Intumescent coatings react to heat by swelling in a controlled manner to many times their original thickness to produce a carbonaceous char which acts as an insulating layer to product the steel section to which it is applied.
Intumescence is a complex process in which chemical reactions take place to convert the coating to a viscous liquid. At the same time, gases are produced and trapped within the liquid. This causes the swelling and formation of the insulating char.
The intumescent mechanism and subsequent char formation absorbs heat from the fire helping to keep the temperature of the steel below its limiting temperature and to provide the required period of structural fire-resistance.