Aerogels are any material of a group of very light or porous solids. The lightest aerogel is less than four times as dense than dry air. They are made by certain gels being heated under pressure, causing the liquid in the gel to lose surface tension and become supercritical, or in a state between a liquid and gas. In this state, the liquid can be removed from the gel by adding additional heat, without messing up the porous structure made by the gel's solid component.
Silica-, melamine-, and carbon- based aerogels have been manufactured in the past. The lightest are silica-based aerogels and some have been appropriately nicknamed 'solid smoke' or 'frozen smoke'. These lightweight aerogels are almost transparent. Heavier aerogels were first produced in 1931 and have since been used to detect fast moving particles given off by particle accelerators
Newer, lighter aerogels with fairly high insulating properties are being tested to replace more dangerous foams as insulation in refrigerators and as substitutes for the air between double pane windows.