Inherent Fire-Retardant Fabrics and Fibers are naturally resistant to combustion when exposed to sparks or flames because of the molecular structure of the fiber and innate natural fire-retardant properties. Wool and silk burn slowly, smoldering and charring are difficult to ignite, and may self-extinguish. Some other inherent FR materials are latex, inherent rayon, A tightly woven wool fabric will take longer to burn than other untreated natural fabrics, such as cotton and linen which can ignite quickly, with a fast-moving flame spread.