A piece of naturally formed glass, Obsidian
Glass is a noncrystalline material that can maintain indefinitely, if left undisturbed, its overall form and
amorphous microstructure at a temperature below its glass transition temperature. Glass synthesis is achieved by quenching a glass forming liquid through its glass transition temperature sufficiently rapidly to avoid the
formation of a regular crystal lattice, producing an
amorphous solid. Amorphous solids may also be formed by methods other than melt quenching, such as vapour deposition or the
sol-gel method. Silica glass may be produced by using
sand as a raw material (or "quartz sand") that contains almost 100 % crystalline
silica in the form of quartz. The most common method for
glass pane production is using molten tin, where the molten glass floats on top of the perfectly
flat molten tin, thus giving it the name "float glass". Glass is sometimes created naturally
from volcanic magma. 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 knives since the stone
age.
The Physics of Glass
The amorphous structure of glassy Silica (SiO
2). No long range order is present, however there is local ordering with
respect to the
tetrahedral arrangement of Oxygen (O) atoms around the Silicon (Si)
atoms.
The standard definition of a glass (or vitreous solid) requires the solid phase to be formed
by rapid melt quenching[1]. Glass is therefore formed via a
supercooled liquid and cooled sufficiently rapidly from its molten state through its
glass transition temperature, Tg, that the supercooled
disordered atomic configuration at Tg, 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 [2]. 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[3].
Glass versus undercooled liquid
Glass is generally treated as an amorphous solid rather than a liquid, though
different views can be justified since characterizing glass as either 'solid' or 'liquid' is not an entirely straightforward
matter.[4] 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
[5]
Some people believe glass is a liquid due to its lack of a first-order phase
transition [4][6] 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. [7]
Although glass is amorphous like a supercooled liquid, it is generally classed as solid
below its glass transition temperature.[8] There is also
the problem that a supercooled liquid is still a liquid — moves and behaves like a liquid,
not a solid — but 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. This explains why glasses and crystalline materials are hard.
Behaviour 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. 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.[9] Occasionally such glass has been found thinner side down, 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 indicate that the 'cathedral glass' theory is misconceived:
- Writing in the American Journal of Physics,[10] physicist Edgar D. Zanotto states
"...the predicted relaxation time for GeO2 at room temperature is
1032 years. Hence, the relaxation period (characteristic flow time) of cathedral glasses would be even longer" (Am. J.
Phys, 66(5):392–5, May 1998). In layman's terms, he wrote that glass at room temperature is very strongly on the solid side of
the spectrum from solids to liquids.
- If medieval glass has flowed perceptibly, then ancient Roman and Egyptian objects should have flowed proportionately more —
but this is not observed. [11] Similarly, prehistoric
obsidian blades should have lost their edge; this is not observed either.
- 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 [4].
Time-dependency as the cause of the confusion
Strictly speaking the glass transition temperature is not a constant but a function of frequency. For example the transition
of polypropylene glycol may well be at -72C measured at a frequency of 1min-1 but at -71C measured at
1.5min-1 . This means that if we observe a material that has a glass transition close to room temperature it
depends how fast we manipulate it. If we smash it on the floor it may break like a solid glass, if we leave it on the 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. This means that
to observe window glass flowing as liquid at room temperature we would have to wait a much longer time than the universe exists.
Therefore it is safe to consider a glass a solid far enough below its transition temperature: Cathedral glass will not drip
because its glass transition 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 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.
Properties and uses
A
vase being created at the Reijmyre glassworks,
Sweden
The most obvious characteristic of ordinary glass is that it is transparent to visible
light, hence its wide application in everyday use. 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 it's transparency as heterogeneities
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, cathode ray tubes, and windows. In scientific research laboratories, flasks, test tubes, lenses
and other laboratory equipment are often made of glass. For these applications, borosilicate
glass (such as Pyrex) is usually used for its strength and low coefficient of thermal expansion, which gives greater resistance to thermal shock and allows for greater accuracy in laboratory measurements when heating and cooling
experiments. For the most demanding applications, quartz glass is used, although it is very
difficult to work. Most such glass is mass-produced using various industrial processes,
but most large laboratories need so much custom glassware that they keep a glassblower on
staff. Volcanic glasses, such as obsidian, have long been used to make stone tools, and flint knapping techniques can easily be adapted to
mass-produced glass.
