Wool is a natural fiber derived primarily from sheep fleece, and its interaction with heat and flame is unique among common textiles. While wool technically burns, it does so with difficulty and does not readily sustain a flame. This natural resistance results from its specific chemical composition and underlying protein structure.
Wool’s Reaction When Exposed to Fire
When exposed to a direct flame, wool fibers exhibit slow ignition compared to many other fabrics. Wool has a high ignition temperature, requiring the heat source to reach 570 to 600 degrees Celsius before combustion begins. This is significantly higher than the ignition point of cotton, which burns at approximately 255 degrees Celsius.
Wool’s most notable characteristic is its tendency to self-extinguish once the external flame source is removed. Instead of bursting into a rapid flame, wool tends to smolder and then stop burning entirely. During combustion, wool shrinks away from the flame and forms a soft, black char or bead.
Unlike synthetic materials, this charred residue does not melt or drip, preventing the burning material from fusing to the skin. A distinct odor of burning hair or feathers is produced, which signals a protein fiber reacting to heat. This charring and self-extinguishing property demonstrate wool’s inherent fire safety.
The Chemical Basis for Fire Resistance
Wool’s fire-resistant behavior stems directly from its primary structural component, the protein keratin. Keratin is rich in nitrogen, which acts as a natural flame retardant. Nitrogen suppresses or dilutes the volatile gases produced during combustion, slowing the chemical reaction required to sustain a flame.
Wool naturally retains a high level of moisture, often up to 10% of its weight. This trapped water must be heated and evaporated before the fiber reaches ignition temperature, absorbing substantial energy from the heat source. When subjected to extreme heat, the fiber’s cross-linked cellular structure swells, forming an insulating char layer on the surface.
This charring mechanism acts as a barrier, protecting the underlying fiber from heat and cutting off the oxygen supply necessary for the flame to spread. Wool’s high Limiting Oxygen Index (LOI) further illustrates its resistance, requiring an oxygen concentration of 25.2% to maintain combustion, whereas the Earth’s atmosphere contains only about 21% oxygen.
Comparing Wool to Common Clothing Fibers
Wool’s fire performance contrasts sharply with both plant-based fibers and petroleum-derived synthetics. Cellulosic fibers, such as cotton and linen, ignite at a much lower temperature and burn quickly, spreading flame rapidly and leaving behind a light, fine ash. These materials lack wool’s self-extinguishing properties and can become fully engulfed quickly.
Synthetic fibers, including polyester and nylon, pose a significant safety risk. These fabrics melt at relatively low temperatures (nylon melts between 160 and 260 degrees Celsius, and polyester between 252 and 292 degrees Celsius). When they melt, these hot, sticky plastics drip and fuse to the skin, often causing severe burns.
Wool’s non-melting char formation is a major safety advantage, avoiding the skin-fusing hazard associated with synthetics. Additionally, wool produces less smoke and fewer toxic fumes during combustion than most synthetic materials. This reduction in smoke density and toxicity is important for fire safety, since inhalation of fumes is a leading cause of fatalities in structure fires.