Manufacturin
g Process
The procedure adopted in the manufacture of
glass may broadly be divided into the following
five stages:
(1) Collection of Raw Materials
(2) Preparation of Batch
(3) Melting in Furnace
(4) Fabrication
(5) Annealing
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
Glass Manufacturing Process # 1.
Collection of Raw Materials:
In glass manufacturing process, the transparent hard
body is created by the application of vast amounts of
heat to sand or quartz. Glass is an amorphous
inorganic, homogeneous transparent or translucent
material which can be molded into any shape.
Based on the type of glass, its raw materials are
collected and different compositions are sent to the
manufacturing process.
Following are the raw materials for different
types of glass.
In addition to the raw materials, the cullet
and decolourisers are also added for each
type of glass.
The cullet indicates waste glass or pieces of
broken glass. They increase the fusibility of
glass and prevent loss of alkali by volatisation
during the reaction in forming new glass. They
also reduce the cost
Glass Manufacturing Process # 2.
Preparation of Batch:
The raw materials, cullet and decolouriser are
finely powdered in grinding machines. These
materials are accurately weighed in correct
proportions before they are mixed together. The
mixing of these materials is carried out in mixing
machines until a uniform mixture is obtained. Such
a uniform mixture is known as the batch or frit and
it is taken for further process of melting in a
furnace.
Glass Manufacturing Process # 3.
Melting in Furnace:
The batch is melted either in a pot
furnace or in a tank furnace. The heating
is continued until the evolution of carbon
dioxide, oxygen, sulphur dioxide and other
gases stops.
Pot Furnace
A pot is a vessel made of fireclay. This process resembles
the crucible steel process.
These pots are placed in
specially prepared holes in the
furnace. The charging and
collecting doors are kept
projecting outside so that raw
materials may be added and
molten glass may be taken out
conveniently.
The pots are filled with raw
materials. The furnace is
heated by means of producer
gas. When the mass has
melted down, it is removed
from the pot and it is taken
for the next operation of
fabrication. The melting of
glass by pot furnace is an
intermittent process. It is
used to melt small quantities
of glass at a time or to
prepare special types of glass.
Tank Furnace
This furnace resembles the
reverberatory furnace adopted for
puddling of the wrought-iron. It
shows the section of a tank furnace
adopted for the melting of glass. It is
constructed with reinforced
masonry. The roof is given special
shape to deflect the flames of
heated gas.
The ports are provided for the
entry of preheated producer gas.
The doors are provided for
charging and for taking out the
molten glass. A bridge separates
the tank into two unequal
compartments.
The batch is heated in large
compartment and it contains
somewhat impure glass. It flows
through opening of bridge into
small compartment. The gall or
floating impurities are collected
at the top of large compartment.
The refractory lining is provided
to the interior surface of tank.
The tank is filled with raw
materials. The furnace is heated
by allowing producer gas through
ports. The charging of raw
materials and taking out of
molten mass are simultaneous.
This is a continuous process and
it is adopted to melt large
quantities of glass at a time.
Glass Manufacturing Process #
4. Fabrication:
The molten glass is given suitable shape or form in this
stage. It can either be done by hand or by machine. The
hand fabrication is adopted for small scale production and
machine fabrication is adopted for large scale production.
Following are the different ways of fabrication:
(i) Blowing
(ii) Casting
(iii) Drawing
(iv) Pressing
(v) Rolling
(vi) Spinning
(i) Blowing:
For this purpose, a blow-pipe is used. Its diameter is
about 12 mm and its length is about 1.80 m. One
end of the blow-pipe is dipped in the molten mass of
glass and a lump of about 50 N weight is taken out.
This lump of glass will then lengthen to some extent
by its own weight.
The operator then blows vigorously from other end
of blow pipe. It can also be done with the help of an
air compressor. This blowing causes the molten mass
to assume the shape of a cylinder. It is then heated
for few seconds and is blown again.
(ii) Casting:
The molten glass is poured in moulds and it is
allowed to cool down slowly. The large pieces
of glass of simple design can be prepared by
this method. It is also adopted to prepare
mirrors, lenses, etc.
(iii) Drawing:
This process consists in simply pulling the molten
glass either by hand or by mechanical
equipment. An iron bar is dipped sideways in the
molten mass of glass. It is lifted up horizontally
and in doing so, it catches up a sheet of molten
glass. This sheet is then allowed to pass over a
large rotating roller. The roller helps the molten
glass to spread in the form of a thin sheet.
(iv) Pressing:
In this process, the molten glass is pressed
into moulds. The pressure may either be
applied by hand or by mechanical means. This
process is adopted for ornamental articles,
hollow glass articles etc.
(v) Rolling:
There are two methods of rolling. In one
method, the molten mass of glass is
passed between heavy iron rollers and flat
glass plate of uniform thickness is
obtained. In another method, the molten
mass of glass is poured on a flat iron
casting table and it is then turned flat with
the aid of a heavy iron roller.
(vi) Spinning:
In this process, the molten glass is spun at
high speed by a machine to form very fine
glass fibres. This glass has tensile strength
equal to that of mild steel. It does not fade,
decay or shrink. It is not attacked by acids,
fire and vermins. It is very soft and flexible.
It is used for providing insulation against
heat, electricity and sound.
Glass Manufacturing Process # 5.
Annealing:
The annealing of glass is a very important process. If
glass articles are allowed to cool down rapidly, the
superficial layer of glass cools down first as glass is a bad
conductor of heat. The interior portion remains
comparatively hot and it is therefore in a state of strain.
Hence, such glass articles break to pieces under very
slight shocks or disturbances.
Following are the two methods of annealing:
(i) Flue treatment
(ii) Oven treatment
(i) Flue Treatment:
In this method, a long flue is provided and it
is constructed in such a way that there is
gradual decrease in temperature from one
end of flue to the other. The red-hot articles
of glass are allowed to enter at the hot end
of flue and they are slowly moved on
travelling bands. They become cool when
they reach the cool end of flue. This method
is useful for large scale production.
(ii) Oven Treatment:
In this method, the red-hot glass articles are
placed in ovens in which arrangement is
made to control the temperature. After
articles are placed in the ovens, the
temperature is slowly brought down. This
method is useful for small scale production.