There are two main methods involved in the production of glass – the float glass process that produces sheet glass, and glassblowing that produces bottles and other containers.
Here, we are primarily interested in the float glass process. Float glass is a sheet of glass which is made by floating molten glass on top of molten metal – usually tin, although it is possible to use lead and other alloys with a low melting point. Modern windows are made using this process as is almost all flat glass we use today.
Between 1953 and 1957, Sir Alastair Pilkington and Kenneth Bickerstaff developed the float glass process, which is often still referred to as the Pilkington process. Before this flat glass was made by either cutting large discs of crown glass or blowing large glass cylinders which were then cut open and flattened. During the process, molten glass is fed into a ‘tin bath’, a bath containing molten tin. A continuous ribbon of molten glass is fed from the melting furnace onto the surface of the tin bath where it literally floats along the surface, forming a ribbon of uniform thickness. Tin is used for this process because it is cohesive, immiscible with molten glass and has a high specific gravity. To prevent oxidisation the tin bath needs to have a positive pressure protective atmosphere of nitrogen and hydrogen.
The other main use of metal in glass manufacture is the colouring process. Glass may be given different colours by the addition of colouring ions, striking glass (precipitation of nanometer sized colloides) and coloured inclusions. Below are some of the main additives and the colours they produce:
· Iron oxide – produces a blueish-green colour and is frequently used in the manufacture of beer bottles. When combined with Chromium, the two produce a richer green which is used in the manufacture of wine bottles.
· Manganese – if used in small amounts manganese will remove the green tint given by iron. In larger amounts it produces an amethyst colour. Manganese is one of the oldest glass additives with a history dating back to the Egyptian period.
· Nickel – the colour produced by nickel depends on the concentration used and it can vary from blue to violet to black. Nickel and cobalt together can be used to decolourise lead glass.
· Gold – in small concentrations gold produces a rich, ruby red colour – ‘Ruby Gold’, or a more cranberry hue in even smaller concentrations. The colour is dependent on the size and dispersion of the gold particles. Sometimes copper can be used in place of gold to produce a similar colour.
· Silver – silver compounds, such as silver nitrate and silver halides can produce a range of colours from orange to red to yellow. The way the glass is treated can affect the end colour, particularly during the heating and cooling process.