I sometimes use “float glass” – have you ever wondered why its called float glass?
Float glass is a sheet of glass made by floating – literally – molten glass on a bed of molten metal, typically tin, although lead and various low melting point alloys were used n the past.
I’m sure we are all pleased that lead isn’t used today!
Tin is a chemical element with the symbol Sn (from the Latin: stannum).
The float glass process is also known as the Pilkington process, named after the British glass manufacturer Pilkington, who pioneered the technique. The inventor was Sir Alastair Pilkington. Here is a brief synopsis of his fascinating story.
Until the 16th century window glass or flat glass was generally cut from large discs (or rondels) of crown glass.
The process of making larger sheets was slow and unreliable.
They were made by blowing large cylinders which were cut opened and flattened then cut into panes.
Most window glass in the early 19th century was made using this cylinder method. The ‘cylinders’ were 6 – 8foot (180 to 240cm) long and 10-14 inches (25 o 36cm) in diameter, limiting the width that panes of glass could be.
This is why you see old windows with small panes of glass. These buildings are very typical examples of this.
The first advances in automating glass manufacturing were patented in 1848 by Henry Bessemer, an English Engineer. His system produced a continuous ribbon of flat glass by forming the ribbon between rollers. This was an expensive process as the surfaces of the glass then needed polishing.
Between 1953 and 1960 Lionel Alexander Bethune Pilkington (Sir Alastair Pilkington; (and others) developed the 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! 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.
The molten glass is fed into a “tin bath” a bath of molten tin (about 3-4m wide 50m long 6cm deep) from a delivery canal and is poured into the tin bath by a ceramic lip known as the spout lip. The amount of glass allowed to pour onto the molten tin is controlled by a gate rather charmingly called a Tweel.
Tin is suitable for the float glass process because it has a high specific gravity, it is cohesive, and immiscible into the molten glass. Tin, however oxidizes in the natural atmosphere to form tin dioxide (SnO2). Known in the production process as dross, the tin dioxide adheres to the glass. To prevent oxidation, the tin bath is provided with a positive pressure protective atmosphere consisting of a mixture of nitrogen and hydrogen.
The glass flows onto the tin surface forming a floating ribbon with perfectly smooth surfaces on both sides and an even thickness.
As the glass flows along the tin bath, the temperature is gradually reduced from 1100 degrees C until the sheet can be lifted from the tin onto rollers at approximately 600 degrees C.
The glass ribbon is pulled off the bath by rollers at a controlled speed. Variation in the flow speed and roller speed enables glass sheets of varying thicknesses to be created.
Once off the bath, the glass sheet passes through a Lehr kiln for approximately 100m where it is further cooled gradually so that it anneals (the molecules “stick together”) without strain and does not crack from the change in temperature. As the process developed, larger and larger sheets of glass were able to be produced. Float glass has a slightly greenish tinge when closely inspected. Here is the edge of my Giant Centre Piece Leaf.
One exiting the “cold end” of the kiln, the glass is cut by machines.
Pilkington Group Limited is now a multinational glass nanufacturing company headquartered in St Helens, United kingdom and a wholly owned subsidiary of the Japan based NSG Group. In Australia, the company changed its name in the recent past to Viridian. Ironically my husband was a consultant on the name change process.
For more information about the process of making float glass – please refer to this pdf: http://www.pilkington.com/resources/datasheet2float.pdf
See how versatile float glass is? I also make these gorgeous nachos plates that double as soup bowls too. Do you like them?
I hope you found this interesting. Talk next week 🙂
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