The standard method to produce an open celled foam is using a mineral foaming agent, which induces crystallisation during foaming. This crystallisation is opening partially the cells. Typical foaming agents for open cell glass foams are CaCO3 and MnO2. However, up to now, 100% open cells are not obtained and low densities around 100 kg/m³ are not possible.

However, an original method was already patented in 1949. Standard closed cell foam (as an example GLAPOR cellular glass foamed with glycerin) is put under an hydrostatic pressure of the order of the compressive strength. The cell walls are broken one by one untill all cells are opened. In this way, an 100% open celled foam is obtained.

This open cell foam can be used as an acoustic absorber, fiber free and non-combustible. I guess this is still a unique product. This application was patented in 1963 and it is shown that some extra holes are made in the cellular glass to absorb better  the lower frequencies (curve B compared with curve A). The absorption is even better with these holes in the back side of the sample (curve C).

The use of hydrostatic pressure  is a difficult step in mass production. It would be much better if we could foam directly a 100% open cell foam from waste glass. At that moment, we would have a unique product at a low cost.

In the race to the holy grail vacuum cellular glass, there is a discussion about which thermal conductivity could be reached. In my opinion, lower than 0.021 W/mK is not possible today and this open cell material in a vacuum bag below 0.1 mbar could prove that.
I suggested this once on a meeting in the University of Freiberg, Germany. A certain well known glass professor claimed  that manufacturing  an open cell foam in the above way is not possible, the foam should be reduced to dust. I guess that was what we call today an “alternative fact“.