Stages of Making Glass

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 Writed Date : 03/16/2014   ( Last Edit : 3/16/2014 )

Stages of Making Glass
Properties of Glass
In fact the glasses are evaluated by three properties:
-The Chemical Properties:
Glasses are resistant against corrosive agents such as acids and alkalis except hydrofluoric acid, so laboratory dishes are made with glass.
-Physical Properties:
A part of the light that shines on the glass is absorbed, a part of it is reflected and a part of that passes. The amount of light which passes through the glass is related to the thickness of the glass, the surface smoothness and the type of coverage; for example glasses with the thickness of 3mm transfer 91% of the light meanwhile glasses with the thickness of 25mm transfer just 78%  of the light.
-Mechanical Properties:
Surface defects have a huge impact on the mechanical properties of the glass. These defects produce stress concentration and consequently they break the glass. In addition, they decrease the real resistance to one hundredth of the theoretical resistance.
For preventing the glass to crack and raising its surface resistance its surface should be polished and cleaned of any defect.
 Glass Materials:
So far about 6500 types of combinations have been introduced for making glass and their properties have been registered such as Sio2, Cao, Nepheline Feldspar Syannyt, Aluminosilicates, na2o, Borates (Boron Compounds).
For producing glass we need a very huge amount of glass sand, Sodium Carbonate, impure Sodium Sulfate and so on.
Melting:
There are two kinds of furnaces in glass-making ;crucible and reservoir furnaces. The crucible furnaces with the average capacity of 2 tons or less are generally applicable for producing specific glasses or when preserving the melt module against the combustion products is obligatory. The crucible furnaces are made of clay or platinum. In reservoir furnaces the module materials enter from the big gate of the furnace which is made of fireproof blocks.
Crucible Furnace:
As it is clear from its name crucible is used for melting. Heat transfer normally goes through conduction to the material inside the crucible. Heat is transferred in three ways:1- conduction 2-moving 3-radiation
The crucible materials are as follows: pure iron crucibles, steel, cast iron, chamot, graphite, silicon carbide. The other pure iron crucibles are specially used for melting the irons that their melting point is less than iron (about 1536-1539 degree centigrade).We
have to use crucible for melting magnesium because of the compounding tension of magnesium with oxygen in the furnaces built of refractory bricks. In fact it draws the refractory bricks oxygen and makes them porous.
The temperature of the newly built furnace is gradually increased depending on the capability of the refractory bricks for tolerating expansion. After heating the recovery furnace, the pleasant temperature which is at least about 1200 degree centigrade is kept all the time. A great part of heat is wasted in the furnace because of the radiation and in fact a very little part of the heat is used for melting.
Anyway the temperature of the furnace walls may so increase that the melting glass may solve or decay them unless the walls be cooled while radiating. For diminishing the action of the melting glass inside the furnace walls the cooling water pipes are usually placed.
Forming:
Glass is generally formed by manual or mechanical molding. The most important factor which should be concerned in mechanical molding is that the favorite article need to be formed during some seconds. During this relatively short time the glass is turned to a clear solid from a viscosity liquid. We could easily understand that solving designing problems such as heat currency, metal resistance and bearings loosing is so complicated and success of such machines helps engineers .Window glass, float glass, safety glass, blown glass and so on are formed with machine.
Annealing:
In all of the glass goods whether molded by machine or hand necessarily they should be put under annealing operations to reduce strain. In summary annealing operations are made of two parts:
First, keeping a mass of glass above a specific critical point until the amount of inner strain will be under the maximum predetermined amount while making a plastic flow.
Second,  gradual cooling of this mass of glass up to room temperature in a way that the amount of strain be under that maximum point.
 Taabdan or annealing device is a hot and precisely designed encasement in which the cooling rate is so controlled that the determined conditions be observed. Making a quantitative relation between tension and repetitive break caused by tension has made the glass producing experts capable of planning such a kind of glass that can tolerate specific conditions of mechanical and thermal tensions.
By applying such information engineers have found some principles for producing integrated equipments. These devices are equipped with automatic temperature regulator and controlled circulation so that they make annealing operations easier with low fuel consumption and less wastes.