Which method is NOT used to make a material flame retardant?

Heating is not a method used to make a material flame retardant because it typically does not alter the material's properties to improve its resistance to flames. Flame retardant materials are designed to withstand combustion, and their effectiveness usually stems from their chemical composition or additional treatments rather than being applied heat.

In contrast, chemical change involves altering the chemical structure of a material to improve its flame resistance, which may include adding flame retardant chemicals that react with the material to enhance its fire performance. Impregnation refers to the process of infusing a substance with flame retardant solutions, enabling deeper penetration and better distribution of the fire-retardant properties throughout the material. Coating involves applying a fire-retardant layer onto the surface of a material, which can create a protective barrier that inhibits combustion.

Therefore, while chemical change, impregnation, and coating can all effectively help to make a material flame retardant, heating does not serve this purpose in the same way.