Glass History and manufacture

Question 1

History of Silicate glass

Answer 1

• Volcanic glass obsidian -used for the production of sharp cutting tools and arrowheads
• First true glass was made in coastal north Syria or ancient Egypt ~ 3500BC
• Earliest known glass objects, were beads
• BY 15TH Century BC
• Glass production in western Asia Crete and Egypt
• fusing of glass from raw materials was a closely guarded secret
• Glass workers in other areas relied on imports of pre-formed glass ingots
• The first glassmaking “manual” dates back to ca. 650 BC.

Question 2

ancient glass production

Answer 2

Made by melting together sand (silicon dioxide), an alkali oxide (which lowers the melting temperature); and lime, which stabilizes the mixture and makes it less soluble in water.
 alkali of Syrian and Egyptian glass was soda ash, sodium carbonate, (extracted from ashes of plants)
‘core-formed’, ductile rope of glass wound round a shaped core of sand and clay then heat fusing glass ropes
Replaced by cast method by 1st century AD

Question 3

hellenistic period many new techniques of glass production

Answer 3

• canes of multi-coloured glass are sliced and the slices arranged together and fused in a mould to create a mosaic-like effect

Question 4

blown glass

Answer 4

 glassblowing in the late first century B.C
 enabled simple vessel to be made in a few minutes.
 No longer a luxury item for the rich alone
 same blowing techniques still used today.
Blue perfume and green oil bottles
Weathering e.g. Shifts in temperature
and moisture are especially harmful to glass,
e.g. a white, chalky layer or rainbowlike colours, (iridescence),
coating the surfaces

Question 5

Roman Empire

Answer 5

used pure white silica sand

• clear glass made by glass blowers in Alexandria circa 100 AD,
• Romans began to use glass for architectural purposes.
• Cast glass windows, with poor optical qualities, began to appear in the most important buildings

Question 6

Middle Ages

Answer 6

• vessels, windows, beads, jewellery
• From the 10th-century onwards, glass was employed in stained glass windows of churches
  • The centre for luxury Italian glassmaking from the 14th century was the island of Murano
  • Bohemian glass, is a decorative glass produced in regions of Bohemia

Question 7

crown glass

Answer 7

 One of the earliest methods of glass window manufacture
 Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool.
 Centrifugal force shaped the hot globe of glass into a round, flat sheet.
 very expensive

Question 8

important advance in glass manufacture

Answer 8

 Addition of lead oxide to the molten glass;
 improved glass appearance and was easier to melt
 easier to manipulate.
 discovered by George Ravenscroft in 1674

Question 9

blown plate glass

Answer 9

• developed in 1620
• used for mirrors and coach plates
• The polishing process was industrialized around 1800 with the adoption of a steam engine to carry out the grinding and polishing of the cast glass.

Question 10

modern industrial processes

Answer 10

• 1832 the British crown glass company developed cylinder method to produce sheet glass

Question 11

fourcault process

Answer 11

 Manufacture of flat glass.
 Developed in Belgium by Émile Fourcault in early 1900s
 “vertical draw” process, glass is drawn against gravity in an upward direction
 Glass rollers hold the ribbon which is drawn upwards where it is rapidly cooled
 flat glass which is suitable for lesser quality uses. can have waves, seeds (small gas bubbles) or stones (undissolved materials).
 Varying thickness
 Replaced by float glass

Question 12

automating glass manufacturing

Answer 12

Patented in 1848 by Henry Bessemer.
 His system produced a continuous ribbon of flat glass using rollers.
 expensive as the surfaces of the glass needed polishing
Bessemer early form of “Float Glass” in 1843, which involved pouring glass onto liquid tin.
1887
the mass production of glass was developed
 semi-automatic process used machines capable of producing 200 standardized bottles per hour

Question 13

what did Pilkington invent in 1898

Answer 13

Wired Cast glass, where the glass incorporates a strong steel-wire mesh for safety and security

Question 14

what did Pilkington improve in 1938

Answer 14

polished plate by using a double grinding polishing process

Question 15

modern float glass

Answer 15

1953 -1957, Pilkington and Bickerstaff developed the float glass process,
 first successful commercial application for forming a continuous ribbon of glass using a molten tin bath on which the molten glass flows unhindered under the influence of gravity
 gave uniform thickness and very flat surfaces.
 Modern windows are made from float glass.

Question 16

the float process

Answer 16

A float line half a kilometre long. Raw materials enter at one end and at the other, plates of glass emerge, at 6,000 tonnes a week.

Question 17

what are the 6 stages of the float process

Answer 17

melting and refining
float bath
coating
annealing
inspection
cutting to order

Question 18

stage 1 melting and refining

Answer 18

Continuous melting process, for 50 hours, glass is free from inclusions and bubbles

Question 19

stage 2 float bath

Answer 19

Glass from the melter flows over a spout on to the mirror-like surface of molten tin, starting at 1,100oC and leaving as a solid ribbon at 600oC.
Thickness range from sub-millimetre to 25mm; with almost optical perfection. Fire finish, the lustre of new chinaware.

Question 20

stage 3 coating

Answer 20

 On-line chemical vapour deposition (CVD) of coatings, less than a micron thick, to reflect visible and infrared wavelengths
 Multiple coatings deposited in the few seconds
 CVD will replace composition as the principal way of varying the optical properties of float glass.

