability,
Hydrolytic class,
after ISO 719^[37] 3 1 3

Color

Main article: Glass_production#Colors

Colors in glass may be obtained by addition of coloring ions that
are homogeneously distributed and by precipitation of finely
dispersed particles (such as in photochromic glasses).^[6] Ordinary
soda-lime glass appears colorless to the naked eye when it is thin,
although iron(II) oxide (FeO) impurities of up to 0.1 wt%^[27]
produce a green tint which can be viewed in thick pieces or with the
aid of scientific instruments. Further FeO and Cr[2]O[3] additions
may be used for the production of green bottles. Sulfur, together
with carbon and iron salts, is used to form iron polysulfides and
produce amber glass ranging from yellowish to almost black.^[38]
Manganese dioxide can be added in small amounts to remove the green
tint given by iron(II) oxide.

History

see also category Glass history

Naturally occurring glass, especially obsidian, has been used by
many Stone Age societies across the globe for the production of
sharp cutting tools and, due to its limited source areas, was
extensively traded. According to Pliny the Elder, Phoenician traders
were the first to stumble upon glass manufacturing techniques at the
site of the Belus River. Agricola, De re metallica, reported a
traditional serendipitous "discovery" tale of familiar type:

"The tradition is that a merchant ship laden with nitrum being
moored at this place, the merchants were preparing their meal on
the beach, and not having stones to prop up their pots, they used
lumps of nitrum from the ship, which fused and mixed with the
sands of the shore, and there flowed streams of a new translucent
liquid, and thus was the origin of glass."^[39]

This account is more a reflection of Roman experience of glass
production, however, as white silica sand from this area was used in
the production of Roman glass due to its low impurity levels. But in
general archaeological evidence suggests that the first true glass
was made in coastal north Syria, Mesopotamia or Old Kingdom
Egypt.^[40] Due to Egypt's favourable environment for preservation,
the majority of well-studied early glass is found in Egypt, although
some of this is likely to have been imported. The earliest known
glass objects, of the mid third millennium BC, were beads, perhaps
initially created as accidental by-products of metal-working slags
or during the production of faience, a pre-glass vitreous material
made by a process similar to glazing.^[41]

During the Late Bronze Age in Egypt and Western Asia there was an
explosion in glass-making technology. Archaeological finds from this
period include coloured glass ingots, vessels (often coloured and
shaped in imitation of highly prized wares of semi-precious stones)
and the ubiquitous beads. The alkali of Syrian and Egyptian glass
was soda ash, sodium carbonate, which can be extracted from the
ashes of many plants, notably halophile seashore plants: (see
saltwort). The earliest vessels were 'core-wound', produced by
winding a ductile rope of metal round a shaped core of sand and clay
over a metal rod, then fusing it with repeated reheatings. Threads
of thin glass of different colours made with admixtures of oxides
were subsequently wound around these to create patterns, which could
be drawn into festoons with a metal raking tools. The vessel would
then be rolled flat ('marvered') on a slab in order to press the
decorative threads into its body. Handles and feet were applied
separately. The rod was subsequently allowed to cool as the glass
slowly annealed and was eventually removed from the centre of the
vessel, after which the core material was scraped out. Glass shapes
for inlays were also often created in moulds. Much early glass
production, however, relied on grinding techniques borrowed from
stone working. This meant that the glass was ground and carved in a
cold state.

By the 15th century BC extensive glass production was occurring in
Western Asia and Egypt. It is thought the techniques and recipes
required for the initial fusing of glass from raw materials was a
closely guarded technological secret reserved for the large palace
industries of powerful states. Glass workers in other areas
therefore relied on imports of pre-formed glass, often in the form
of cast ingots such as those found on the Ulu Burun shipwreck off
the coast of Turkey.

