Glass Industry

Introduction:

Glass has been manufactured in New Zealand for over one hundred years, and is a common part
of our daily lives. It is used commonly in windows, bottles, jars and domestic glassware, as well
as as a material for artwork, such as stained glass. It is manufactured from cheap and abundant
raw materials, and is readily recycled.

When silica or quartz is heated up to 16500C it melts to a colourless liquid which on cooling
gives glass. This fused mass is highly sensitive to temperature change therefore it requires
special heat treatment so that ordinary glass can be manufacture which is much stable to
temperature change. The glass of various commercial qualities is prepared by heating sand or
quartz along with metal oxide or carbonates.

Glass has two important properties.

Firstly it does not have a definite melting point but softens gradually over a range of
temperatures.

Secondly it does not cleave in a plane face like diamond or table salt.

The explanation of both these properties lies in the fact that glass has no ordered structure, but is
instead a super cooled liquid. A sheet of glass left to stand for a long time, perhaps one hundred
years, will actually flow and change its dimensions slightly.

Raw materials:

Glass consists mainly of silica, obtained from the pure sands of the Parengarenga Harbour, just
north of North Cape. This sand is washed and sifted to remove shells, stones and exceptionally
large grains of sand, before it is mixed with other materials which control the colour and other
properties, and lower the 1730oC melting point of pure silica. Between 10 and 80% of the
finished product is from recycled glass (known as cullet) which is collected both in kerb-side
collections and in the approximately 1000 yellow recycling bins established by ACI New
Zealand Glass Manufacturers throughout the country.

The main raw material used to make glass is sand. To make clear glass, a special sand called
silica sand is used.

This fine white sand is needed because it is very pure and does not contain other unwanted
chemicals. Glass production also needs limestone, soda ash and other chemicals to colour the
glass. The production of glass uses energy, both during the extraction of the sand as well as
during transportation and processing. Large amounts of fossil fuels (coal, oil and gas) are used
during these stages, which in turn produce the greenhouse gas, carbon dioxide.

The main sources of the silica sand needed to make clear glass are in Australia and South East
Asia in countries like Indonesia, Malaysia and Vietnam. Silica sand also comes from other
countries including China, India and Egypt. Some sand is from Britain and Ireland and most of
the glass factories in the U.K. use this sand.

There are different types of glass and each uses a slightly different mix of raw materials. The
sand, limestone and other ingredients are carefully weighted and melted together in a furnace at
around 1500º C (water boils at 100º C). The molten glass is then formed into the final product,
for example sheets for windows or shaped into bottles and jars.

The Manufacturing Process:

Glass is made from its raw materials in a carefully controlled two-step process, and is then
molded to form either sheet glass or bottles. A schematic diagram of this process is given in
Figure 1.

Step 1 – Batch mixing:

 Glass is made of different ingredients in differing proportions depending on the desired end
product, but most glass (except for some specialist glass) consists of all the "majors" mixed with
small quantities of some of the minors. Thus the minors are weighed first in a special weighing
hopper, and added to the majors with a little water. Water is necessary as in a very dry mix the
fines can blow off the batch as it enters the furnace and clog up the furnace flues. The two tonne
batch is then mixed for between two and three minutes in a rotary mixer, before being
transported to a batch hopper, from which it is slowly fed into a furnace.

The mix of raw materials is dependent on the type of glass desired. Window glass is made from
72% SiO2, 13% Na2CO3 and 12% CaCO3, while bottle glass has more SiO2 and less CaCO3.
Crystal is made from 45% SiO2 and 44% PbO with 9% K2CO3, and pyrex (used for laboratory
equipment and ovenwear because of its heat resistance) from 80% SiO2 and 12% B2O3. The
remainder in each of these mixtures is made up of the various minors.

Glass colour The choice of minors at this point determines the colour of the final product.
Colour results from two factors: the oxidation state of the glass, and the specific colourant
additives used. Glass oxidation is promoted by the addition of carbon, and the degree of
oxidation is measured on an arbitrary scale known as the carbon number. Clear glass has a
carbon number of zero, dark green glass is -28 and amber is -52.

Other variations of colour are achieved through the action of coloured materials that act as dyes.
For example, the iron (II) ions naturally present in sand results in the green tinge seen in clear
glass, and this can be masked by the addition of selenium. Moreover, the amber and green
colours of glass bottles are caused not only by the degree of oxidation, but also by the addition of
iron chromite and an ironsand / saltcake mix respectively. As glass is fed continuously into the
furnaces, each furnace has to be dedicated to producing glass of a particular recipe, and it takes
12-48 hours and a number of steps to alter the mix to change to producing a different type of
glass of an acceptable standard.

