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.
