We found a nice recent paper paper about the foaming of glass, sponsored by the Russian federation, cited as follows:

This research work is carried out with the financial support of the Scholarship of the President of the Russian Federation for young scientists and postgraduates (2016-2018, #SP-1074.2016.1).

Maybe, that is the most important fact. For many years, the development was only in the hands of private capital but now governments are aware of the importance of cellular glass for the society. Today Russia and Slovenia, yesterday Denmark and it is clear, more will come. That gives a good feeling: many brains and huge piles of money are increasing the knowledge about this wonderful product. Today it is only available for the happy few but that will clearly change.

The paper focuses on foaming of waste glass and selects the glycerin / water glass because in this system, a furnace with neutral atmosphere will perform well contrary to carbon (black) foaming. The water glass protects the glycerin for oxidation while the glycerin delivers the carbon by decomposition.

More in detail, the paper selects the best ratio glass / water glass and glycerin. Some experiments are shown here under:

B3 at 825°C seems the best candidate.