Saturday, 8 January 2011

Float Glass Characteristics in Relation to Kiln Forming

A reported 90% of the world's flat glass is produced by the float glass process invented in the 1950's by Sir Alastair Pilkington of Pilkington Glass. Molten glass is “floated” onto one end of a molten tin bath. The glass is supported by the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in a lehr. The finished product has near-perfect parallel surfaces.

An important characteristic of the glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch. It also becomes apparent when compressed.
Float glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.

The heat characteristics of Float glass depend in large part on which company manufactures the glass being used, so the temperature characteristics are given in ranges.

The softening point is around 760C

The annealing point is around 560—540C

The strain point is around 525-505C. The strain point being the temperature below which no further annealing can occur, but the glass can still be thermally shocked below this range.

The characteristic of float glass having a molecular level of tin left on the “tin side” but not the “air side” is important to distinguish. If any forming of the glass is planed after fusing, the tin side in compression will show a “tin bloom” similar to devitrification.

The fact that there are many manufacturers of float glass means that they are not all made to the same specifications. It is not advisable to fuse float glass from different suppliers in kiln forming, so the best advice is to fuse only from one sheet for each piece.

Due to the robustness of float glass, it can be fired with a quicker initial temperature rise than glasses formulated for kiln forming. The down side is that it devitrifies very easily and very badly. Rarely can you perform more than two firings before the devitrification begins to become troublesome.