Glass remained a luxury material, and the disasters that overtook
Late Bronze Age civilisations seem to have brought glass-making to a
halt. It picked up again in its former sites, in Syria and Cyprus,
in the ninth century BC, when the techniques for making colourless
glass were discovered. In Egypt glass-making did not revive until it
was reintroduced in Ptolemaic Alexandria. Core-formed vessels and
beads were still widely produced, but other techniques came to the
fore with experimentation and technological advancements. During the
Hellenistic period many new techniques of glass production were
introduced and glass began to be used to make larger pieces, notably
table wares. Techniques developed during this period include
'slumping' viscous (but not fully molten) glass over a mould in
order to form a dish and 'millefiori' (meaning 'thousand flowers')
technique, where canes of multi-coloured glass were sliced and the
slices arranged together and fused in a mould to create a
mosaic-like effect. It was also during this period that colourless
or decoloured glass began to be prized and methods for achieving
this effect were investigated more fully.

During the first century BC glass blowing was discovered on the
Syro-Palestinian coast, revolutionising the industry and laying the
way for the explosion of glass production that occurred throughout
the Roman world. Over the next 1000 years glass making and working
continued and spread through southern Europe and beyond.

South Asia

Indigenous development of glass technology in South Asia may have
begun in 1730 BCE.^[42] Evidence of this culture includes a
red-brown glass bead along with a hoard of beads dating to 1730 BCE,
making it the earliest attested glass from the Indus Valley
locations.^[42] Glass discovered from later sites dating from
600-300 BCE displays common color.^[42]

Chalcolithic evidence of glass has been found in Hastinapur,
India.^[43] Some of the texts which mention glass in India are the
Shatapatha Brahmana and Vinaya Pitaka.^[43] However, the first
unmistakable evidence in large quantities, dating from the 3rd
century BCE, has been uncovered from the archaeological site in
Taxila, Pakistan.^[43]

By the beginning of the Common Era, glass was being used for
ornaments and casing in South Asia.^[43] Contact with the
Greco-Roman world added newer techniques, and Indians artisans
mastered several techniques of glass molding, decorating and
coloring by the early centuries of the Common Era.^[43] Satavahana
period of India further reveals short cylinders of composite glass,
including those displaying a lemon yellow matrix covered with green
glass.^[44]

Romans

A full discussion of Roman glass making and working can be found on
the Roman glass page.

Anglo-Saxon world

Evidence for glass making, working and use in the 5th to 8th
centuries in England is discussed in the Anglo-Saxon glass page.

Islamic world

In the medieval Islamic world, the first clear, colourless,
high-purity glasses were produced by Muslim chemists, architects and
engineers in the 9th century. Examples include Silica glass and
colourless high-purity glass invented by Abbas Ibn Firnas (810-887),
who was the first to produce glass from sand and stones.^[45] The
Arab poet al-Buhturi (820-897) described the clarity of such glass,
"Its colour hides the glass as if it is standing in it without a
container."^[46]

Stained glass was also first produced by Muslim architects in
Southwest Asia using coloured glass rather than stone. In the 8th
century, the Arab chemist Jabir ibn Hayyan (Geber) scientifically
described 46 original recipes for producing coloured glass in Kitab
al-Durra al-Maknuna (The Book of the Hidden Pearl), in addition to
12 recipes inserted by al-Marrakishi in a later edition of the
book.^[47]

The parabolic mirror was first described by Ibn Sahl in his On the
Burning Instruments in the 10th century, and later described again
in Ibn al-Haytham's On Burning Mirrors and Book of Optics
(1021).^[48] By the 11th century, clear glass mirrors were being
produced in Islamic Spain. The first glass factories were also built
by Muslim craftsmen in the Islamic world. The first glass factories
in Christian Europe were later built in the 11th century by Muslim
Egyptian craftsmen in Corinth, Greece.^[49]

Medieval Europe

Glass objects from the 7th and 8th centuries have been found on the
island of Torcello near Venice. These form an important link between
Roman times and the later importance of that city in the production
of the material. Around 1000 AD, an important technical breakthrough
was made in Northern Europe when soda glass, produced from white
pebbles and burnt vegetation was replaced by glass made from a much
more readily available material: potash obtained from wood ashes.
> From this point on, northern glass differed significantly from that
made in the Mediterranean area, where soda remained in common
use.^[50]

Until the 12th century, stained glass -- glass to which metallic or
other impurities had been added for coloring -- was not widely used.

