GLASS

AKHIL KRISHNAN G

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 pointSoftens over a temperature range

PROPERTIES OF GLASSGlass is:

AmorphousBrittleTransparent / TranslucentGood electrical insulatorUnaffected by air, water, acid or chemical reagents No definite crystal structure means glass has high Compressive strengthCan absorb, transmit and reflect light

RAW MATERIALS USED IN MANUFACTURING GLASS

Raw MaterialsSodium 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 PRODUCTION AND PROCESSINGMAJOR STEPS1.Melting and refining.2.Forming and shaping.3.Heat treatment4.Finishing

1.MELTING AND REFININGThe mixed batch of raw materials, along with

broken or reclaimed glass, is fed into one end of a continuous –type furnace where it melts and remains molten at around 1500 deg.

Molten glass is drawn continuously from the furnace and runs in troughs to the working area, where it is drawn off for fabrication at a temperature of about 1000 deg.

2.FORMING METHODSPRESSINGlow cost fabrication method.Used to manufacture table- and ovenware,

insulators, lenses, and reflectors.Gobs of glass are fed into molds on a rotating

press.The molds are moved beneath a plunger

which forces the glass into final shape.

GLASS BLOWINGUsed to produce hollow products such as

bottles, jars, and light bulbs.Molten glass in ribbon form sags through

holes as air is blown in from above.At the same time, moulds move up from

below and clamp around the molten glass.More puffs of air forces the glass into the

mould to form final shape.

3.HEAT-TREATMENT OF GLASSANNEALINGWhen glass cools from the forming range to

room temperature , thermal stresses develop that adversely affect strength properties.

Annealing is done to eliminate this stress.Involves heating the glass to annealing range,

holding it there for a period of time, and then cooling it slowly to room temperature.

TEMPERINGInvolves heating glass to around the

softening point and then cooling it rapidly with blasts of air or by quenching it in oil.

Heat tempered glass is 3 to 5 times stronger than annealed glass while still retaining its initial clarity, hardness, and expansion co-efficient.

4.FINISHING GLASS SURFACEHydrofluoric acid is used for polishing and

etching.Also be stained by copper and silver

compounds.Ferric or cerium oxides are used to produce

smooth, accurately finished surfaces.

VARIETIES OF 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 GLASSPotash 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.

SAFETY GLASSIt 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 GLASSIt 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.

TOUGHENED GLASSIt 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.

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

PHOTOSENSITIVE GLASSThese 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

RECENT DEVELOPMENTSBioactive glasses : A group of surface

reactive glass-ceramic biomaterials and include the original bioactive glass, Bioglass. The biocompatibility of these glasses has led them to be investigated extensively for use as implant materials in the human body to repair and replace diseased or damaged bone.