Step 2 - Batch melting:

The ingredients mixture is fed continuously into a furnace fired by natural gas, boosted by
electricity when necessary. The glass is initially heated to 1400oC, then raised to 1540oC, at
which temperature the mixture melts. The glass is then held above 1400oC while it is refined
and CO2 and SO3 are evolved. When no more gases are evolved the liquid is ready to be formed
into the desired endproduct. The furnaces are kept at these precise temperatures by a cross-fired
system which reduces heat loss and promotes a more even heat distribution in the molten glass.
It functions as follows:

1. Preheated air (which has been heated by passing up the regenerator packing and so cooling the
packing) is blown into the furnace by large fans

2. The air mixes with natural gas and combusts

3. The flow of air pushes the flue gasses across the furnace, over the glass and down the opposite
regenerator, heating up its packing.
4. After half an hour, the opposite regenerator is used and the cycle reverses.

Shaping plate glass:

The cooled, molten glass from the furnace flowed into an extension of the tank known as the
drawing canal, where it cooled to 1000oC before being drawn up into a tower, the drawing
tower, by dipping an iron grille into the glass, onto which the glass stuck. The 2.5 metre wide
sheet of glass was drawn up into the tower by asbestos rollers, cooling as it rose. Plate glass can
be made as thin as 2mm, with this thickness determined by the speed of its progress up the
drawing tower - 2mm thick glass moves up at approximately 170 metres an hour, while the
average is about 40 metres per hour.

By the time the glass reaches the top of the tower it is ten metres above the molten glass, and
only 280oC. On the top floor of the factory the glass is monitored to ensure its constant
thickness, and then scored and snapped off by the break-off machine. The individual sheets
weigh 22kg, and are lifted by rubber suction pads and placed on a conveyor belt where they are
cooled and have their rough edges snapped off, before being transported to the warehouse for
storage.

Molding glass containers:

Molten glass is removed from the furnace through forehearths (heated channels) where the glass
is cooled to between 1100 and 1150oC, the exact temperature varying depending on the product
to be formed. It is then fed into a forming machine where shears cut off weighed "gobs" of red-
hot glass, one two or three at a time as required. These are molded in "sections" within the
machine, held in the air for a short time to cool (to prevent them from losing their shape
immediately) and transported to the annealing lehrs.

The annealing lehrs are a further stage in the cooling process, where the bottles are reheated to
600oC and then slowly cooled to remove stress points and prevent the glass from becoming
brittle. Finally the bottles are coated with a shiny, slippery spray-on coating that temporarily
protects them from becoming scratched, and they are packed for delilvery to clients.

Types of Glass:
1. Soda-lime or soft glasses:

The raw materials are silica (sand), calcium carbonate and soda ash. Their approximate
composition is Na2O.CaO.6SiO2. About 90% of all glasses produced belong to soda lime glass.
The low cost, low melting point soda-lime glass has resistant to de-vitrification and to water.
However, they have poor resistance to common reagents like acids.

Uses:

They are used as window glass, electric bulbs, plate glass, bottles, jars, building blocks and
cheaper tablewares, where high temperature resistance and chemical stability are required.

2. Potash-lime or hard glasses:

Silica (sand), calcium carbonate and potassium carbonate are the basic raw material for potash
lime glass. Their approximate composition is K2O.CaO.6SiO2. They possess high melting point,
fuse with difficulty and have good resistance to acids, alkalis and other solvents compare to
ordinary glasses.

Uses:

These glasses are costlier than soda-lime glasses and are used for chemical apparatus,
combustion tubes, etc., which are to be used for heating operations.

3. Lead glass or Flint glass:

Instead of calcium oxide, lead oxide is fused with silica. As much as 80% of lead oxide is
incorporated for dense optical glasses. In addition K2O is used instead of sodium oxide. So,its
approximate composition is K2O.PbO.6SiO2. Lead glass is more expensive than ordinary lime-
soda glass, but it is much easier to shape and to work with. Lead glass has a lower softening
temperature and higher refractive index than soda glass. It has excellent electrical properties. It is
bright, lustrous and possesses high specific gravity (3 to 3.3).

Uses:
Lead glasses are used for high quantity table wares, optical lenses, neon sign tubing, cathode ray
tubes, electrical insulators and in the art objects because of their high luster. High lead content
glasses are used for extra dense optical glasses, for windows and shields to protect personnel
from X-rays and gamma rays in medical and atomic energy fields respectively.

