Float Glass Annealing LehrAfter the formed glass ribbon leaves the tin bath, the glass is at a temperature of 530C. If the glass ribbon were allowed to cool in free air, the surfaces would cool much more rapidly than the internal body of the glass. This would cause the surface to be in severe compression and would also cause unwanted internal stresses in the ribbon.
The thermal history of glass during and immediately after forming is such that internal stresses are bound to be present. It is therefore necessary to bring the glass to ambient temperature gradually by a controlled thermal treatment - annealing. In practice, annealing is carried out in a long annealing lehr oven, 6m wide and 120m long, with a predetermined temperature gradient through which the glass passes. In the case of containers and table ware, the product is transported on a conveyor belt and for flat glass on a special roller conveyor. Controlled electric heating elements are precisely located in the lehr to maintain a consistent temperature profile across the width of the glass ribbon.
The end result of the annealing process is glass which has been carefully cooled to ambient temperatures over a long distance, without the introduction of temporary or residual stresses.