**6. Charring rate of wood**

The charring rate, speed at which charring depth advance in the material when exposed to high temperature, is a critical parameter for flammability of wooden samples because it allows the determination of the size of the residual section of wood. It depends on wood species, density, moisture content, permeability, composition and direction of burning [28]. For example, the charring rate of flame retardant-treated wood is linearly proportional to the applied heat flux in cone calorimeter and inversely proportional to the density of wood. Charred wood is bounded by the transition between the pyrolysis layer, the zone where thermal degradation of wood and char formation is actually occurring and the char layer, a zone of cracked charcoal that has no relevant strength or stiffness properties. Charring depth is the distance between the outer surface of the original member and the position of the char line. The base of char layer is widely occurs between 280 and 300°C. Beikircher et al. determined the charring rate, mass loss and temperature development of Norway wood coated with transparent and colored intumescent coatings using cone calorimeter and ISO 834 furnace test. They found that the intumescent coatings reduce the charring rate significantly at all irradiance and cellulosic fire exposure condition (ISO 834 test curve) in comparison with the uncoated (REF) wood sample (**Figure 2**). Intumescent coatings can delay the onset of charring and reduce the charring rate of wood [29].

**Figure 2.** Charring rate of intumescent coated and uncoated wood exposed to different test conditions [29].
