About 50 years ago, an invention was made and patented in the VEB Schaumglaswerk Taubenbach, East-Germany. I give the abstract hereunder.

A process for making foam glass wherein mixtures of sulfur trioxide-containing glass and active carbon are first sintered in a pure steam atmosphere having a partial pressure above 200 mm. Hg and subsequently heated at a temperature ranging from 800° to 900 °C to form a foam structure.

Glass powder is sintered in a steam atmosphere before foaming. The glass powder is a mixture of a sulphate containing glass and active coal. Due to this steam atmosphere, a better foaming behaviour is obtained and the cells contains up to 11% H2S instead of less than 1%.

It seems that this procedure allows to foam a lower density with improved thermal conductivity.