The coupling of the oxidation of SiC, Si3N4 and AlN powders, added as foaming agents to ground cullet, with the reduction of oxides of metals with multiple valence states (such as MnO2, Fe2O3 and CeO2) has been proven to be particularly effective for the foaming of recycled soda-lime-silica glass. The foaming generated by oxidation/reduction couples is more intensive and above all it leads to a more homogeneous microstructure in comparison to carbide or nitride foaming agents used alone. Nitrides, coupled with MnO2, provide the best compromise between expansion and mechanical strength, and some significant microstructural differences observed in the fabricated glass foams can be attributed to the differences in their size compared to that of the cullet powder.

Reference: GARCÍA-TEN, J.; SABURIT, A.; ORTS, M.J.; BERNARDO, E.; COLOMBO, P. Glass foams from oxidation/reduction reactions using SiC, Si3N4 and AlN powders. Glass Technol.: Eur. J. Glass Sci. Technol. A, 52(4), 103-110, 2