Glass Flashcards
ancient glass production
melting together sand (silicon dioxide), an alkali oxide, which lowers the melting temp; and line, which stabilises the mixture and makes it less soluble in water
core formed, ductile rope of glass wound round a shaped core of sand and clay then heat fusing glass ropes
hellenistic period
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
enables simple vessel to be made in few mins
no longer luxury item for rich
weathering e.g. shifts in temp and moisture are harmful to glass
roman empire
used pure white silica sand
cast glass windows with poor optical qualities, began to appear
anglo-saxon
vessels, windows, beads, jewellery
from 10th century onwards, glass was employed in stained glass windows of churches
bohemian glass is a decorative glass produced in regions pf Bohemia
crown glass
hot blown gas cut open opposite the pipe, then rapidly spun on table before cooling down
centrifugal force shaped the hot globe of glass into a round, flat sheet
very expensive and could not be used to make large panes
importance in glass manufacture
addition of lead oxide to the molten glass
improved glass appearance and was easier to melt
increased working period of the glass, making it easier to manipulate
blown plate glass
rolled molten glass poured on an iron table allowing the manufacture of very large plates
The polishing process was industrialised 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
- blowing long cylinders of glass, then cut along the length and flattened onto a cast-iron table, before being annealed.
fourcault process
manufacture of flat glass
“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 cooles
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
polished plate
1938, Pilkington improved polished plate by using a double grinding polishing process
modern float glass
1953 -1957, Sir Alastair Pilkington and Kenneth Bickerstaff developed the revolutionary 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 success of this process lay in the careful balance of the volume of glass fed onto the bath, where it was flattened by its own weight
Full scale profitable sales of float glass were first achieved in 1960.
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.
the float process stage 1
Melting and refining
- continuous melting process, for 50 hours, glass is free from inclusions and bubbles
the float process stage 2
float bath
- Glass from the melter flows gently 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.
the float process 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.
the float process 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
the float process 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 the unaided eye would be unable to see.
the float process 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 (water spreads evenly)
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 for self cleaning glass
UV light shines on the titanium dioxide coating, electrons are released
Electrons interact with water molecules (H2O) in the air, breaking them up into hydroxyl radicals (OH-), which are highly reactive, short-lived
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
H2O +e +H+ = OH. + H2
toughened (tempered) glass - safety glass
physically and thermally stronger (less brittle) 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, of polyvinyl butyral (PVB) or ethylene-vinyl acetate (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
Glass sheets may be of different colours
Silicon dioxide “silica” SiO2
Occurs naturally as sand and in its purest form as quartz crystals
Mpt 1710 degrees Celsius
Extended 3D 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
melting SiO2
On melting SiO2 becomes disordered as bonds break and it achieves an amorphous structure
cooling SiO2
On cooling the disorder is retained “glassy” structure
explains the irregular breakage pattern of glass (no crystal plane)
soda lime glass
Adding soda lowers mpt and viscosity
Increased electrical conductivity
Adding lime makes it insoluble
Na+ and Ca+ ions terminate the network and soften the glass
Soda and lime decrease the softening point of the glass from 1600 degrees Celsius to 700 degrees Celsius 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
sodium borosilicate glass, pyrex - special glass
silica (81%)+ boric oxide (12%B2O3) + soda (4.5%Na2O) + alumina (2.5%Al2O3)
Used for chemical glassware, cooking glass, car head lamps, etc.
low coefficients of thermal expansion 3.25×10−6/°C (typical soda-lime glass 9×10−6/°C)
aluminosilicate glass - special glass
silica + alumina + lime + magnesia (MgO) + barium oxide (BaO) + boric oxide
Fiberglass, glass-reinforced plastics (GRP) e.g. boats,
resistant to weathering
lead oxide glass - special glass
silica + lead oxide (24%PbO) + potassium oxide (K2O) + soda (Na2O) + zinc oxide (ZnO) + 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.
geranium oxide glass - special glass
alumina + germanium dioxide (GeO2).
Extremely clear, diameter of a human hair
fibre-optic waveguides in communication networks.
Only 5% of light intensity lost through
1 km of glass fibre
cadmium oxide - special glass
good slow-neutron absorbers
arsenic sulphide glass - special glass
transmittance in IR out to ~13um (769c-1)
recycles glass - special glass
saves on raw materials and energy
impurities in the cullet can lead to product and equipment failure
fused quartz - special glass
very low thermal expansion, can be used to high temperatures
where was the first glass made and when
First glass made in north Syria or ancient Egypt – 3500BC
what are the earliest known glass objects
beads
what was the alkali of syrian and egypitian glass
soda ash, sodium carbonate
what was ancient glass prodcution replace by and when
replaced by cast method by 1st century AD
what was the earliest method of glass window manufacture
crown glass
when and where was blown plate glass developed
developed in 1620 in London
what is blown plate glass used for
used for mirrors and coach plates
who and when developed automating glass manufacture
Patented in 1848 by Henry Bessemer
system produced a continuous ribbon of flat glass using rollers.
Bessemer also introduced what
an early form of “Float Glass” in 1843, which involved pouring glass onto liquid tin.
when and where was the mass production of glass devloped?
1887, the mass production of glass was developed in Castleford, Yorkshire.
- semi-automatic process used machines capable of producing 200 standardised bottles per hour
wired cast glass
1898, Pilkington invented Wired Cast glass, where the glass incorporates a strong steel-wire mesh for safety and security
what percentage of commerical glass is soda lime glass
80%
what is soda lime glass used for
Used for windows, jars, jugs, and everyday drinking glass
what is the composition of soda lime glass
Composition
50-75% silica
12-18% soda
5-12% CaO
how is soda lime glass produced
Produced by heating above raw materials to 1675 degrees Celsius
