Cotton is a natural fiber primarily composed of cellulose, a plant polymer that dictates its reaction to heat. Its widespread use in textiles is due to its breathability and durability. While not fireproof, cotton’s chemical structure provides a different type of heat tolerance than synthetic fabrics. Understanding its thermal properties is key to safely caring for cotton items and recognizing the risks of extreme heat exposure.
The Thermal Threshold of Cotton
Cotton possesses high resistance to the controlled heat encountered in everyday use. This resilience stems from its cellulosic structure, which responds to heat by degrading rather than melting. Unlike synthetic materials, cotton will char, turning black and brittle.
The upper limit for safe, prolonged exposure is approximately 150°C (302°F). Sustained temperatures above this range, such as leaving a hot iron stationary, begin thermal degradation, appearing as scorching or yellowing. This discoloration is an irreversible chemical change caused by the breakdown of cellulose chains.
Irreversible thermal decomposition, or pyrolysis, generally begins in pure cotton around 260°C to 270°C (500°F to 518°F). The material undergoes a chemical transformation at this point, releasing volatile compounds. This high decomposition threshold means cotton is often considered safer than synthetics near heat sources, as it will not melt onto the skin.
Practical Heat Handling and Care
Cotton’s thermal stability allows it to withstand the high temperatures of household appliances. White or heavily soiled items can handle a hot wash (60°C/140°F or higher for sanitization). However, warm water (around 40°C/104°F) is recommended for colored or everyday items to minimize dye fading and control shrinkage.
Moisture, heat, and agitation in a tumble dryer cause cotton fibers to contract, leading to shrinkage. For heavy items like towels and denim, a high-heat setting (60°C to 65°C/140°F to 150°F) is appropriate for efficient drying. Lighter garments, such as t-shirts, benefit from a medium heat setting to reduce excessive shrinkage.
When ironing, cotton requires the highest heat setting, typically ranging from 180°C to 220°C (350°F to 430°F). Steam is often used to quickly loosen the hydrogen bonds within the cellulose structure, effectively smoothing out wrinkles. Moving the iron constantly prevents the surface temperature from exceeding the scorching point and causing permanent damage.
What Happens When Cotton Burns
When cotton is exposed to a direct flame or spark, its flammability characteristics become apparent. The ignition temperature, where the fabric sustains a flame, is low, typically 210°C to 235°C (410°F to 455°F) for pure cotton. This low ignition point means cotton is easy to ignite.
Once ignited, cotton burns quickly and vigorously with a yellow flame, similar to how paper burns. The fire is propagated by the rapid decomposition of cellulose into flammable gases and tar. Unlike most synthetic materials that melt and drip, cotton maintains its structure while burning, leading to a char that turns into a soft, light-gray ash.
The residue is easily crushed between the fingers, distinguishing it from the hard, plastic-like bead left by melted synthetics. Cotton can also undergo self-ignition, or spontaneous combustion, if heated internally to about 400°C to 420°C (752°F to 788°F) without an external spark. Specialized flame-retardant treatments can be applied to alter its combustion process, causing it to form a heavier char layer that slows fire spread.