The 11th century saw the emergence in Germany of new ways of making
sheet glass by blowing spheres. The spheres were swung out to form
cylinders and then cut while still hot, after which the sheets were
flattened. This technique was perfected in 13th century Venice.

The Crown glass process was used up to the mid-19th century. In this
process, the glassblower would spin approximately 9 pounds (4 kg) of
molten glass at the end of a rod until it flattened into a disk
approximately 5 feet (1.5 m) in diameter. The disk would then be cut
into panes.

Late medieval Northern Europe

Glass making in late medieval Northern Europe is discussed in the
article on Forest glass.

Murano glassmaking

Main articles: Murano glass and Venetian glass

The center for glassmaking from the 14th century was the island of
Murano, which developed many new techniques and became the center of
a lucrative export trade in dinnerware, mirrors, and other luxury
items. What made Venetian Murano glass significantly different was
that the local quartz pebbles were almost pure silica, and were
ground into a fine clear sand that was combined with soda ash
obtained from the Levant, for which the Venetians held the sole
monopoly. The clearest and finest glass is tinted in two ways:
firstly, a small or large amount of a natural coloring agent is
ground and melted with the glass. Many of these coloring agents
still exist today; for a list of coloring agents, see below. Black
glass was called obsidianus after obsidian stone. A second method is
apparently to produce a black glass which, when held to the light,
will show the true color that this glass will give to another glass
when used as a dye. ^[51]

The Venetian ability to produce this superior form of glass resulted
in a trade advantage over other glass producing lands. Murano's
reputation as a center for glassmaking was born when the Venetian
Republic, fearing fire might burn down the city's mostly wood
buildings, ordered glassmakers to move their foundries to Murano in
1291. Murano's glassmakers were soon the island's most prominent
citizens. Glassmakers were not allowed to leave the Republic. Many
took a risk and set up glass furnaces in surrounding cities and as
far afield as England and the Netherlands.

Glass art

Main article: Glass art

Beginning in the late 20th century, glass started to become highly
collectible as art. Works of art in glass can be seen in a variety
of museums, including the Chrysler Museum, the Museum of Glass in
Tacoma, the Metropolitan Museum of Art, the Toledo Museum of Art,
and Corning Museum of Glass, in Corning, NY, which houses the
world's largest collection of glass art and history, with more than
45,000 objects in its collection.^[52]

Several of the most common techniques for producing glass art
include: blowing, kiln-casting, fusing, slumping, pate-de-verre,
flame-working, hot-sculpting and cold-working. Cold work includes
traditional stained glass work as well as other methods of shaping
glass at room temperature. Glass can also be cut with a diamond saw,
or copper wheels embedded with abrasives, and polished to give
gleaming facets; the technique used in creating Waterford crystal
^[53]. Art is sometimes etched into glass via the use of acid,
caustic, or abrasive substances. Traditionally this was done after
the glass was blown or cast. In the 1920s a new mould-etch process
was invented, in which art was etched directly into the mould, so
that each cast piece emerged from the mould with the image already
on the surface of the glass. This reduced manufacturing costs and,
combined with a wider use of colored glass, led to cheap glassware
in the 1930s, which later became known as Depression glass^[54]. As
the types of acids used in this process are extremely hazardous,
abrasive methods have gained popularity.

Objects made out of glass include not only traditional objects such
as vessels (bowls, vases, bottles, and other containers),
paperweights, marbles, beads, but an endless range of sculpture and
installation art as well. Colored glass is often used, though
sometimes the glass is painted, innumerable examples exist of the
use of stained glass.

