Looks like cotton thread. Actually Nomex or Kevlar wrapped in cotton. Does not melt. Decomposes at 700F.
|Use fire retardant Nomex (tm) and Kevlar (tm) for sewing seams and attaching patches to clothing and gear used by first responders, motor sports participants, and others who need protection from fire and heat. It has a flat, cotton-like finish, and meets NFPA standards: 1975-1999, 1971-1997, 1976-1992, and 1977-1998.||