On my survival trip in June, I spent every night of the trip in and around campfires of various sizes in my 80/20 (wool/acrylic) ebay tank, and saw no problems of any kind. Sparks flew on it, it landed in the fire once or twice and didn't even smolder. It may have been that the kilt had dirt in it, or could have been damp that prevented it from burning, but I experienced no negative reaction of the kilt to the fire.

It could also have been that I has a magical invisible fireproof cloak on...who knows.