Engineering Chemistry S Datta

426 ENGINEERING CHEMISTRY
This yellow colour is rectified by the pale violet colour due to manganese silicate and a
perfect white glass results.
Manufacturing procedure
The manufacturing procedure is divided into four major phases:
(i) Melting
(ii) Shaping
(iii) Annealing
(iv) Finishing
(i) Melting. Two types of furnaces are in use:
(a) Pot furnaces (b) Tank furnaces
Pot furnaces are employed principally in the manufacture of optical glass, art glass and
plate glass and in the small scale units. The pots are really crucibles made of selected clay,
high alumina fire clay such as mulite (3Al 2
O 3
. 2SiO 2
) or platinum. The material of the pot is
attacked by prolonged action of molten glass and hence has to be changed after a number of
operations except when platinum is used.
In a tank furnace, batch materials are charged into one end of a large ‘tank’ built of
refractory blocks. The tank measures 125′ × 30′ × 5′ with a capacity of 1400 tons. The glass
forms a pool in the hearth of the furnace across which the flames play alternately from one
side to the other.
The ‘formed’ glass is worked out of the opposite end of the tank. The operation is
continuous. The walls of tank furnace gradually wear away under the action hot glass.
The furnaces may be of either regenerative or recuperative design. In either of the cases
the heat from the hot gases is utilised in preheating the air, for combustion of the fuel. Once
the furnace has been heated, a temperature 1205°C is maintained all the time. The larger part
of the heat is lost by radiation from the furnace walls.
This sort of heat loss is necessary to reduce the action of the molten glass on the furnace
wall. Otherwise the furnace wall will corrode. Sometimes water cooling pipes are used to cool
the furnace wall.
(ii) Shaping or forming. The glass articles are shaped either manually or by machine.
In order to produce a sheet glass first a cylinder of approximate size is blown into shape. It is
allowed to stand vertically on a stand. The cylinder is cut open by running a sharp knife
lengthwise. At present every stage of shaping has been mechanised. The blowing is done by
compressed air.
(iii) Annealing. To reduce strains, it is necessary to anneal all glass objects. Annealing
involves two operations:
(i) Holding a mass of glass above a certain critical temperature, long enough to reduce
internal strain by plastic flow.
(ii) Cooling the mass to room temperature slowly.
The annealing oven is nothing more than a carefully designed heated chamber in which
the rate of cooling can be controlled.
(iv) Finishing. All types of glass articles must undergo certain finishing operations.
These include cleaning, grinding, polishing, cutting, enameling etc. Although all these
are not required for every glass object, one or more is almost always necessary.