Question 21

stage 4 annealing

Answer 21

 considerable stresses are developed in the ribbon as it cools.
 Too much stress and the glass will break beneath the cutter.
 To relieve these stresses the ribbon undergoes heat-treatment in a long furnace known as a lehr.
 heated above a transition point then allowed to cool slowly.
 Annealed glass breaks into large, jagged shards that can cause serious injury

Question 22

stage 5 inspection

Answer 22

 Occasionally a bubble is not removed during refining, a sand grain refuses to melt, a tremor in the tin puts ripples into the glass ribbon
 Automated on-line inspection does two things.
 It reveals process faults upstream that can be corrected.
 it enables computers downstream to steer cutters round flaws.
 100 million measurements a second locating flaws

Question 23

stage 6 cutting to order

Answer 23

Diamond wheels trim off selvedge- stressed edges- and cut the ribbon to size dictated by computer

Question 24

self cleaning glass

Answer 24

 In 2001Pilkington Activ™,
 clean themselves through the action of water,
 hydrophobic by rolling droplets
 hydrophilic by sheeting water
 Hydrophilic coatings based on titania, chemically break down absorbed dirt in sunlight by photocatalysis
 20–30 nm layer of nanocrystalline anatase form of titanium dioxide deposited by CVD onto float glass.
 20% more expensive, self-cleaning is slow process, need sun and rain, can be abraded off

Question 25

mechanism

Answer 25

• UV light shines on the titanium dioxide coating, electrons are released.
• electrons interact with water molecules, breaking them up into hydroxyl radicals (OH·), which are highly reactive
• OH· attacks the organic molecules of dirt, breaking apart their chemical bonds and oxidising them into smaller, harmless substances such as carbon dioxide and water.
• OH· also make the glass hydrophilic (water-loving). When it rains, water molecules spread evenly across it and wipe it clean

Question 26

toughened (tempered) glass

Answer 26

 physically and thermally stronger than normal glass
 No sharp edges on breakage
 Rapid cooling of hot glass puts outer surfaces in compression and interior in tension
 Reduced surface flaws

Question 27

laminated glass

Answer 27

 interlayer, (PVB) or (EVA), between its two or more layers of tempered glass
 A typical laminated makeup is 2.5 mm glass, 0.38 mm interlayer, and 2.5 mm glass.
“5.38 laminated glass”
 Resist penetration “security glass”
 Restrain passengers in a vehicle collision
 Invented in France “Triplex”
 Originally used cellulose acetate (1902) or cellulose nitrate(1903-36) for interlayer but discoloured
 Sheets can be different colours

Question 28

silicon dioxide (silica) SiO2

Answer 28

• Occurs naturally as sand and in its purest form as quartz crystals
• Mpt 1710 degrees Celsius
• Extend 3 D structure which is very ordered
• The Si atom shows tetrahedral coordination, with four oxygen atoms surrounding a central Si atom
• Number of distinct crystalline forms (polymorphs) as well as amorphous forms

Question 29

Crystalline silica

Answer 29

ordered and arranged

Question 30

amorphous silica

Answer 30

disordered and unarranged

Question 31

melting and cooling silica

Answer 31

melting becomes disordered as bonds break and it achieves amorphous structure

on cooling the disorder is retained ‘glassy’ structure

explains the irregular breakage pattern of glass (no crystal plane)

Question 32

soda lime glass

Answer 32

• 80% of all commercial use
• Used for windows, jars, jugs and everyday drinking glass
• Composition: 60-75% silica, 12-18% soda, 5-12% lime
• Produced by heating above raw materials to 1675 degrees Celsius

Question 33

glass structure

Answer 33

see powerpoint

Question 34

soda lime glass properties

Answer 34

• Adding soda lowers mpt and viscosity
• Increases electrical conductivity
• Adding lime makes it insoluble
• Na+ and ca 2+ ions terminate the network and soften the glass
• Soda and lime decrease the softening point of the glass from 1600 to 700 degrees making it easier to fabricate in objects
• Addition of 1-4% MgO to prevent cracks
• Addition of 0.5-1.5% Al2O3 to increase durability of glass
• Expands on heating
• Breaks easily on heating and cooling

Question 35

special glasses Sodium borosilicate glass, Pyrex

Answer 35

silica + boric oxide + soda (Na2O) + alumina

• Used for chemical glassware, cooking glass, car head lamps, etc.
• low coefficients of thermal expansion

Question 36

special glasses Aluminosilicate glass

Answer 36

• silica + alumina + lime + magnesia (MgO) + barium oxide (BaO) + boric oxide
• Fiberglass, glass-reinforced plastics(GRP) e.g. boats
• resistant to weathering

Question 37

special glass Lead-oxide glass,

Answer 37

silica + lead oxide + potassium oxide + soda (Na2O) + zinc oxide + alumina.
- high density therefore a high refractive index, looks “brilliant”(called “crystal“).
- If less than 24%PbO called “crystal glass”
- high elasticity, making glassware “ring”
- fragile and is easier to cut
- Glass with 65% PbO used as radiation shielding
 lead absorbs gamma rays etc.

Question 38

special glass Germanium-oxide glass

Answer 38

• alumina + germanium dioxide (GeO2)
• Extremely clear , diameter of a human hair
• fiber-optic waveguides in communication networks.
• Only 5% of light intensity lost through 1 km of glass fiber

Question 39

special glass cadmium oxide

Answer 39

glasses are good slow neutron absorbers

Question 40

special glass arsenic sulfide glass

Answer 40

transmittance in IR out to ~13µm (769cm-1)

Question 41

special glass recycled glass

Answer 41

saves on raw materials and energy. Impurities in the cullet can lead to product and equipment failure.

Question 42

special glass fused quartz

Answer 42

very low thermal expansion, can be used at high temperatures(1000–1500 °C) e.g.as furnace tubes, lighting tubes, melting crucibles, etc.