Every physicist, working with glass, is fascinated by the glass transition. This glass transition range is responsibe for the residual stress on the glass. In this range where the glass is not purely elastic but also not purely viscous, behaves the glass  strange. Its properties depends on the way it is cooled. Thermal expansion, viscosity and specific heat depends largely on the cooling rate through the transition range. The first ones to find that experimentally were L.R.Troussart and P.Gérard from Belgium.

The first ones, who could describe theoretically the glass transition were  P.Gilard and J. De Bast, also from Belgium. With their equations, the residual stress, being the sum of relaxed stress and structural stress could be calculated. The structural stress is a consequence of the fact that the surface of the glass has another cooling history as the bulk of the glass through the transition range and so slightly other properties.

The structural part becomes very large for tempered glass, due to the very fast cooling but becomes negligible for the relative slow cooling of cellular glass. This slow cooling is a consequence of the fact that temperature gradients are easily developed in cellular glass due to its low thermal conductivity.

For most cellular glass people, the annealing is the most difficult job because it is a black box where you can’t see what is happening. Also for float glass, CNUD EFCO is market leader for annealing furnaces. The gravity point of cellular glass and for float glass lehrs is already a long time in Belgium. Probably is that the heritage of Gilard and De Bast, great guys ….