Slag tapped from pressurized slagging gasifiers has unique properties not associated with similar slag formed under nonpressurized conditions. Upon heating to a temperature in the 800 to 900 °C range, this slag expands to as much as twenty times its initial volume, producing a glass foam that is relatively inert and could be used to produce commercially viable insulating products. Initial attempts to fabricate loosefill aggregate insulation have resulted in a light-weight foam with a thermal conductivity and density comparable to those of vermiculite. Spectrochemical analysis of gas released upon crushing a specimen detected no toxic or noxious components.