There are many different types of glass with different chemical and
physical properties. Each can be made by a suitable adjustment to chemical
compositions, but the main types of glass are:
·
Borosilicate Glass
·
Commercial Glass
·
Glass Fibre
·
Lead Glass
Glasses may be devised to meet almost any imaginable requirement. For
many specialised applications in chemistry, pharmacy, the electrical and
electronics industries, optics, the construction and lighting industries,
glass, or the comparatively new family of materials known as glass ceramics,
may be the only practical material for the engineer to use.
Types of special glass include:
·
Alkali-barium Silicate Glass
·
Aluminosilicate Glass
·
Glass Ceramics
·
Optical Glass
·
Sealing Glass
·
Technical Glass
·
Vitreous Silica
Alkali-barium Silicate Glass
Without this type of glass, watching TV would be very dangerous. A
television produces X-rays that must be absorbed, otherwise they could in the
long run cause health problems. The X-rays are absorbed by glass with minimum
amounts of heavy oxides (lead, barium or strontium). Lead glass is commonly
used for the funnel and neck of the TV tube, while glass containing barium is
used for the screen.
Aluminosilicate Glass
A small, but important type of glass, aluminosilicate, contains 20%
aluminium oxide (alumina-Al2O3) often including calcium oxide, magnesium oxide
and boric oxide in relatively small amounts, but with only very small amounts
of soda or potash. It is able to withstand high temperatures and thermal shock
and is typically used in combustion tubes, gauge glasses for high-pressure
steam boilers, and in halogen-tungsten lamps capable of operating at
temperature as high as 750°C.
Borosilicate Glass
Most of us are more familiar with this type of glass in the form of
ovenware and other heat-resisting ware, better known under the trade name
Pyrex. Borosilicate glass, the third major group, is made mainly of silica (70-80%)
and boric oxide (7-13%) with smaller amounts of the alkalis (sodium and
potassium oxides) and aluminium oxide. This type of glass has a relatively low
alkali content and consequently has good chemical durability and thermal shock
resistance (it doesn't break when changing temperature quickly.)As a result it
is widely used in the chemical industry, for laboratory apparatus, for ampoules
and other pharmaceutical containers, for various high intensity lighting
applications and as glass fibres for textile and plastic reinforcement.
Commercial Glass
Most of the glass we see around us in our everyday lives in the form of
bottles and jars, flat glass for windows or for drinking glasses is known as
commercial glass or soda-lime glass, as soda ash is used in its manufacture.
The main constituent of practically all commercial glass is sand. Sand
by itself can be fused to produce glass but the temperature at which this can
be achieved is about 1700°C. Adding other minerals and chemicals to sand can
considerably reduce the melting temperature.
The addition of sodium carbonate (Na2CO3), known as soda ash, to produce
a mixture of 75% silica (SiO2) and 25% of sodium oxide (Na2O), will reduce the
temperature of fusion to about 800°C. However, a glass of this composition is
water-soluble and is known as water glass. In order to give the glass
stability, other chemicals like calcium oxide (CaO) and magnesium oxide (MgO)
are needed. These are obtained by adding limestone which results in a pure
inert glass.
Commercial glass is normally colourless, allowing it to freely transmit
light, which is what makes glass ideal for windows and many other uses.
Additional chemicals have to be added to produce different colours of glass
such as green, blue or brown glass.
Most commercial glasses have roughly similar chemical compositions of:
70% - 74% SiO2 (silica) 12% - 16% Na2O (sodium oxide) 5% - 11% CaO
(calcium oxide) 1% - 3% MgO (magnesium oxide) 1% - 3% Al2O3 (aluminium oxide)
Flat glass is similar in composition to container glass except that it
contains a higher proportion of magnesium oxide.
Within these limits the composition is varied to suit a particular
product and production method. The raw materials are carefully weighed and
thoroughly mixed, as consistency of composition is of utmost importance in
making glass.
Nowadays recycled glass from bottle banks or kerbside collections, known
as cullet, is used to make new glass. Using cullet has many environmental
benefits, it preserves the countryside by reducing quarrying, and because
cullet melts more easily, it saves energy and reduces emissions.
Almost any proportion of cullet can be added to the mix (known as
batch), provided it is in the right condition, and green glass made from batch
containing 85% to 90% of cullet is now common.
Although the recycled glass may come from manufacturers around the
world, it can be used by any glassmaker, as container glass compositions are
very similar. It is, however, important that glass colours are not mixed and
that the cullet is free from impurities, especially metals and ceramics.
