The fabrics used in the outer layer of firefighting clothing are blends of fibers that are very resistant to high temperatures, such as para-aramid and meta-aramid. These materials are inherently flame and heat resistant, and also have the ability to self-extinguish when the ignition source is removed. However, the outer layer fabrics are extremely difficult to dye, and the dye used to color the material is not able to withstand the same high temperatures as the fibers. In turn, the dyes will react to exposure to ultraviolet rays, found in both natural and artificial light, causing the material to often show a change in color.
When the outer layer fabric shows signs of discoloration, usually what has occurred is a sublimation of the dye – a physicochemical phenomenon that consists in the direct passage of a substance from the solid to the gaseous state and vice-versa, without passing through the liquid state.
The change in color of the outer layer material is usually an indication of exposure to excessive ultraviolet light or the result of a thermal incident (contact of the garment with the flame). When sublimation is the result of thermal exposure, laboratory tests show that the dye usually begins to exhibit change at temperatures in the region of 232°C, with or without direct flame contact.
Discoloration alone is no cause for despair, since the heat and flame characteristics are inherent and will not be lost by this factor. However, the material must be inspected for mechanical strength. This can be accomplished by applying pressure with your bare hands to pull and push the fabric in the discolored areas, if the fabric tears it should be considered inoperable and discarded.
If any part of the outer layer is exposed to enough heat to cause discoloration, then the other barriers should also be carefully checked. Especially the moisture barrier, which is located closest to the outer layer.