Technological applications
The types and uses of glass for scientific and technical purposes are myriad, and range from applications involving the smallest
of devices such as
DNA microarrays to football field sized enormously powerful
neodymium doped glass (as shown above)
lasers used for
laser fusion applications.
Pure SiO2 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. This type of glass can be made so pure that when combined with Germanium Oxide glass hundreds of kilometers of
fibre optic cables can be manufactured which are transparent at infrared wavelengths. Individual fibres are given an equally transparent core of
SiO2/GeO2 glass, which has only slightly different optical properties
(the germanium contributing to a higher index of refraction). Undersea cables have sections doped with erbium, which
amplify transmitted signals by laser emission from
within the glass itself. Amorphous SiO2 is also used as a dielectric material in
integrated circuits due to the smooth and electrically neutral interface it forms
with silicon.
Glasses used for making optical devices 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 the possibility of embedding active molecules, such as
enzymes, to add a new level of functionality to glass vessels.
Glass in buildings
-
Glass has been used in buildings since the 11th century. Uses for glass in buildings include 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.
Glass ingredients
Pure silica (SiO2) has a melting
point of about 2,000°C (3,632°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
(Na2CO3), which lowers the melting point to about 1,000°C (1,832°F); "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 aluminum oxide are added to provide for a better chemical
durability. The resulting glass contains about 70 to 72 percent silica by weight and is called a soda-lime glass. 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).
Properties such as density and melting point vary greatly depending on the material added to
the silica: density can range from light display glass with 2.37 g/cm³ to high lead-content flint glass with 7.2 g/cm³, while
melting points can range from 500 to 1650 °C.[12] These
ranges can be exceeded, but usually at the cost of stability or practicality.
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[13] . In 2006 Italian scientists created an amorphous phase of carbon
dioxide using extreme pressure. The substance was named amorphous
carbonia(a-CO2) and exhibits an atomic structure resembling that of ordinary window glass [14].
Colors
Metallic additives in the glass mix can produce a variety of colors. Here
cobalt has been added to produce a bluish colored decorative glass.
The inside of a blue glass cup.
Ordinary glass appears colorless to the naked eye when it is thin, although iron
oxide impurities produce a green tint which can be viewed in thick pieces or with the aid of
scientific instruments. Further metals and metal oxides can be
added to glass during its manufacture to change its color, examples of which are listed below.
- Iron(II) oxide results in bluish-green glass, frequently used for beer bottles.
Together with chromium it gives a richer green color, used for wine bottles.
- Sulphur, together with carbon and iron salts, is used to form
iron polysulphides and produce amber glass ranging from yellowish to almost black. In borosilicate glasses rich in boron, sulphur imparts a blue color. With calcium it yields a deep yellow color. [15]
- Manganese can be added in small amounts to remove the green
tint given by iron, or in higher concentrations to give glass an amethyst color.
Manganese is one of the oldest glass additives, and purple manganese glass was used since early Egyptian history.
- Magnanese dioxide, which is black, is used to remove the green color from the glass; in a very
slow process this is converted to sodium permanganate, a dark purple compound. In
New England some houses built more than 300 years ago have window glass
which is lightly tinted violet because of this chemical change; and such glass panes are
prized as antiques.
- Selenium, like manganese, can be used in small concentrations to decolorize glass, or in
higher concentrations to impart a reddish color, caused by selenium atoms dispersed in glass. It is
a very important agent to make pink and red glass. When used together with cadmium sulfide [16], it yields a brilliant red color known as "Selenium Ruby".