Glass Ceramics
Some of these "Glass ceramics", formed typically from lithium
aluminosilicate glass, are extremely resistant to thermal shock and have found
several applications where this property is important, including cooker hobs,
cooking ware, windows for gas or coal fires, mirror substrates for astronomical
telescopes and missile nose cones.An essential feature of glass is that it does
not contain crystals. However, by deliberately stimulating crystal growth in
glass it is possible to produce a type of glass with a controlled amount of
crystallisation that can combine many of the best features of ceramics and
glass.
Glass Fibre
Glass fibre has many uses from roof insulation to medical equipment and
its composition varies depending on its application.For building insulation and
glass wool the type of glass used is normally soda lime. For textiles, an
alumino-borosilicate glass with very low sodium oxide content is preferred
because of its good chemical durability and high softening point. This is also
the type of glass fibre used in the reinforced plastics to make protective
helmets, boats, piping, car chassis, ropes, car exhausts and many other
items.In recent years, great progress has been made in making optical fibres
which can guide light and thus transmit images round corners. These fibres are
used in endoscopes for examination of internal human organs, changeable traffic
message signs now on motorways for speed restriction warnings and
communications technology without which telephones and the internet would not
be possible.
Lead Glass
Commonly known as lead crystal, lead glass is used to make a wide
variety of decorative glass objects.It is made by using lead oxide instead of
calcium oxide, and potassium oxide instead of all or most of the sodium oxide.
The traditional English full lead crystal contains at least 30% lead oxide
(PbO) but any glass containing at least 24% PbO can be described as lead
crystal. Glass containing less than 24% PbO, is known simply as crystal glass.
The lead is locked into the chemical structure of the glass so there is no risk
to human health.Lead glass has a high refractive index making it sparkle
brightly and a relatively soft surface so that it is easy to decorate by
grinding, cutting and engraving which highlights the crystal's brilliance
making it popular for glasses, decanters and other decorative objects.Glass
with even higher lead oxide contents (typically 65%) may be used as radiation
shielding because of the well-known ability of lead to absorb gamma rays and
other forms of harmful radiation.
Optical Glass
Optical glasses will be found in scientific instruments, microscopes,
fighter aircraft and most commonly in spectacles.
The most important properties are the refractive index and the
dispersion. The index is a measure of how much the glass bends light. The
dispersion is a measure of the way the glass splits white light into the
colours of the rainbow. Glass makers use the variations in these
characteristics to develop optical glasses.
Sealing Glass
A wide variety of glass compositions are used to seal metals for
electrical and electronic components. Here the available glasses may be grouped
according to their thermal expansion which must be matched with the thermal
expansions of the respective metals so that sealing is possible without
excessive strain being induced by differing levels of expansion.For sealing to
tungsten, in making incandescent and discharge lamps, borosilicate alkaline
earths-aluminous silicate glasses are suitable. Sodium borosilicate glasses may
be used for sealing to molybdenum and the iron-nickel-cobalt (Fernico) alloys
are frequently employed as a substitute, the amount of sodium oxide permissible
depending on the degree of electrical resistance required. With glasses
designed to seal to Kovar alloy, relatively high contents of boric oxide
(approximately 20%) are needed to keep the transformation temperature low and
usually the preferred alkali is potassium oxide so as to ensure high electrical
insulation.Where the requirement for electrical insulation is paramount, as in
many types of vacuum tube and for the encapsulation of diodes, a variety of
lead glasses (typical containing between 30% and 60% lead oxide) can be used.
Technical Glass
Technical is the term given to a range of glasses used in the
electronics industry.
Without borate glass the computer revolution would not have been
possible as it's vitally important in producing electrical components. This
type of glass, contains little or no silica and is used for soldering glass,
metals or ceramics as it melts at the relatively low temperature of 450-550°C,
well below that of normal glass, ceramics and many metals.
Glass of a slightly different composition is used for protecting silicon
semi-conductor components against chemical attack and mechanical damage. Known
as passivation glass it is vital in microelectronics technology and the
production of the silicon chips inside computers.
Another type of glass - Phosphate Glass - which is a semi conductor, is
used in the construction of secondary electron multipliers.
Chalcogenide glass - Similar semi conductor effects are also
characteristic of a type of glass that can be made without the presence of
oxygen. Some of them have potential use as infrared transmitting materials and
as switching devices in computer memories because their conductivity changes
abruptly when particular threshold voltage values are exceeded.
Vitreous Silica
Silica glass or vitreous silica is of considerable technical importance
as it has a very low thermal expansion. This difficult to make glass contains
tiny holes created using acids and is used for filtration. Porous glasses of
this kind are commonly known as Vycor.
