A highly porous material based on metal oxides or silica. It has a very low density below 10 kg m^–3 and has excellent heat and electrical resistance as well as acoustic properties. Aerogels can be formed using the process of supercritical drying using carbon dioxide to remove a solvent such as ethanol used in their formation. Being supercritical and without a gas–liquid interface, it avoids the crushing effects of capillary forces on the porous structure during a conventional drying process.