These intumescents produce a light char, a poor conductor of heat, which thus delays heat transfer. Ablative coatings, for example, contain a significant amount of hydrates. When the hydrates are heated, they decompose, and water vapour is released, which has a cooling effect. Once the water is spent, the insulation characteristics of the char that remains can slow down heat transfer from the exposed side to the unexposed side of an assembly.
Soft char products are typically used in thin-film intumescent coatings for fireproofing structural steel and firestop pillows. The expansion pressure created for these products is usually very low because the soft carbonaceous char has little substance, which is beneficial if the aim is to produce a layer of insulation.