I recently knit a sweater out of 100% wool (Patons Classic Merino, to be exact) and after a few wears, there is considerable felting in the armpit area. I guess it's not very surprising, considering that I made it out of a yarn that is highly recommended for felting projects and the armpit is a pretty warm, high-friction area. Has anyone else experienced this? Have you found that armpit felting is more common with certain fibre blends (I would imagine that acrylic-wool blends would be better off, and superwash should, by definition, be immune)? What about sweaters that you only wear on top of t-shirts? Are my armpits just freakish felting machines? It doesn't really bother me that much, since only I'm the only who would ever notice it, but I feel like I need to know more about this phenomenon!
If nothing else, at least now you all know: if you're knitting a sweater out of the same wool you once used to make a felted purse, be forwarned!