1) History:

The first window was made blowing large bottles. There after the bottles were cut in half and flattened. Windows were then cut out of these panes. These bottles were 2 to 2.5m long and 250 to 350mm in diameter.

Alastair Pilkington is the inventor of the float glass process. Before float glass larger sheets of plate glass were made casting glass as a liquid from onto an iron surface. There after it is grinded and polished on both sides to even it out.

2) General information:

Float glass is made melting raw materials consisting of sand, limestone, soda ash, dolomite, iron oxide and salt cake. The raw materials are mixed together and put into a furnace. The mixture is heated up to form a large pool of molten glass. It is then fed into a float bath. The amount of glass to flow into the molten tin is controlled by a refactory gate called a tweel. The tin bath is pressurized that causes the glass to flatten out to form a flat smooth surface.

As the glass flows into the bath it is cooled gradually until the sheet can be lifted and placed onto the rollers. It travels about a further 100m while it is cooled completely. After this process machines takes the sheet of float glass and cuts it to size.

Annealed glass means slowly cooled glass. Thus what we would refer to as normal float glass and not safety glass. Any glass has a greenish color; the more panes you put on top of each other will cause the greenish color to become darker

Did you know: Glass stays in a liquid form even after the toughening process. Signs of this you can see in all the old buildings were you will find that a piece of glass will be about 1-2mm thicker at the bottom than at the top. This means that glass is constantly flowing downwards.

Glass weight: = 2.5Kg/mm/m².

  • 6mm glass would be 2.5 x 6 = 15Kg/m²,
  • 8mm = 20Kg/m² (Our panel of 650 x 2100 would thus weigh 27Kg)
  • 10mm = 25Kg/m²

Glass tensile strength (N/mm²):

  • Annealed = 40Mpa
  • Toughened 120 – 200Mpa with an average of 160Mpa for our application which makes is 4 times stronger than normal glass

Young’s module for glass (E) = 7×10(to the power of 10) PA = 70Gpa

3) Toughened glass

Toughened glass is a form of safety glass that is 400% stronger than your normal float glass that will shatter into many small pieces when broken and can deflect up to 150mm. To test if a piece of toughened glass to see if it is compatible to specification you need to take this piece of glass and mark out a square (50mm x 50mm) on the surface of the glass. Holding a board on opposite sides of the glass it is shattered. The pieces it broke into within the square marked on the glass are counted. 50 pieces or more is the amount that it needs to break into for it to be compatible to specification. If the count is below 50 it means that toughening process weren’t long enough. The longer a piece of glass stays in the furnace or oven the more pieces it will break into. Toughened glass is a form of annealed glass put trough a tempered process.

The glass is placed on a roller table, taking it trough a furnace heating it up to 700 degrees. The glass surface is then rapidly cooled forcing cool air onto the glass. Rapidly cooling the glass surface causes it to harden while the inner layer of the glass is still free to flow for a short period. The final contraction of the inner layer causes compressive stresses in the surface balanced by tensile stresses in the body of the glass. The compressive stress gives the glass its increased stress. Any surface flaws are pressed closed by the compressive force. The cooling pattern during this process can be shown using a polarized light, which shows the strain patterns in the glass.

Toughened glass is most vulnerable to breakage on the edge of a pane of glass. This is due to the fact that the tensile stress being the greatest at this point. Shattering of toughened glass would not necessarily happen right away when the surface of the glass suffers a hard impact. Any minor stresses like the sudden change in temperature will cause the glass to brake over time. Toughened glass needs to be cut, drilled and polished before toughening. On the downside toughened glass are more susceptible to scratches than float glass. The reason for this is that toughened glass surface is not as hard as normal float glass.

You get two different types of toughening, fully tempered and rim tempered were only the outside edge of a piece of glass will be toughened.

4) Usages:

Toughened glass has many usages here are just a few:

  • Aluminum widows
  • Sliding doors
  • Showers
  • Glass dinnerware
  • Drinking glasses
  • Any frameless products
  • Microwave safe dinnerware
  • Transport industry
  • Any application that can cause a safety risk