4. Borosilicate glass or Pyrex glass or Jena glass:

It is the most common of the hard glasses of commerce which contain virtually only silica and
borax with a small amount of alumina and still less alkaline oxides. Borosilicate glass has the
following composition.

Component: SiO2 B2O3 Al2O3 K2O Na2O

Percentage: 80.5 13 3 3 0.5

Boron and aluminium oxides substitutes Na2O and CaO used the lime-soda glasses which results
in a glass of low thermal coefficient of expansion, and high chemical resistance. Borosilicate
glasses have a very much higher softening point and excellent resistivity to shock.

Uses:

They are used in pipelines for corrosive liquids, gauge glasses, superior laboratory apparatus,
kitchenwares, chemical plants, television tubes, electrical insulators etc.

5. 96% Silica glass:

It is produced and shaped as typical borosilicate glass, having dimensions bigger than desired.
The heat treatment to the article, separate the glass into two layers, one consisting mainly of
silica and the other of the alkali oxides and borates. Then article is dipped in hot acid which
dissolves away the alkali oxides and boron oxide layer, leaving behind a fine porous structure
consisting of about 96% silica, 3% B2O3 and traces of other materials. This glass is then washed
carefully and annealed to 12000C. The shrinkage of about 14% takes place and hard firm shape
is produced which is almost gaslight. The translucent 96% glass, if it is so desired heated to a
higher temperature and made almost transparent or clear.
It is expensive than other types of glasses. The expansion coefficient is very low which accounts
for its high resistance to thermal shot. The softening temperature is about 15000C. They possess
high chemical resistance to most corrosive agents. They are corroded by only HF, hot H3PO4
and concentrated alkaline solutions.

Uses:

They are used where high temperature resistance is required and articles can be safely used at
temperature up to 8000C. They are used for the constructed chemical plants, laboratory
crucibles, induction furnace linings, electrical insulators.

6. 99.5% silica glass or Vitreosil:

It is produced by heating SiO2 to its melting point (1,7500C). Because of absence of fluxing
agents, it is extremely difficult to get rid of the bubbles. Shaping of the glass is difficult due to
high viscosity at its working temperature. The final product is translucent. It has high softening
temperature about 16500C, compare to 96% silica glass. Its thermal expansion is very low. Due
to their opaque nature, they tend to be mistaken for pipe when dirty and are, therefore, often
broken accidentally.

If vitreosil glass is heated for long periods above its melting point, it finally becomes transparent
and is then known as ―clear silica glass‖. It has considerable transmission properties e.g. 1mm
of this material allows no less than 93% of light to pass corresponding figure for good optical
glass is only 6%.

Uses:

uses are similar to 96% silica glass. It is exposed for the construction of pipelines for hot
concentrated acid. Clear silica glass is used mainly for plant ware, chemical laboratory wares,
electrical insulating materials, and in electrical heat furnaces.

7. Alumino-silicate glass:

It possess exceptionally high softening temperature and having the typical constituent as follow

Component: SiO2 Al2O3 B2O3 MgO CaO Na2O & K2O

 Percentage: 55 23 7 9 5 1

Uses:

It is used for high pressure mercury discharge tubes chemical combustion tube, certain domestic
equipment etc.

8. Safety glass:

Thin layer of vinyl plastic is introduced between two or three flat sheets of glass and the whole
is subjected to slight pressure. It is then heated till the glass layers and plastic layers merge into
one another to give a sandwich. On cooling the glass becomes quite tough. When such a glass
breaks it does not fly into pieces as the inner plastic layer tends to hold back the broken pieces of
the glass.

Uses:

It is mostly used in automobile and aero plane industries as wind shield.

9. Optical or Crookes glasses:

They contain phosphorus and lead silicate, together with a little cerium oxide, is capable of
absorbing harmful UV light. Very careful manufacturing process of heating the molten mass for
prolonged time secured the homogeneity of the glass. In general optical glasses have low melting
points and are relatively soft. Their chemical resistant and durability are appreciably lower than
those of ordinary glasses.

Uses:

Used for manufacture of lenses.

10. Polycrystalline glass or Pyroceram:

It is the most recent development of producing glass by adding one or more nucleating agents to
a special or convectional glass batch. Then it is shaped into desired form and subjected to
controlled heat treatment.
The nucleating agents induced the formation of a large number of submicroscopic crystalline
which act as centers for further crystal growth. Crystalline glass is not ductile, but it has much
greater impact strength than ordinary glass. It exhibits high strength and considerable hardness
and can be formed and shaped into articles by any methods of manufacturing.

