Glass

Question 1

5 major forms of flat glass and description

Answer 1

1. Spun Glass, molten glass rotated and formed as a circular sheet. Ornamental
2. Blown Glass, blown into a cylindrical mould and allowed to cool. Ornamental
3. Drawn Sheet Glass, molten glass is drawn up from the furnace and passed through metal rollers
4. Plate Glass, drawn sheet glasses which is ground and polished after manufacture
5. Float Glass, Glass from furnace is poured in a continuous ribbon and floats along the top of a bath of molten tin

Question 2

Raw material of glass

Answer 2

Sand

Question 3

Other materials in glass

Answer 3

Cullet, which is broken scrap glass,
Limestone
Dolomite, a double carbonate containing calcium and magnesium carbonate
Soda Ash

Question 4

Why are other materials added to glass

Answer 4

They lower the melting point of glass from 1700C to as low as 800C

Question 5

How does iron oxide in glass change it

Answer 5

Gives it a green tint, which can be removed by adding manganese dioxide

Question 6

How does lead change glass

Answer 6

Adds surface lustre or in high quantities a glass resistant to x-rays

Question 7

How does borax change glass

Answer 7

Produces a glass with low thermal movement

Question 8

Describe the production of float glass

Answer 8

1. Glass from furnace is poured in a continuous ribbon and floats along the top of a bath of molten tin
2. The glass is held in a chemically controlled atmosphere at a high enough temperature and for sufficient time for irregularities to melt out
3. The ribbon of glass is then cooled, whilst still advancing across the tin until the surfaces are hard enough to be annealed

Question 9

What does annealed mean

Answer 9

Heating of glass then allowing it to cool slowly in order to remove internal stresses and toughen it

Question 10

How does the density of concrete compare to that of glass

Answer 10

Similar density

Question 11

Strength of glass

Answer 11

• Needs to resist wind, impact and thermal stresses
• Glass is elastic up to breaking point
• Completely brittle and gives no warning of failure

Question 12

Glass Properties : Thermal Movement

Answer 12

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

Question 13

Glass Properties: Thermal Insulation

Answer 13

-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

Question 14

Glass Properties: Behaviour in Fire

Answer 14

Glass is non-combustible and doesn’t break or melt/ The typical fire resistance of glass is 30mins

Question 15

Difference between toughened glass laminated glass

Answer 15

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

Question 16

Has is toughened glass produced

Answer 16

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

Question 17

What is the optimum size gap in double glazing

Answer 17

20mm

Question 18

How can the greenhouse effect be overcome

Answer 18

• 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

Question 19

How are glass fibres made

Answer 19

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