- Small concentrations of cobalt (0.025 to 0.1%) yield blue glass. The best results are achieved when using glass containing potash. Very small amounts can be used for decolorizing.
- Tin oxide with antimony and
arsenic oxides produce an opaque white glass, first used
in Venice to produce an imitation porcelain.
- 2 to 3% of copper oxide produces a turquoise
color.
- Pure metallic copper produces a very dark red, opaque glass, which is sometimes used as a
substitute for gold in the production of ruby-colored glass.
- Nickel, depending on the concentration, produces blue, or violet, or even black glass. Lead
crystal with added nickel acquires purplish color. Nickel together with small amount of cobalt was used for decolorizing
of lead glass.
- Chromium is a very powerful colorizing agent, yielding dark green [17] or in higher concentrations even black color. Together with tin oxide and
arsenic it yields emerald green glass. Chromium aventurine, in which aventurescence was achieved by growth of large
parallel chromium(III) oxide plates, was also made from glass with added
chromium.
- Cadmium together with sulphur results in deep yellow color, often used in glazes. However,
cadmium is toxic.
- Adding titanium produces yellowish-brown glass. Titanium is rarely used on its own, is more often employed to intensify and brighten other colorizing
additives.
- Metallic gold, in very small concentrations (around 0.001%), produces a rich ruby-colored glass
("Ruby Gold"), while lower concentrations produces a less intense red, often marketed as
"cranberry". The color is caused by the size and dispersion of gold particles. Ruby gold
glass is usually made of lead glass with added tin.
- Uranium (0.1 to 2%) can be added to give glass a fluorescent yellow or green color [18]. Uranium glass is typically not radioactive enough to be
dangerous, but if ground into a powder, such as by polishing with sandpaper, and inhaled, it can be carcinogenic. When used with lead glass with very high proportion of lead, produces a deep red color.
- Silver compounds (notably silver nitrate) can produce
a range of colors from orange-red to yellow. The way the glass is heated and cooled can
significantly affect the colors produced by these compounds. The chemistry involved is complex and not well understood.
History of glass
Phoenicia and Egypt
Naturally occurring glass, such as obsidian, has been used since the stone age. According to Pliny the Elder, the Phoenicians made the first glass:[19]
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.
Glass used as a glaze for pottery is known as early as
3000 BC. However, there is archaeological evidence to support the claim that the first glass was
made in Mesopotamia [citation needed]. Glass beads, seals, and architectural decorations date from around
2500 BC. Glass was also discovered by Native Americans during
the same time period.
The color of glass made from naturally occuring sand is green to bluish green which is caused by iron impurities. Common glass
today usually has a slight green or blue tint, arising from these same impurities. Glassmakers learned to make colored glass by
adding metallic compounds and mineral oxides to produce brilliant hues of red, green, and blue; the colors of gemstones. When
gem-cutters learned to cut glass, they found clear glass was an excellent lifter of light. The earliest known beads from
Egypt were made during the New Kingdom around 1500 BC
and were produced in a variety of colors. They were made by winding molten glass around a metal bar and were highly prized as a
trading commodity, especially blue beads, which were believed to have magical powers.
Core-formed amphoriskos (17 cm / 6.7
in tall) 1st century BC, Cyprus
The Egyptians also made small jars and clothing using the core-formed method. Glass threads
were wound around a bag of sand tied to a rod. The glass was continually reheated to fuse the threads together. The glass-covered
sand bag was kept in motion until the required shape and thickness was achieved. The rod was allowed to cool, then finally the
bag was punctured and the rod removed. The Egyptians also created the first colored glass rods which they used to create colorful
beads and decorations. They also worked with cast glass, which was produced by pouring molten glass into a mold, much like
iron and the more modern crucible
steel.[20] By the 5th century BC this technology had spread to Greece and beyond. In the first century BC there were many
glass centres located around the Mediterranean. Around this time, at the eastern end of the Mediterranean, glass blowing, both free-blowing and
mould-blowing, was discovered.