11. Toughened glass

It is made by dipping articles still hot in an oil bath, so that certain chilling takes place. There so,
outer layers of the articles shrink and acquire a state of compression; while the inner layers are in
a state of tension. Such a glass is more elastic and capable of withstanding mechanical and
thermal shocks. When such a glass breaks, it does not fly but is reduced to fine powder.

Uses:

It is used for window shields of fast moving vehicles like cars, trucks, aeroplane; window shields
of furnaces, automatic opening doors and large show cases.

12. Insulating glass :

It is a transparent unit prepared by using two or more plates of glass separated by 6-13 mm thick
gap, field up with dehydrated air and then thematically sealing around the edges. This provides a
high insulation against heat. Thus, if such a glass is used for separating apartments, it does not
transmit heat and consequently the apartments will remain cool during summer and warm during
winter.

Uses:

It is used as thermal insulating materials

13. Wired glass:

It is formed by embedding a wire mesh at the center of the glass sheet during casting due to this
when glass breaks it do not fall into splinters. Additionally, it is more fire resistant than ordinary
glass.

Uses:
It is used mainly for making fire-resisting doors, windows, skylights, roofs 14. Laminated glass
It is made by pressing or bonding together two or more sheets /plates of glass with one or more
alternating layer of bonding material like plastic resin, asphalt or synthetic rubber.

The essential qualities of the laminated glass are

 It is shatter-proof, i.e. its pieces do not fly off when suddenly broken.

 It is shock-proof, i.e. it can with stand sudden changes of temperature and pressure without
breaking.

A bullet-resistant laminated glass is manufactured by pressing together several layers of glass

with vinyl resins in alternate layers. Ordinary, thickness of such glass varies from 12.7 mm - 76.5
mm. Even thicker types are made for specific uses.

Uses:

As safety glass in aircrafts, automobiles, helicopters, submarines. Bullet resistant lamination

glass finds application in making automobile wind screens, looking windows etc.

15. Glass wool:

It is a fibrous wool-like material composed of intermingled fine threads or filaments of glass

which is completely free from alkali. Glass filaments are obtained by forcing molten mass of
glass through small orifice of average diameter of 0.005 - 0.007mm continuously which is sent to
rapidly revolving drum resulting in wool like form. It has low electrical conductivity and eight
times higher tensile strength than steel. It does not absorb moisture and it is completely heat
proof.

Uses:

It is employed for heat insulation purpose, e.g., insulation of metal pipe lines, motors, vacuum
cleaners, walls and roofs of houses. Being resistant to chemicals, glass wool is used for filtration
of corrosive liquids like acids and acidic solution. It is used for electrical and sound insulation. It
is also employed in air filter as dust filtering material. It is also used for manufacturing fiber-
glass, by blending with plastic resins.
Photosensitive Glass:

It is UV sensitive high alumina soda lime glass. The positive in UV region on glass is developed
by thermal treatment only at 540-550°C. The desired photo activity in UV region can be obtained
by admixture of high alumina soda lime glass with small amounts of Cu2O NaCN.SnO2 and
abeitic acid in appropriate amounts. A blue colour is promoted by NaCN absence of tin oxide. In
presence of tin oxide an impression in red is observed. By manipulation the ingredients in glass,
brown and yellow images can also be possible. A potash alumina glass mixed with LiSiO3,
cerium and silver, salts in appropriate proportions have also been used as photosensitiveglass.

17. Photochromic glass:

Large number of microscopic particles of silver halides trapped in the three dimensional silicate
networks in fixed concentration. On exposure light, temporary colour centers consisting of silver
particles only are produced and these add quickly producing total darkness. The intensity of
darkness depends upon the concentration of silver. Because reversible darkening is controlled by
the radiations in the UV region quite abundant in day light, the photo blackening does not occurs
markedly in the lamp light night.

18. Fiber glass:

Fiber glass is nothing but molten glass process mechanically to a flexible thread of filament. A
hot platinum nozzle filled with molten glass forces out the fluid in the form of a thin continuous
thread which when caught by a rapidly moving disc gets converted into fiber through elongation
and twist given by the disc fabrics made of glass are bad conductors of heat and electricity and
are noninflammable. Hence articles made of fiber glass are fire proof.

Uses:

Such type of glass is used in textiles and reinforcing and can be spun into yarn, gathered into a
mat, and made into insulation and a great variety of other products may be with it.