Glass Flashcards
5 major forms of flat glass and description
- Spun Glass, molten glass rotated and formed as a circular sheet. Ornamental
- Blown Glass, blown into a cylindrical mould and allowed to cool. Ornamental
- Drawn Sheet Glass, molten glass is drawn up from the furnace and passed through metal rollers
- Plate Glass, drawn sheet glasses which is ground and polished after manufacture
- Float Glass, Glass from furnace is poured in a continuous ribbon and floats along the top of a bath of molten tin
Raw material of glass
Sand
Other materials in glass
Cullet, which is broken scrap glass,
Limestone
Dolomite, a double carbonate containing calcium and magnesium carbonate
Soda Ash
Why are other materials added to glass
They lower the melting point of glass from 1700C to as low as 800C
How does iron oxide in glass change it
Gives it a green tint, which can be removed by adding manganese dioxide
How does lead change glass
Adds surface lustre or in high quantities a glass resistant to x-rays
How does borax change glass
Produces a glass with low thermal movement
Describe the production of float glass
- Glass from furnace is poured in a continuous ribbon and floats along the top of a bath of molten tin
- The glass is held in a chemically controlled atmosphere at a high enough temperature and for sufficient time for irregularities to melt out
- The ribbon of glass is then cooled, whilst still advancing across the tin until the surfaces are hard enough to be annealed
What does annealed mean
Heating of glass then allowing it to cool slowly in order to remove internal stresses and toughen it
How does the density of concrete compare to that of glass
Similar density
Strength of glass
- Needs to resist wind, impact and thermal stresses
- Glass is elastic up to breaking point
- Completely brittle and gives no warning of failure
Glass Properties : Thermal Movement
The coefficient of thermal expansion for glass is lower than the materials in which it is fixed to (e.g. window frame). Therefore allowance needs to be made of 3mm between frame and pane of glass
Glass Properties: Thermal Insulation
-Glass has a high thermal conductivity, so offers little resistance to conducted heat flow. The main resistance comes from air layers which impede convection heat flow
Glass Properties: Behaviour in Fire
Glass is non-combustible and doesn’t break or melt/ The typical fire resistance of glass is 30mins
Difference between toughened glass laminated glass
Toughened glass is specially treated so that it will break into small, blunt pieces under pressure, making it much safer than shards of normal glass. and is up to five times stronger than regular glass. Laminated glass features a plastic interlayer which holds the glass shards in place should it break, laminated glass is thicker than toughened glass and, therefore, offers better insulation and resistance against any shock or damage
Has is toughened glass produced
Glass is heated uniformly to its softening point and then is cooled rapidly by jets of air. The surface initially shrinks relative to the core, when the core cools down and shrinks the surface layer are placed under compression and the core in tension . This makes the glass more resistant to loading in bending.
Glass can also be toughened by chemical treatment or ion diffusion
What is the optimum size gap in double glazing
20mm
How can the greenhouse effect be overcome
- Using tinted heat absorbent types of glass which absorb up to 50% of the heat
- Heat reflecting glass, usually has a thin metallic coating applied to the inner face of the outer sheet of glass
How are glass fibres made
By drawing filaments from the base of a platinum crucible containing molten glass. These fibres are collected into strands and then formed into rovings, woven into cloth or chopped to form matting
