The Manufacture of Float Glass
A non-stop process, nearly half a kilometer long.
First of all the ingredients for ‘Soda Lime Glass’ Float Glass are collected, tested and measured, then they are carefully mixed, under computer control, into the correct ratios, (all are approximate), Silica sand (73%), Soda (14%), Calcium oxide (9%) and Magnesium (4%). The mixture is divided into batches and ‘Cullet’ (bits of recycled glass) is added, the raw materials then pass from the mixing silos into the furnaces.
The furnaces are usually sectioned into chambers, then at around 1500’C the raw materials become molten, this molten material drains from the furnace onto a bed of molten tin, with a temperature of around 1000’C. The molten tin is very fluid and ‘watery’, and the molten glass is very viscous, so the two liquids do not mix, and the contact surface between them is incredibly flat.
As the glass reaches the end of this bed of molten tin, it cools to about 600’C, and at 600’C the glass is hard enough to pass over rollers without being marked, the glass is then annealed, which changes the internal stresses on the glass and ensures a degree of flatness, it also allows the glass to be cut and worked in a predictable way.
This method of glass production sees both surfaces ‘fire finished’, so for normal applications, they need no further grinding or polishing. After further cooling, the glass is quality checked, cut and stacked for distribution. In most plants in the world today, the entire process is automatic and computer controlled, producing many million tonnes of ‘plate’ or ‘float glass per year.