The Harvard Museum of Natural History has a collection of extremely
detailed models of flowers made of painted glass. These were
lampworked by Leopold Blaschka and his son Rudolph, who never
revealed the method he used to make them. The Blaschka Glass Flowers
are still an inspiration to glassblowers today. ^[55]

See also

* Aluminium oxynitride
* Favrile iridescent glass
* Glass makers and brands
* Glass recycling
* Glass fibre
* Magnifying glass
* Opaline glass

References

1. ASTM definition of glass from 1945; also: DIN 1259, Glas - Begriffe für 
Glasarten und Glasgruppen, September 1986
2. Zallen, The Physics of Amorphous Solids, John Wiley, New York, (1983).
3. The physics of structurally disordered matter: an introduction, Adam 
Hilger in association with the University of Sussex press (1987)
4. Elliot, Physics of amorphous materials, Longman group ltd (1984)
5. Horst Scholze: "Glass - Nature, Structure, and Properties"; Springer, 
1991, ISBN 0-387-97396-6
6. Werner Vogel: "Glass Chemistry"; Springer-Verlag Berlin and Heidelberg 
GmbH & Co. K; 2nd revised edition (November 1994), [640]ISBN 3540575723
7. See article: Samsung Corning Precision Glass, TFT-LCD Glass substrates
8. Sol-Gel Technologies Ltd.
http://www.solgel.com/biz/featcom/solgel.asp
9. B. H. W. S. de Jong, "Glass"; in "Ullmann's Encyclopedia of Industrial 
Chemistry"; 5th edition, vol. A12, VCH Publishers, Weinheim, Germany, 
1989, ISBN 3-527-20112-5, p 365-432.
10. Cut Glass And Glass Blowing History And Development Article - 
Entertainment Article at www.redsofts.com
http://www.redsofts.com/articles/read/151/61135/Cut_Glass_And_Glass_Blowing_History_And_Development.html
11. Calumite Limited, United Kingdom
http://www.calumite.co.uk/
12. PFG Glass
http://www.pfg.co.za/about%20glass.htm
13. Glass melting, Pacific Northwest National Laboratory
http://depts.washington.edu/mti/1999/labs/glass_ceramics/mst_glass.html
14. Glass melting in the laboratory
http://glassproperties.com/melting/
15. McMillan, P.F. Journal of Materials Chemistry, 14, 1506-1512 (2004)
16. carbon dioxide glass created in the lab 15 June 2006  Retrieved 3 
August 2006
http://www.newscientisttech.com/article.ns?id=dn9339
17. "Folmer, J. C. W.; Franzen, Stefan." Study of polymer glasses by 
modulated differential scanning calorimetry in the undergraduate physical 
chemistry laboratory. Journal of Chemical Education (2003), 80(7), 
813-818. CODEN: JCEDA8 ISSN:0021-9584.
18. Salmon, P.S., Order within disorder, Nature Materials, 1(87), (2002)
19. M.I. Ojovan, W.E. Lee. Topologically disordered systems at the glass 
transition. J. Phys.: Condensed Matter, 18, 11507-11520 (2006)
20. Philip Gibbs. "Is glass liquid or solid?". Retrieved on 2007-03-21.
http://math.ucr.edu/home/baez/physics/General/Glass/glass.html
21. "Philip Gibbs" Glass Worldwide, (may/june 2007), pp 14-18
22. Jim Loy. "Glass Is A Liquid?". Retrieved on 2007-03-21.
http://www.jimloy.com/physics/glass.htm
23. Florin Neumann. "Glass: Liquid or Solid -- Science vs. an Urban 
Legend". Retrieved on 2007-04-08.
http://dwb.unl.edu/Teacher/NSF/C01/C01Links/www.ualberta.ca/~bderksen/florin.html
24. Chang, Kenneth (2008-07-29). "The Nature of Glass Remains Anything but 
Clear", New York Times. Retrieved on 2008-07-29.
http://www.nytimes.com/2008/07/29/science/29glass.html
25. Dr Karl's Homework: Glass Flows
http://www.abc.net.au/science/k2/homework/s95602.htm
26. "Do Cathedral Glasses Flow?" Am. J. Phys., 66 (May 1998), pp 392-396
27. "High temperature glass melt property database for process modeling"; 
Eds.: Thomas P. Seward III and Terese Vascott; The American Ceramic 
Society, Westerville, Ohio, 2005, ISBN 1-57498-225-7
28. Soda-lime glass for containers is slightly different from soda-lime 
glass for windows (also called flat glass or float glass). Float glass has 