Romans
Roman
Cage Cup from the 4th Century A.D.
During the Roman Empire craftsmen working as non-citizens developed many new techniques
for the creation of glass. Through conquest and trade, the use of glass objects and the techniques used for producing them were
spread as far as Scandinavia, the British Isles and China.[21] This spreading of technology resulted in glass artists congregating in areas such as
Alexandria in Egypt where the famous Portland Vase was
created, the Rhine Valley where Bohemian glass was
developed and to Byzantium where glass designs became very ornate and where processes such as
enamelling, staining and gilding were developed. At
this time many glass objects, such as seals, windows, pipes, and vases were manufactured. Window glass was commonly used during
the 1st century BC. Examples found in Karanis, Egypt
were translucent and very thick. After the fall of the Empire, the Emperor Constantine
moved to Byzantium where the use of glass continued, and spread to the Islamic world, the masters of glass-vessel making in the
later Middle Ages. However, in Europe, the use of glass declined and many techniques were forgotten. The production of glass did
not completely stop; it was used throughout the Anglo-Saxon period in Britain. But it
did not become common again in the West until its resurgence in the 7th century.
Islamic world
In the medieval Islamic world, the first clear, colourless, high-purity glass were
produced by Muslim chemists, architects
and engineers in the 9th century. One example is quartz glass, a colourless high-purity glass invented by Abbas Ibn
Firnas (810-887), who was the first to produce glass from stones such as
quartz.[22] The Arab poet al-Buhturi (820-897) described the clarity of such glass as follows:[23]
"Its colour hides the glass as if it is standing in it without a container."
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.[24]
The refracting 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).[25] 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.[26]
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.[27]
A 16th Century Stained Glass Window
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. Until the 12th century, stained glass, glass with metallic and other impurities for coloring, was not widely used.
The Crown glass process was used up to the mid-1800s. 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. Venetian glass was highly prized between the 10th and 14th centuries.
Murano glassmaking
-
The center for glass making 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: 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; see for a list
of coloring agents below. Those include gold for ruby-red colored glass, silver for a multitude of colors. Black glass was called
obsidianus after obsidian stone. A second method is apparently to produce a black glass
which, when held against the sun, will show the true color that this glass will give to another glass when used as a dye.
[28]
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 weren't allowed to leave the
Republic, however. Many craftsmen, however, took a risk and set up glass furnaces in surrounding cities and as far afield as
England and the Netherlands.
Renaissance glassmaking
Around 1688, a process for casting glass was developed, which
led to its becoming a much more commonly used material.
Industrial revolution glassmaking
The invention of the glass pressing machine in 1827 allowed the
mass production of inexpensive glass products.
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. This and other types of hand-blown sheet glass was replaced in the 20th century by
rolled plate glass, and then again in the 1960s by float
glass, at first in the UK and then elsewhere.
Glass art
-
Beginning in the late 20th century, glass started to become highly collectable as art. While earlier modern glass masters such
as Rene Lalique, Louis Comfort Tiffany,
Emile Gallee, Carlo Scarpa and Paul Venini were sought after for important glass collections, the scale and ambition of glass art
scaled new heights. Some important contemporary glass artists in glass include Dale
Chihuly, Lino Tagliapietra, William Morris, Stanislaw Libensky, Bertil Vallien,
Livio Seguso, Harvey Littleton, Dante Marioni, Dan Dailey, Sonja Blomdahl, Tom Patti, Stephen Rolfe Powell, Richard Marquis,
Therman Statom, Hiroshi Yamano, Ann Robinson, Paul Marioni, Nancy Callan to name just a few.
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 [29].
Several of the most common techniques for producing glass art include: blowing,
kiln-casting, fusing, slumping, pate-de-verre, 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 [30]. 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[31]. 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,
