While some researchers argue that the higher lignin content of softwood makes it inherently more fire-resistant than hardwood due to increased char formation, others point out that softwood generally has higher thermal conductivity, which can accelerate heat transfer and contribute to faster fire spread within a structure. However, studies on cellulose char have shown that at higher densities, the char layer becomes more cohesive, restricting oxygen flow to the underlying fuel and significantly reducing fire spread regardless of conductivity. This suggests that the density of the char layer, rather than the initial lignin content or thermal conductivity of the wood, might be the most crucial determinant of a timber structure’s fire resistance.
The statement that “studies on cellulose char have shown that at higher densities, the char layer becomes more cohesive, restricting oxygen flow…” plays which one of the following roles in the argument?
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