a higher magnesium oxide content as compared to container glass, and a 
lower silica and calcium oxide content. For further details see main 
article Soda-lime glass.
29. Leadbetter et al, Journal of non-crystalline solids, 7:37-52 (1972)
30. Micoulaut et al, Physical Review E, 73:031504 (2006)
31. 35 T[g] data for GeO[2] from SciGlass 6.7
32. Kotkata et al., J. Phys. D: Appl. Phys. 27 pp 623-627 (1994)
33. Salmon et al, Physical Review Letters, 96, 235502 (2006)
34. The subscript D indicates that the refractive index n was measured at 
a wavelength l of 589.29 nm, F and C indicate 486.13 nm (blue) and 656.27 
nm (red) respectively (see
article Fraunhofer lines)
35. Hwa et al, Materials Chemistry and Physics, 94, 1, 37-41 (2005)
36. Valid for glass composition, wt%: 80.7 SiO[2], 13.1 B[2]O[3], 4.1 
Na[2]O, 2.1 Al[2]O[3]; Reference: Baak N. T. E. A. and Rapp C. F., GB 
Patent No. 1132885 Cl C 03 C 3/04, Abridg. Specif., 1968; Assignee: 
Owens-Illinois, Inc. (US).
37. International Organization for Standardization, Procedure 719 (1985)
http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=4948
38. Substances Used in the Making of Coloured Glass 1st.glassman.com 
(David M Issitt). Retrieved 3 August 2006
http://1st.glassman.com/articles/glasscolouring.html
39. Agricola, Georgius, De re metallica, translated by Herbert Clark 
Hoover and Lou Henry Hoover, Dover Publishing. De Re Metallica Trans. by 
Hoover Online Version Page 586 Retrieved = 12 September 2007
http://www.farlang.com/gemstones/agricola-metallica/page_001
http://www.farlang.com/gemstones/agricola-metallica/page_621
40. Glass Online: The History of Glass". Retrieved on 2007-10-29.
http://www.glassonline.com/infoserv/history.html
41. True glazing over a ceramic body was not used until many centuries 
after the production of the first glass.
42. Gowlett 1997, page 276-277
43. Ghosh 1990, page 219
44. "Ornaments, Gems etc." (Ch. 10) in Ghosh 1990
45. Lynn Townsend White, Jr. (Spring, 1961). "Eilmer of Malmesbury, an 
Eleventh Century Aviator: A Case Study of Technological Innovation, Its 
Context and Tradition", Technology and Culture 2 (2), pp. 97-111. "Ibn 
Firnas was a polymath: a physician, a rather bad poet, the first to make 
glass from stones, a student of music, and inventor of some sort of 
metronome."
46. Ahmad Y Hassan, Assessment of Kitab al-Durra al-Maknuna, History of 
Science and Technology in Islam.
http://www.history-science-technology.com/Articles/articles%2093.htm
47. Ahmad Y Hassan, The Manufacture of Coloured Glass, History of Science 
and Technology in Islam.
http://www.history-science-technology.com/Articles/articles%209.htm
48. Roshdi Rashed (1990), "A Pioneer in Anaclastics: Ibn Sahl on Burning 
Mirrors and Lenses", Isis 81 (3), p. 464-491 [464-468].
49. Ahmad Y Hassan, Transfer Of Islamic Technology To The West, Part III: 
Technology Transfer in the Chemical Industries, History of Science and 
Technology in Islam.
http://www.history-science-technology.com/Articles/articles%2072.htm
50. Donny L. Hamilton. "Glass Conservation". Conservation Research 
Laboratory, Texas A&M University. Retrieved on 2007-03-21.
http://nautarch.tamu.edu/class/anth605/File5.htm
51. Georg Agricola De Natura Fossilium, Textbook of Mineralogy, M.C. 
Bandy, J. Bandy, Mineralogical Society of America, 1955, Page 111 Section 
on Murano Glass, De Natura Fossilium Retrieved 12 September 2007
http://www.farlang.com/gemstones/agricola-metallica/page_001
52. Corning Museum of Glass". Retrieved on 2007-10-14.
http://www.cmog.org/index.asp?pageId=1276
53. "Waterford Crystal Vistors Centre". Retrieved on 2007-10-19.
http://www.waterfordvisitorcentre.com/
54. Depression Glass". Retrieved on 2007-10-19.
55. the Harvard Museum of Natural History's page on the exhibit
http://www.hmnh.harvard.edu/exhibitions/glassflowers.html

