BELGLAS gives education to glass engineering companies, who want to extend their scope to cellular glass. Indeed, from float glass to endless ribbon foamed cellular glass is only a small step.

A typical question is always: “Can we temper cellular glass?”. The answer is “No” and in the following I give the “Why”.

By this occasion, I want to introduce the very nice work about glass tempering by Jonathan Barr, namely “The Glass Tempering Handbook“. After a good introduction about glass, soda lime glass, the float glass method, stress in glass, he comes to the tempering of glass. It can be a good help to understand the following.

The first thing to understand is why tempering is done. The following figure shows you that the glass plate comes much more resistant against bending. After tempering, the compressive stress on the surface works against the bending tensile strain on the bottom surface. Like already explained, the weakness of glass is located on the surface,when tension is applied, but the bulk is extremely strong. The large tension stress in the bulk is for that reason not at all a problem.

Like can be find on the PRODUCTS page, also cellular glass has compressive stress on the surface and tensile stress in the bulk after annealing or fast cooling. However, cellular glass is in fact one big glass surface, also in the bulk. For that reason, it is rather weak to tension forces in the bulk, contrary with not-foamed glass. This is why tempering is not possible for cellular glass. Too fast cooling in the annealing range induces too much residual tension in the bulk, causing spontaneous breakage.

If we want a higher bending strength, we need to work with thicker cellular glass at a higher density. Tempering is never a solution for cellular glass.