GLASS

Glass• Definition• Preparation• Composition• Variety• Uses

DefinitionGlass is an amorphous, hard, brittle, transparent or translucent super cooled liquid of infinite viscosity, having no definite melting point obtained by fusing a mixture of a number of metallic silicates or borates of Sodium, Potassium, Calcium, and Lead.

It possess no definite formula or crystalline structure.

• “An inorganic product of fusion which has cooled to a rigid condition without crystallizing”

• Does not have a specific melting point• Softens over a temperature range

Properties of glass

Glass is:

• Amorphous• Brittle• Transparent / Translucent• Good electrical insulator• Unaffected by air, water, acid or chemical

reagents except HF• No definite crystal structure means glass

has high Compressive strength• Can absorb, transmit and reflect light

Raw materials used in manufacturing glass

Raw Materials• Sodium as Na2Co3 (used in soft glass).

• Potassium as K2Co3 (used in Hard Glass).• Calcium as lime stone, chalk and lime.• Lead as litharge, red lead (flint glass).• Silica arc quartz, white sand and ignited

flint.• Zinc is zinc oxide (Heat and shock proof

glass).• Borates are borax, Boric acid (Heat and

shock proof glass).• Cullets or pieces of broken glass to increase

fusibility.

Glass Components• Formers – Network Formation

SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5

• Fluxes – Softeners

Na2O, K2O, LiO, Al2O3, B2O3, Cs2O

• Stabilizers – Provide Chemical Resistance

CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO

Manufacturing steps Melting

Forming and Shaping

Annealing

Finishing

Melting process

Raw materials in proper proportions are mixed with cullets. It is finely powdered and intimate mixture called batch is fused in furnace at high temperature of 1800°C this charge melts and fuses into a viscous fluid.

CaCO3 + SiO2 CaSiO3 + CO2

Na2CO3 + SiO2 Na2SiO3 + CO2

After removal of CO2 decolorizes like MnO2 are added to remove traces of ferrous compounds and Carbon. Heating is continued till clear molten mass is free from bubbles is obtained and it is then cooled to about 800°C.

Forming, shaping, and annealing• Forming and Shaping

The viscous mass obtained from melting is poured into moulds to get different types of articles of desired shape by either blowing or pressing between the rollers.

• Annealing

Glass articles are then allowed to cool gradually at room temperature by passing through different chambers with descending temperatures. This reduces the internal Strain in the glass.

finishing

Finishing is the last step in glass manufacturing. It involves following steps.

Cleaning

Grinding

Polishing

Cutting

Sand Blasting

Varieties of glass

Soda lime or soft glass

Potash lime or hard glass

Lead glass or flint glass

Borosilicate or Pyrex glass

Alumino- Silicate glass

96% Silica glass 99.5% Silica glass(Vitreosil)

Safety glass

Optical or Crook’s glass

Poly-crystalline glass

Toughened glass Colored glass

Wired Glass Glass Wool Fiber glass Photosensitiveglass

Photo-chromic glass

Neutral glass Laminated glass Insulating glass

Sodalime or soft glass• About 90% of all glass is soda-lime glass made with

silica (sand), Calcium carbonate and soda ash. • The approximate composition is Na2CO3.CaO.6SiO2. • They are low cost, resistant to water but not to

acids. • They can melt easily and hence can be hot worked.

• Uses: Window glass, Electric bulbs, Plate glass, Bottles, Jars, cheaper table wares, test tubes, reagent bottles etc

Potash lime or hard glass• Potash lime glass is made with silica (sand),

Calcium carbonate and potassium carbonate. • The approximate composition is K2CO3.CaO.6SiO2. • They posses high melting point, fuse with difficulty

and are less acted upon by acids, alkaline and other solvents than ordinary glass.

• Uses: These glasses are costlier than soda lime glass and are used for chemical apparatus, combustion tubes and glassware which are used for heating operations.

Lead glass or flint glass• It is made up of lead oxide fluxed with

silica and K2CO3 is used instead of sodium oxide.

• Its approximate composition is K2Co3.PbO.SiO2.

• To get dense optical glasses about 80% lead oxide is used. Lead glasses has a lower softening temperature than soda glass and higher refractive index and good electrical properties. It is bright lustrous and possess high specific gravity.

• Uses: High quality table wares, optical lenses, neon sign tubing, cathode ray tubes, electrical insulators, crystal art objects or cut glass, Windows and Shields for protection against X-rays and Gamma rays in medical and atomic energy fields etc.

Borosilicate / pyrex / jena glass• It is common hard glass

containing silica and boron with small amount of alumina and less alkaline solids.

• It contains SiO2(80.5%), B2O3(13%), Al2O3(03%), K2O(3%) and Na2O(0.5%). These glass have low thermal coefficient of expansion, and high chemical resistance i.e..shock proof.

• Uses: Industrially used for pipeline of corrosive liquids, gauge glasses, superior laboratory apparatus, kitchen wares, chemical plants, television tubes, electrical insulators etc.

