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
