Optical microscopy was used to characterize the structure of the asproduced foam using an Olympus BX 60 microscope. The pores of the specimens were filled with a cold-hardening epoxy resin before grinding and polishing for optical examination. Additionally, the surface morphology and elemental composition in cross-sections of the specimens were examined by a JEOL 840A scanning electron microscope (SEM) equipped with an OXFORD INCA energy-dispersive spectroscopy (EDS) analyzer.