Glass History and manufacture Flashcards
History of Silicate glass
- 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.
ancient glass production
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
hellenistic period many new techniques of glass production
- canes of multi-coloured glass are sliced and the slices arranged together and fused in a mould to create a mosaic-like effect
blown glass
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
Roman Empire
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
Middle Ages
- 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
crown glass
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
important advance in glass manufacture
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
blown plate glass
- 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.
modern industrial processes
- 1832 the British crown glass company developed cylinder method to produce sheet glass
fourcault process
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
automating glass manufacturing
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
what did Pilkington invent in 1898
Wired Cast glass, where the glass incorporates a strong steel-wire mesh for safety and security
what did Pilkington improve in 1938
polished plate by using a double grinding polishing process
modern float glass
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.
the float process
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.
what are the 6 stages of the float process
melting and refining float bath coating annealing inspection cutting to order
stage 1 melting and refining
Continuous melting process, for 50 hours, glass is free from inclusions and bubbles
stage 2 float bath
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.
stage 3 coating
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.
stage 4 annealing
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
stage 5 inspection
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
stage 6 cutting to order
Diamond wheels trim off selvedge- stressed edges- and cut the ribbon to size dictated by computer
self cleaning glass
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
mechanism
- 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
toughened (tempered) glass
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
laminated glass
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
silicon dioxide (silica) SiO2
- 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
Crystalline silica
ordered and arranged
amorphous silica
disordered and unarranged
melting and cooling silica
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)
soda lime glass
- 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
glass structure
see powerpoint
soda lime glass properties
- 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
special glasses Sodium borosilicate glass, Pyrex
silica + boric oxide + soda (Na2O) + alumina
- Used for chemical glassware, cooking glass, car head lamps, etc.
- low coefficients of thermal expansion
special glasses Aluminosilicate glass
- silica + alumina + lime + magnesia (MgO) + barium oxide (BaO) + boric oxide
- Fiberglass, glass-reinforced plastics(GRP) e.g. boats
- resistant to weathering
special glass Lead-oxide glass,
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.
special glass Germanium-oxide glass
- 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
special glass cadmium oxide
glasses are good slow neutron absorbers
special glass arsenic sulfide glass
transmittance in IR out to ~13µm (769cm-1)
special glass recycled glass
saves on raw materials and energy. Impurities in the cullet can lead to product and equipment failure.
special glass fused quartz
very low thermal expansion, can be used at high temperatures(1000–1500 °C) e.g.as furnace tubes, lighting tubes, melting crucibles, etc.
