This is a timely post! I recently got a bunch of fabric, and it all smelled "old." I washed it with Borax, and most of it came out fine. Some of it, however, had the bad smell "revive" when I ironed it. It was thinner fabric, and I wondered if it was perhaps a poly-cotton blend and that's what made the smell stay. The rest was fine, no smell at all.
Interesting...