Bibliography

* Noel C. Stokes; The Glass and Glazing Handbook; Standards
Australia; SAA HB125-1998
* Brugmann, Birte. Glass Beads from Anglo-Saxon Graves: A Study on
the Provenance and Chronology of Glass Beads from Anglo-Saxon
Graves, Based on Visual Examination. Oxbow Books, 2004. ISBN
1-84217-104-6
* Gowlett, J.A.J. (1997). High Definition Archaeology: Threads
Through the Past. Routledge. ISBN 0415184290.
* Ghosh, Amalananda (1990). An Encyclopaedia of Indian
Archaeology. BRILL. ISBN 9004092625.

External links

Look up [783]Glass in
[784]Wiktionary, the free dictionary.
[785]Wikimedia Commons has media related to:
[786]Glass
* Glass Encyclopedia - A comprehensive guide to all types of antique and 
collectable glass, with information, pictures and references
http://www.20thcenturyglass.com/glass_encyclopedia_home.htm
* Free information and articles about Designer Glassware, Vintage Art 
Glass, Depression Glass & Collectible Glass
http://www.justglass-online.com/
* The Canadian Museum of Civilization - The Story of Glass Making in 
Canada
http://www.civilisations.ca/hist/verre/veint01e.html
* Corning Museum of Glass
http://www.cmog.org/
* A comprehensive guide to art glass and crystal around the world
http://www.worldartglass.com/index.asp
* Working Description Furnace & Moleria - Murano Glass
http://venixe.com/en/glass-working-descriptions/description-of-the-art-of-murano-glass-furnace-and-mol.html
* Informative website about the glass industry
http://www.glassonweb.com/
* Substances used in the Making of Colored Glass
http://1st.glassman.com/articles/glasscolouring.html
* Glass property measurement and calculation
http://glassproperties.com/
* Almost 400 articles and images about glass (mostly art glass)
http://www.glassfacts.info/
* Dual personality of glass explained at last - New Scientist, 22 June 
2008
http://technology.newscientist.com/channel/tech/dn14179-dual-personality-of-glass-explained-at-last.html?feedId=online-news_rss20

References

783. http://en.wiktionary.org/wiki/Special:Search/glass
784. http://en.wikipedia.org/wiki/Wiktionary
785. http://en.wikipedia.org/wiki/Wikimedia_Commons
786. http://commons.wikimedia.org/wiki/Category:Glass

* This page was last modified on 30 July 2008, at 07:35.
[And the NYT piece on Tuesday, July 29 was already included!]