A thermal insulator is a material which does not allow the passage of heat; therefore the particles which make up the material are not free to move. Plastics, wood, and air can be thermal insulators providing that NO thermal currents can develop. Air in trapped clothing, air between the panes of glass in double glazing are examples, as are a lid to cover the coffee mug. Ionic compounds are good thermal insulators. Magnesium carbonate, an ionic compound, is sometimes used. Ionic compounds have extremely high melting points and therefore, if they were heated they would not melt.