Alumino-silicate glass• The typical approximate

composition of this type of glass is SiO2(55%), Al2O3(23%), MgO(09%), B2O3(07%), CaO(05%) and Na2O, K2O(01%).

• This type of glass possess exceptionally high softening temperature.

• Uses: It is used for high pressure mercury discharge tubes, chemical combustion tubes and certain domestic equipments.

96% silica glass• It contains 96% Silica, 03% B2O3

and traces of other materials. • It is translucent, the coefficient of

thermal expansion is very low hence it has high resistance to thermal shock, have high chemical resistance to corrosive agents and are corroded only by Hydrofluoric acid, hot phosphoric acids and concentrated alkaline solutions.

• Uses: Used only where high temperature resistance is required (800°C). They are used in construction of chemical plants, laboratory crucibles, induction furnace lining and electrical insulators.

99.5% silica glass / vitreosil• It contains pure silica heated

to its melting point. It is translucent, the coefficient of thermal expansion is very low hence it has high resistance to thermal shock, have high chemical resistance to corrosive agents.

• If Vitreosil glass is heated above its melting point, it becomes transparent and is known as clear silica glass.

• Uses: They are used in construction of chemical plants, laboratory crucibles, induction furnace lining, electrical insulators and heaters and have high light transmission properties.

Safety glass• It is made by fusing two to

three flat sheets of glass and in between them alternate thin layer of vinyl plastic is introduced. It is heated where both the layers merge together and glass is toughened.

• Uses: It is used as wind shield in automobiles and airplanes. On breaking it pieces does not fly apart because of the presence of the plastic layer in between the glass layers.

Optical or crook’s glass• It contains Phosphorus, PbCO3, silicates and

Cerium oxide which has the property to absorb harmful ultra-violet light. This glass is given through homogeneity by heating it for a prolonged period of time. These glasses have low melting point and are relatively soft.

• Uses: They are used for making optical lenses.

Poly-crystalline glass• It is new type of glass which is produced

by adding nucleating agents to a conventional glass batch and then shaped into desired form. It is then subjected to heating where nucleating agents forms large number of micro crystallites. It is not ductile. It exhibits high strength and considerable hardness.

• Uses: For making specialized articles.

Toughened glass• It is made by dipping articles

still hot in an oil bath, so that chilling takes place. This results in outer layer of articles shrink and acquire a state of compression while inner layer are in state of tension. Such glass is more elastic to mechanical and thermal shock. It breaks into a fine powder.

• Uses: For making window shields of fast moving vehicles, windows of furnace and automatic opening doors.

Coloured glass

Addition of transition metal compounds to glass gives color to the glass. They are outlined below.

Yellow: Ferric Salts Green: Ferrous and Chromium salts

Purple: Magnese dioxide salt

Red: Nickel and cuprous salts Cu2O

Lemon Yellow: Cadmium sulphide

Fluorescent greenish yellow: Uranium oxide

Blue: Cobalt Salts, CuO

Greenish Blue Color:Copper Sulphate

Brown: Iron

Opaque milky white: Cryolite of Calcium

phosphate

Ruby : Auric Chloride

Wired glass

• Wired glass does not fall apart into splinters when it breaks and is fire resistant. It is made by fusing wire in between the two glass layers.

Uses: For making fire resistant doors, roofs, skylights and windows

Fibre glass• It is transformed into a fine thread of filament and

has got a high tensile strength.

Uses: Found extensive use for the manufacture of fabric, reinforcing plastics and production of thermal insulation materials etc

Photosensitive glass• These are glasses by

which a colored picture may be developed by exposing the glass to black and white negative in ultra violet light. The appropriate proportions of potash-alumina glass, mixed with LiSO3, cerium and Silver salts have also been used as photosensitive glass.

• Uses: Photographic development

Glass wool• Glass wool consists of tiny

fibers formed by action of steam jets on dripping molten glass down from very fine hole.

• Uses: Heat Insulation, for filtration of Corrosive chemicals, sound insulation etc

Photo-chromic glass• The three dimensional silicate

network contains large no. of microscopic particles of silver halide which on exposure to light produce color.

• Uses: In making tinted car glasses and goggles.

Neutral glass• These glasses are highly resistant to chemical attacks

and they are specialized soda lime glass where alkali has been replaced by alumina, boron oxide and zinc oxide.

• Uses: Making Syringes, Injection Ampoules and vials etc.

Laminated glass• The sheets of glass fiber or glass wool are soaked in a

solution of thermosetting plastic like phenol formaldehyde resin and placed one above the other and then cured under heat or pressure. It is strong as steel. Non flammable and insulating. In bullet resistant glass vinyl resins are added in alternate layers.

• Uses: Shatter, shock and Bullet proof Glass

Insulating glass• Two or more plates of glass are filled with

dehydrated air and the edges are sealed air-tightly.

• Uses: Provides thermal insulating and so houses remain cool in summer and warm in winter.

Fracture / failure of glass• Glass does not have crystal lattice structure hence

it breaks. Fracture is caused by small imperfections, flaws and irregularity on the surface of the glass. Flaws are very fine cracks cause concentration of stress and the crack proceeds quickly causing a fracture.

THE END