Was the fabric ever washed? Whether it was washed or not, did you ever spray it with sizing or starch? I ask because it's possible that you simply scorched the sizing or starch that is on the fabric. If this is the case, the mark will wash out with soap and water. Unwashed fabric will have sizing still in it from the factory. If you have sprayed with sizing or starch, that adds an additional layer that separates the fabric from the heat of the iron. I once scorched the starch on a fabric I was ironing; washing took the starch (and the scorch mark) out!